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Biological Membrane

Biological membrane

A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. It is, almost invariably, a lipid bilayer (except for Archaea which have isoprene membranes), being composed of a double layer of lipid-class molecules, specifically phospholipids, with occasional proteins intertwined, some of which function as channels. Such membranes typically define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside. For example, the membrane around peroxisomes shields the rest of the cell from peroxides, and the plasma membrane separates a cell from its surrounding medium. Most organelles are defined by such membranes, and are called membrane-bound organelles. Probably the most important feature of a biomembrane is that it is a selectively permeable structure. This means that the size, charge and other chemical properties of the atoms and molecules attempting to cross it will determine whether they succeed to do so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings. If a particle is too large or otherwise unable to cross the membrane by itself, but is still needed by a cell, it could either go through one of the protein channels, or be taken in by means of endocytosis. More general information on this topic may be found in the article at cell membrane.

Structure

Composition

The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol.

cholesterol

Phospholipids and glycolipids consist of two long, nonpolar (hydrophobic) hydrocarbon chains linked to a hydrophilic head group. In the phospholipids the head consist of phosphorylated either: :
- Glycerol (and hence the name phosphoglycerides given to this group of lipids). :
- Sphingosine (with only one member - sphingomyelin).

In the glycolipids the head contains of sphingosine with one or several sugar units attached to it. The hydrophobic chains belong either to: :
- two FAs - in the case of the phosphoglycerides. :
- one FA and the hydrocarbon tail of sphingosine - in the case of sphingomyelin and the glycolipids.

The FAs in phospho- and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon FAs are the most common ones. FAs may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of FAs chains have a profound effect on membranes fluidity.

In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the FAs. The C-3 hydroxyl group is esterified to phosphoric acid. The resulting compound, called phosphatidate, is the simplest phosphoglycerate. Only small amounts of phosphatidate are present in membranes. However, it is a key intermediate in the biosynthesis of the other phosphoglycerides.

Sphingosine is an amino alcohol that contains long, unsaturates hydrocarbon chain. In sphingomyelin and glycolipids, the amino group of sphingosine is linked to a FAs by an amid bond. In sphingomyelin the primary hydroxyl group of sphingosine is esterified to phosphoryl choline. In glycolipids, the sugar component is attached to this group. The simplest glycolipid is cerebroside, in which there is only one sugar residue, either Glc or Gal. More complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues.

Membrane

A membrane is a thin, typically planar structure or material that separates two environments. Because it sits between environments or phases and has a finite volume, it can be referred to as an interphase rather than an interface. Membranes selectively control mass transport between the phases or environments. Biological membranes include:
- Cell membrane and intracellular membranes
- mucous membrane
- S-layer Artificial membranes are used in:
- Reverse osmosis
- Filtration (Microfiltration, Ultrafiltration)
- Pervaporation
- Dialysis
- Electrodialysis
- Emulsion liquid membranes
- Membrane-based solvent extraction
- Membrane reactors
- Gas permeation
- supported liquid membranes Theoretical membranes are used in:
- M-theory (simplified) simple:Membranes

Cell (biology)

(red) and DNA (green)]] The cell is the structural and functional unit of all living organisms, and are sometimes called the "building blocks of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,000 billion or 10¹⁴ cells). The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells and that cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word cell comes from the Latin cella, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to small rooms monks lived in.

Overview

Properties of cells

cork Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities. All cells share several abilities:
- Reproduction by cell division.
- Metabolism, including taking in raw materials, building cell components, creating energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from metabolic pathways.
- Synthesis of proteins, the functional workhorses of cells, such as enzymes. A typical mammalian cell contains up to 10,000 different proteins.
- Response to external and internal stimuli such as changes in temperature, pH or nutrient levels.
- Traffic of vesicles.

Types of cells

vesicle One way to classify cells is whether they live alone or in groups. Organisms vary from single cells (called single-celled or unicellular organisms) that function and survive more or less independently, through colonial forms with cells living together, to multicellular forms in which cells are specialized. 220 types of cells and tissues make up the multicellular human body. Cells can also be classified into two categories based on their internal structure.
- Prokaryotic cells are structurally simple. They are found only in single-celled and colonial organisms. In the three-domain system of scientific classification, prokaryotic cells are placed in the domains Archaea and Eubacteria.
- Eukaryotic cells have organelles with their own membranes. Single-celled eukaryotic organisms such as amoebae and some fungi are very diverse, but many colonial and multicellular forms such as plants, animals, and brown algae also exist.

Subcellular components

brown alga (2) nucleus (3) ribosome (4) vesicle,(5) rough endoplasmic reticulum (ER), (6) Golgi apparatus, (7) Cytoskeleton, (8) smooth ER, (9) mitochondria, (10) vacuole, (11) cytoplasm, (12) lysosome, (13) centrioles]] centriole All cells whether prokaryotic or eukaryotic have a membrane, which envelopes the cell, separates its interior from its environment, controls what moves in and out, and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes and RNA, which contain the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell then briefly describe their function.

Cell membrane - a cell's protective coat

Main article: Cell membrane The cytoplasm of a eukaryotic cell is surrounded by a plasma membrane. A form of plasma membrane is also found in prokaryotes, but is usually referred to as the cell membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (fat-like molecules) and proteins. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell.

Cytoskeleton - a cell's scaffold

Main article: Cytoskeleton The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell's structure by directing, bundling, and aligning filaments.

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long term information storage, but some viruses (retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g. mRNA) and enzymatic functions (e.g. ribosomal RNA) in organisms that use RNA for the genetic code itself. Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory). A human cell, e.g. has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). The nuclear genome is divided into 46 linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins. Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is.

Organelles

Main article: Organelle The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called organelles, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are only found in eukaryotes.
- Cell nucleus - a cell's information center: The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm.
- Ribosomes - the protein production machine: Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an mRNA. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called translation. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.
- Mitochondria and chloroplasts - the power generators: Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex metabolic pathways. Chloroplasts are larger than mitochondria, and convert solar energy into a chemical energy ("food") via photosynthesis. Like mitochondria, chloroplasts have their own genome. Chloroplasts are found only in photosynthetic eukaryotes like plants and algae. There are a number of plant organelles that are modified chloroplasts; they are broadly called plastids and are often involved in storage.
- Endoplasmic reticulum and Golgi apparatus - macromolecule managers:: The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface, and the smooth ER, which lacks them. Translation of the mRNA for those proteins that will either stay in the ER or be exported from the cell occurs at the ribosomes attached to the rough ER. The smooth ER is important in lipid synthesis, detoxification and as a calcium reservoir. The Golgi apparatus, sometimes called a Golgi body or Golgi complex is the central delivery system for the cell and is a site for protein processing, packaging, and transport. Both organelles consist largely of heavily folded membranes.
- Lysosomes and peroxisomes - the cellular digestive system: Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.
- Centrioles - They help in the formation of mitotic appratus. Two centrioles are present in the animal cells. They are also found in some fungi and algae cells.
- Vacuoles-They store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane.

Anatomy of cells

Prokaryotic cells

Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
- The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon.
- Most prokaryotes have a cell wall (some exceptions are Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like fungi, but has a different chemical composition
- A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.

Eukaryotic cells

There are two types of cells, eukaryotic and prokaryotic. Eukaryotic cells are usally found in multi-cellular organisms, while prokaryotic cells are usually on their own. Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus." Other differences include:
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
- The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (i.e. folded around histones). All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA.
- Eukaryotes can move using cilia or flagella. The flagella are more complex than those of prokaryotes.

Cell functions

Cell growth and metabolism

Main articles: Cell growth, Cell metabolism Between successive cell divisions cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions; catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, where the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into a less chemically complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways. The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy. The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.

Making new cells

Main article: Cell division Cell divisions (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.]] Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells. DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.

Protein synthesis

Main article: Protein biosynthesis Protein synthesis is the process in which the cell builds proteins. DNA transcription refers to the synthesis of a messenger RNA (mRNA) molecule from a DNA template. This process is very similar to DNA replication. Once the mRNA has been generated, a new protein molecule is synthesized via the process of translation. The cellular machinery responsible for synthesizing proteins is the ribosome. The ribosome consists of structural RNA and about 80 different proteins. When the ribosome encounters an mRNA, the process of translating an mRNA to a protein begins. The ribosome accepts a new transfer RNA, or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—bearing an amino acid, the building block of the protein. Another site binds the tRNA that becomes attached to the growing chain of amino acids, forming the a polypeptide chain that will eventually be processed to become a protein.

Origins of cells

Main article: Origin of life The origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.

Origin of first cell

If life is viewed from the point of view of replicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If freely-floating DNA molecules that code for enzymes that are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example, by producing nucleotides) will automatically advantage the neighbouring DNA molecules. This might be viewed as "parasitism by default". Therefore the selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged. If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules won't benefit from a positive mutation in a neighbouring molecule: this in turn means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it. (Note. This is more a metaphor given for simplicity than complete accuracy, since the earliest molecules of life, probably up to the stage of cellular life, were most likely RNA molecules, acting both as replicators and enzymes: see RNA world hypothesis . But the core of the reasoning is the same.) Biochemically, cell-like spheroids formed by proteinoids are observed by heating amino acids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth. Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water tend to burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The Phospholipid is a good example of a common oily compound prevalent in the prebiotic seas. Phospholipids can be constructed in ones mind as a hydrophilic head on one end, and a hydrophobic tail on the other. Phospholipids also possess an important characteristic, that is being able to link together to form a bilayer membrane. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Phospholipids.html] can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multi-celluar organisms could be achieved. This theory is expanded upon in the book, "The Cell: Evolution of the First Organism" by Joseph Panno Ph.D.

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory. There is still considerable debate on if organelles like the hydrogenosome predated the origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.

History

- 1632-1723: Antony van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
- 1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope. ::...I could exceedingly plainly perceive it to be all perforated and porous, much like honeycomb...these pores or cells, were not very deep, but consisted of a great many little boxes... – Hooke describing his observations on a thin slice of cork.
- 1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the Cell Theory.
- The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822-1895).
- Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).
- 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935 he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
- 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.

References

- Category:Cell biology Category:Biology ko:세포 ms:Sel ja:細胞 simple:Cell th:เซลล์ (ชีววิทยา)

Lipid bilayer

In biology and chemistry, a lipid bilayer is a membrane or zone of a membrane composed of lipid molecules (usually phospholipids). The lipid bilayer is a critical component of all biological membranes, including cell membranes, and is a prerequisite for cell-based organisms.

Structure and function

The structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that are insoluble in water. There are two important regions of a lipid that provide the structure of the lipid bilayer: the hydrophilic region, also called a polar head region, and the hydrophobic, or nonpolar tail region. The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is repelled from such conditions. Since a lipid molecule contains regions that are both polar and nonpolar, they are called amphipathic molecules. Within a critical range of concentrations, certain kinds of lipids alone in a test tube of water will self-organize to form a "bilayer". The bilayer is composed of two opposing layers of lipid molecules arranged so that their hydrocarbon tails face one another to form the oily bilayer core, while their electrically charged or polar heads face the watery or "aqueous" solutions on either side of the membrane. Because of the oily core, a pure lipid bilayer is permeable to small hydrophobic solutes but has only a very low permeability barrier to inorganic ions and other hydrophilic molecules.

Other lipid structures

Lipids can assume self-organizing structures other than bilayers, depending on their concentration and type:
- Micelles
- Monolayers
- vesicles.

Isoprene

Isoprene
Chemical name	2-Methyl-1,3-butadiene
Chemical formula	C₅H₈
Molecular mass	68.11 g/mol
Density	0.681 g/ml
Melting point	-145.95 °C
Boiling point	34.067 °C
CAS number	78-79-5
SMILES	CC(=C)C=C
Image:Isoprene-Structure.png
200px

Isoprene is a common synonym for the chemical compound 2-methyl-1,3-butadiene. It is commonly used in industry, is an important biological material, and can be a harmful environmental pollutant and toxicant when present in excess quantities. At room temperature, isoprene is a colorless liquid which is highly flammable and easily ignited. It can form explosive mixtures in air and is highly reactive, capable of polymerizing explosively when heated. The United States Department of Transportation considers isoprene a hazardous material and requires special marking, labeling, and transportation for it. It is most readily available industrially as a by-product of the thermal cracking of naphtha or oil. About 95% of isoprene production is used to produce cis-1,4-polyisoprene - a synthetic version of natural rubber. Natural rubber is a polymer of isoprene - most often cis-1,4-polyisoprene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials are found in high quality natural rubber. Some natural rubber sources are composed of trans-1,4-polyisoprene, a structural isomer which has similar, but not identical properties.

Biological roles and effects

Isoprene is formed naturally in plants and animals and is generally the most common hydrocarbon found in the human body. The estimated production rate of isoprene in the human body is 15 µmol/kg/h, equivalent to approximately 17 mg/day for a 70 kg person. Isoprene is also common in low concentrations in many foods. Isoprene is produced in the chloroplasts of leaves of certain tree species through the DMAPP pathway; the enzyme isoprene synthase is responsible for its biosynthesis. The amount of isoprene released from isoprene-emitting vegetation depends on leaf mass, leaf area, light (particularly photosynthetic photon flux density), and leaf temperature. Thus, during the night, little isoprene is emitted from tree leaves while daytime emissions are expected to be substantial (~5 ppbV) during hot and sunny days. With a global biogenic production in the range of 350–500 Tg of carbon/year, isoprene has a large impact on atmospheric processes and is thus an important compound in the field of Atmospheric Chemistry. Isoprene affects the oxidative state of large air masses, is an important precursor for ozone, a pollutant in the lower atmosphere. Furthermore, isoprene forms secondary organic aerosols through photooxidation with OH radicals which also have wide-ranging health effects, particularly for the respiratory tract, and reduce visibility due to light scattering effects. Because of its atmospheric importance, much work has been devoted to emission studies from isoprene-emitting vegetation, and, kinetic and mechanistic studies of isoprene oxidation via OH radicals, ozone, and NO₃ radicals. It is a common structural motif in biological systems. The terpenes (for example, the carotenes are tetraterpenes) are derived from isoprene, as are the terpenoids and coenzyme Q. Also derived from isoprene are phytol, retinol (vitamin A), tocopherol (vitamin E), dolichols, and squalene. Heme A has an isoprenoid tail, and lanosterol, the sterol precursor in animals, is derived from squalene and hence from isoprene. The functional isoprene units in biological systems are dimethylallyl pyrophosphate (DMAPP) and its isomer isopentenyl pyrophosphate (IPP), which are used in the biosynthesis of terpenes and lanosterol derivatives. In virtually all organisms, isoprene derivatives are synthetised by the HMG-CoA reductase pathway. Addition of these chains to proteins is termed isoprenylation. According to the United States Department of Health and Human Services Eleventh Edition Report on Carcinogens, isoprene is reasonably expected to be a human carcinogen. Tumors have been observed in multiple locations in multiple test species exposed to isoprene vapor. No adequate human studies of the relationship between isoprene exposure and human cancer have been reported.

Biosynthesis and Its Inhibition by Statins

HMG-CoA reductase inhibitors, also known as the group of cholesterol-lowering drugs called statins, inhibit the synthesis of mevalonate. Mevalonate is a precursor to isopentenyl pyrophosphate, which combines with its isomer, dimethyl allyl pyrophosphate, in repeating alternations to form isoprene (or polyprenyl) chains. Statins are used to lower cholesterol, which is synthesized from the 15-carbon isoprenoid, farnesyl pyrophosphate, but also inhibit all other isoprenes, including coenzyme Q10. This [http://www.cholesterol-and-health.com/Synthesis-Of-Cholesterol.html flow chart]shows the biosynthesis of isoprenes, and the point at which statins act to inhibit this process.

Reference

Merck Index, Eleventh Edition, ISBN 911910-28-X. Poisson, N., M. Kanakidou, and P. J. Crutzen, "Impact of nonmethanehydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modelling results," Journal of Atmospheric Chemistry, vol. 36, pp. 157–230, 2000. Monson, R. K., and E. A. Holland, "Biospheric trace gas fluxes and their control over tropospheric chemistry," Annual Review of Ecological Systems, vol. 32, pp. 547–+, 2001. Claeys, M., B. Graham, G. Vas, W. Wang, R. Vermeylen, V. Pashynska, J. Cafmeyer, P. Guyon, M. O. Andreae, P. Artaxo, and W. Maenhaut, "Formation of secondary organic aerosols through photooxidation of isoprene," Science, vol. 303, pp. 1173–1176, 2004. Pier, P. A., and C. McDuffie, "Seasonal isoprene emission rates and model comparisons using whole-tree emissions from white oak," Journal of Geophysical Research, vol. 102, pp. 23,963–23,971, 1997. Poschl, U., R. von Kuhlmann, N. Poisson, and P. J. Crutzen, "Development and intercomparison of condensed isoprene oxidation mechanisms for global atmospheric modeling," Journal of Atmospheric Chemistry, vol. 37, pp. 29–52, 2000.

External link

[http://www.cholesterol-and-health.com/Synthesis-Of-Cholesterol.html Flow Chart Showing the Biosynthesis of Isoprenes] [http://ntp.niehs.nih.gov/ntp/roc/toc11.html Report on Carcinogens, Eleventh Edition; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program] Category:Dienes ja:イソプレン

Molecules

A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. The science of molecules is called molecular chemistry or molecular physics, depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. According to the strict definition, molecules can consist of one atom (as in noble gases) or more atoms bonded together. The concept of monatomic (single-atom) molecule is used almost exclusively in the kinetic theory of gases. In molecular sciences, a molecule consists of a stable system (bound state) comprising two or more atoms. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, Van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates. A peculiar use of the term molecular is as a synonym to covalent, which arises from the fact that, unlike molecular covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above. No typical "smallest particle" can be defined for covalent crystals, or network solids, which are composed of repeating unit cells that extend indefinitely either in a plane (such as in graphite) or three-dimensionally (such as in diamond). Although the concept of molecules was first introduced in 1811 by Avogadro, and was accepted by many chemists as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808), with notable exceptions (Boltzmann, Maxwell, Gibbs), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin (1911), and was strenuously resisted by early positvists such as Mach. The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry. Dozens of molecules have now been identified in interstellar space by microwave spectroscopy. microwave spectroscopy (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres; white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single bonds.]]

Chemical bond

:See main article chemical bond In a molecule, the atoms are joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O₂), or of different elements, as with water (H₂O).

Size

Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes. The smallest molecule is the hydrogen molecule. The interatomic distance is 0.15 nanometres (1.5 Å). But the size of its electron cloud is difficult to define precisely. Under standard conditions molecules have a dimension of a few to a few dozen Å.

Empirical formula

:See main article empirical formula The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.

Chemical formula

:See main article chemical formula The chemical formula reflects the exact number of atoms that compose a molecule. The molecular mass can be calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a ¹²C isotope atom. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular geometry

:See main article molecular geometry Molecules have fixed equilibrium geometries—bond lengths and angles—. A pure substance is composed of molecules with the same geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.

Molecular spectroscopy

:See main article spectroscopy Molecular spectroscopy is the study of the response (spectrum) of a molecule to a signal of known energy (or frequency, according to Planck's formula). This signal is usually an electromagnetic wave or a beam of electrons, but new molecular spectroscopies, such as the positron spectroscopy, are under development. The molecular response can be signal absorption (absorption spectroscopy), emission of another signal (emission spectroscopy), fragmentation, or a change in its chemical nature. Spectroscopy is recognized as the most powerful tool in the investigation of the microscopic properties of molecules, and, in particular, their energy levels. Nowadays, in order to extract the maximum microscopic information from the experimental results, spectroscopical studies are very often coupled with computational chemical investigations. The theoretical background of spectroscopy is the scattering theory.

Phospholipid

Phospholipids are formed from four components: fatty acids, a negatively-charged phosphate group, an alcohol and a backbone. Phospholipids with a glycerol backbone are known as glycerophospholipids or phosphoglycerides. There is only one type of phospholipid with a sphingosine backbone; sphingomyelin. Phospholipids are a major component of all biological membranes, along with glycolipids and cholesterol.

Phosphoglycerides

In phosphoglycerides, the carboxyl group of each fatty acid is esterified to the hydroxyl groups on carbon-1 and carbon-2. The phosphate group is attached to carbon-3 by an ester link. This molecule, known as a phosphatidate, is present in small quantities in membranes, but is also a precursor for the other phosphoglycerides.

Phosphatidyl choline

Image:Phosphatidyl-Choline.png
Phosphatidyl choline is the major component of lecithin.

Phosphatidyl ethanolamine

Image:Phosphatidyl-Ethanolamine.png
Phosphatidyl ethanolamine is the major component of cephalin.

Synthesis

In phosphoglyceride synthesis, phosphatidates must be activated first. Phospholipids can be formed from an activated diacylglycerol or an activated alcohol. Phosphatidyl serine and phosphatidyl inositol are formed from a phosphoester linkage between the hydroxyl of an alcohol (serine or inositol) and cytidine diphosphodiacylglycerol (CDP-diacylglycerol). In the synthesis of phospatidyl ethanolamine, the alcohol is phosphorylated by ATP first, and subsequently reacts with cytidine diphosphate (CDP) to form the activated alcohol. The alcohol then reacts with a diacylglycerol to form the final product. In mammals, phosphatidyl choline can be synthesized via two separate pathways; a series of reactions similar to phosphatidyl ethanolamine synthesis, and the methylation of phosphatidyl ethanolamine, which is catalyzed by phosphatidyl ethanolamine methyltransferase, an enzyme produced in the liver.

Sphingomyelin

ATP
The backbone of sphingomyelin is sphingosine, an amino alcohol formed from palmitate and serine. The amino terminal is acylated with a by a long-chain acyl CoA to yield ceramide. Subsequent substitution of the terminal hydroxyl group by phosphatidyl choline forms sphingomyelin. Sphingomyelin is present in all eukaryotic cell membranes, but is mainly present in cells of the nervous system. Sphingomyelin is wrapped around nerve cells by Schwann cells to form the myelin sheath. Multiple Sclerosis is a disease characterised by deterioration of the myelin sheath and so nerve impulses cannot be conducted along the nerve.

Amphipathic character

Due to its polar nature, the head of a phospholipid is hydrophilic (attracted to water); the nonpolar tails are hydrophobic (not attracted to water). When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other, forming a membrane with hydrophilic heads on both sides extending out into the water. This allows it to form liposomes spontaneously, or small lipid vesicles, which can then be used to transport materials into living organisms and study diffusion rates into or out of a cell membrane. This membrane is partially permeable, very flexible, and has fluid properties, in which embedded proteins and phospholipid molecules are constantly moving laterally across the membrane because of the forces generated by their vibrations. Such movement can be described by the Fluid Mosaic Model, which describes the membrane as a "mosaic" of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane... oh - and your mom goes to college!

References

# Berg, J.M., J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pagings)
- th:ฟอสโฟไลปิด

Ion channel

Another, unrelated ion channeling process is part of ion implantation. Ion channels are pore-forming proteins that help establish the small voltage gradient that exists across the membrane of all living cells (see cell potential), by controlling the flow of ions. They are present in the membranes that surround all biological cells.

Basic features

An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi-subunit" assemblies usually involve a circular arrangement of identical or related proteins closely packed around a water-filled pore through the plane of the membrane or lipid bilayer. While large-pore channels permit the passage of ions more or less indiscriminately, the archetypal channel pore is just one or two atoms wide at its narrowest point, it conducts a specific species of ion, such as sodium or potassium, and conveys them through the membrane single file--nearly as fast as the ions move through free fluid. In some ion channels, access to the pore is governed by a "gate," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel.

Biological role

Because "voltage-gated" channels underlie the nerve impulse and because "transmitter-gated" channels mediate conduction across the synapses, channels are especially prominent components of the nervous system. Indeed, most of the offensive and defensive toxins that organisms have evolved for shutting down the nervous systems of predators and prey (e.g. the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails and others) work by plugging ion channel pores. But ion channels figure in a wide variety of biological processes that involve rapid changes in cells. In the search for new drugs, ion channels are a favorite target.

Diversity and activation

- Voltage-gated channels open or close, depending on the transmembrane potential. Examples include the sodium and potassium voltage-gated channels of nerve and muscle, that are involved in the propagation of the action potential, and the voltage-gated calcium channels that control neurotransmitter release in pre-synaptic endings.
- Ligand-gated channels open in response to a specific ligand molecule on the external face of the membrane in which the channel resides. Examples include the "nicotinic" Acetylcholine receptor, AMPA receptor and other neurotransmitter-gated channels.
- Cyclic nucleotide-gated channels, Calcium-activated channels and others open in response to internal solutes and mediate cellular responses to second messengers.
- Stretch-activated channels open or close in response to mechanical forces that arise from local stretching or compression of the membrane around them; for example when their cells swell or shrink. Such channels are believed to underlie touch sensation and the transduction of acoustic vibrations into the sensation of sound.
- G-protein-gated channels open in response to G protein-activation via its receptor.
- Inward-rectifier K channels allow potassium to flow into the cell in an inwardly rectifying manner, i.e, potassium flows into the cell but not out of the cell. They are involved in important physiological processes such as the pacemaker activity in the heart, insulin release, and potassium uptake in glial cells.
- Light-gated channels like channelrhodopsin are directly opened by the action of light
- Resting channels remain open at all times. Certain channels respond to multiple influences. For instance, the NMDA receptor is partially activated by interaction with its ligand, glutamate, but is also voltage-sensitive and only conducts when the membrane is depolarized. Some calcium-sensitive potassium channels respond to both calcium and depolarization, with an excess of one apparently being sufficient to overcome an absence of the other.

Detailed structure

Channels differ with respect to the ion they let pass (for example, Na⁺, K⁺, Cl⁻), the ways in which they may be regulated, the number of subunits of which they are composed and other aspects of structure. Channels belonging to the largest class, which includes the voltage-gated channels that underlie the nerve impulse, consists of four subunits with six transmembrane helices each. On activation, these helices move about and open the pore. Two of these six helices are separated by a loop that lines the pore and is the primary determinant of ion selectivity and conductance in this channel class and some others. The channel subunits of one such other class, for example, consist of just this "P" loop and two transmembrane helices. The determination of their molecular structure by Roderick MacKinnon using X-ray crystallography won a share of the 2003 Nobel Prize in Chemistry. Because of their small size and the difficulty of crystallizing integral membrane proteins for X-ray analysis, it is only very recently that scientists have been able to directly examine what channels "look like." Particularly in cases where the crystallography required removing channels from their membranes with detergent, many researchers regard images that have been obtained as tentative. An example is the long-awaited crystal structure of a voltage-gated potassium channel, which was reported in May 2003. One inevitable ambiguity about these structures relates to the strong evidence that channels change conformation as they operate (they open and close, for example), such that the structure in the crystal could represent any one of these operational states. Most of what researchers have deduced about channel operation so far they have established through electrophysiology, biochemistry, gene sequence comparison and mutagenesis.

History

The existence of ion channels was hypothesized by the British biophysicists Alan Hodgkin and Andrew Huxley as part of their Nobel Prize-winning theory of the nerve impulse, published in 1952. Channel's existence was confirmed in the 1970s with an electrical recording technique known as the "patch clamp," which led to a Nobel Prize to Erwin Neher and Bert Sakmann, the technique's inventors. Hundreds if not thousands of researchers continue to pursue a more detailed understanding of how these proteins work. In the last years the development of automated patch clamp devices helped to increase the throuput in ion channel screening signigicantly.

References

# Two textboks that discuss ion channels are: Neuroscience (2nd edition) Dale Purves, George J. Augustine, David Fitzpatrick, Lawrence. C. Katz, Anthony-Samuel LaMantia, James O. McNamara, S. Mark Williams, editors. Published by Sinauer Associates, Inc. (2001) [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.chapter.227 online textbook] and Basic Neurochemistry: Molecular, Cellular, and Medical Aspects (6th edition) by George J Siegel, Bernard W Agranoff, R. W Albers, Stephen K Fisher and Michael D Uhler published by Lippincott, Williams & Wilkins (1999): [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=bnchm.chapter.421 online textbook]

External links

- [http://physrev.physiology.org/cgi/content/full/80/2/555 The Voltage Sensor in Voltage-Dependent Ion Channels]
- [http://www.cellbio.wustl.edu/faculty/huettner/69.pdf X-ray crystal structure of a potassium channel]
- [http://www.ionchannels.org Ion Channel, Biophysics and Electrophysiology Resources] Category:membrane biology Category:Biochemistry ja:イオンチャンネル

Compartment

In heraldry, a compartment is a design placed under the shield, usually rocks, a grassy mount, or some sort of other landscape upon which the supporters are depicted as standing. Care must be taken to distinguish true compartments from items upon which supporters are merely resting one or more feet. It is sometimes said to represent the land held by the bearer. As an official part of the blazon it is a comparatively late feature of heraldry, often derived from the need to have different supporters for different families or entities, although sometimes the compartment is treated in the blazon separately from the supporters. The decorative flourish which was often placed by heraldic artists under the feet, hooves or paws of supporters, chiefly in the 19th century, was disparagingly known by some as the "gas bracket," although this term never had any official currency; the only case in which something similar was ever actually mentioned in the blazon was the "arabesque" vert on which the whale supporters of Zaanstad, Noord Holland, the Netherlands, balance. Usually when arms are augmented by supporters, a compartment will be added too. In rare cases, a compartment might be granted as an augmentation. A compartment without supporters is possible but practically unknown, with the exception of the Coat of Arms of South Australia. A compartment is usually some kind of landscape (in the case of Scottish chiefs it is generally a "mount vert" [grassy mount] covered with the clan's flower) or seascape, and these can be quite elaborate, particularly in more recent Canadian grants, such as the compartment of the University of Northern British Columbia, in which the female kermodei bear and woodland caribou buck stand on a forest, mountain peaks and ears of wheat, all rising out of the conventionalised heraldic representation of water, which is itself charged with an orca as designed by Ron Sebastian. (Compartments can have a specific piece of geography; Kenya's compartment is Mt. Kilimanjaro and the compartment of Arbelaez, Cundinamarca, Colombia is a globe.[http://www.fotw.net/flags/co-cunar.html]) However, there are some unusual compartments. The compartment of the Association of Universities and Colleges of Canada is a quadrangle.[http://www.gg.ca/heraldry/pub-reg/project-pic.asp?lang=e&ProjectID=382&ProjectElementID=1333] [http://www.civicheraldry.co.uk/cumberland.html The arms of] the Cumberland County Council has a wall as a compartment, while the Canadian Academy of Engineering [http://collection.nlc-bnc.ca/100/200/301/ic/can_digital_collections/governor/heraldry/CanAcadEngine.html has a bridge spanning water]. The chief of Clan Donnachaidh has a main in chains as a compartment, while that of Dundas of that Ilk is "a salamander in flames of fire". [http://www.ngw.nl/int/aus/gisborne.htm The arms of] Gisborne, New Zealand contain another unique compartment. Category:Heraldry

Chemistry

Chemistry (derived from the Arabic word kimia, alchemy, where al is Arabic for the) is the science of matter that deals with the composition, structure, and properties of substances and with the transformations that they undergo. In the study of matter, chemistry also investigates its interactions with energy and itself (see physics, biology). Because of the diversity of matter, which is mostly composed of different combinations of atoms, chemists often study how atoms of different chemical elements interact to form molecules and how molecules interact with each other. molecules

Introduction

Chemistry is a large field encompassing many subdisciplines that often overlap with significant portions of other sciences. The fundamental component of chemistry is that it involves matter in some way (this explains its broad reach). It may involve the interaction of matter with non-material phenomena such as energy. More central to chemistry is the interaction of matter with other matter such as in the classic chemical reaction where chemical bonds are broken and made, forming new molecules. Matter, such as the chair you are sitting on or the air you breathe, is known today to consist of molecules. Each molecule consists of small bits of matter known as atoms that are connected together through chemical bonds. Each atom consists of smaller bits of matter known as subatomic particles. The structure of the world we commonly experience and the properties of the matter we commonly interact with are determined by the nature of this matter on the chemical level. Steel is hard because of how the atoms are bound together. Wood will burn because it can react with oxygen in a chemical reaction. Water is a liquid at room temperature because of how each molecule of water interacts with its neighbors. In fact, you are a thinking, sentient being because of an on-going series of chemical reactions and other chemical interactions. You can see this text because of how light interacts with molecules called proteins in the back of your eye. Chemistry is often called the central science because it is what connects most of the other sciences together. Chemistry is in some ways physics on a larger scale and in some ways is biology or geology on a smaller scale. Chemistry is used to understand and make better materials for engineering. It is used to understand the chemical mechanisms of disease as well as to create pharmaceuticals to treat disease. Chemistry is somehow involved in almost every science, every technology and every "thing". With such a large area of study, it is impossible to know everything about chemistry and very difficult to summarize the field concisely. Even the most knowledgable, experienced chemist only knows a very narrow area of chemistry better than others. Of course, most chemists have a broad general knowledge of many areas of chemistry as well. Chemistry is divided into many areas of study called subdisciplines in which chemists specialize. The chemistry taught at the high school or early college level is often called "general chemistry" and is intended to be an introduction to a wide variety of fundamental concepts and to give the student the tools to continue on to more advanced subjects. Many concepts presented at this level are often incomplete and technically inaccurate yet of extraordinary utility. Chemists regularly use these simple, elegant tools and explanations in their work when they suffice because the best solution possible is often so overwhelmingly difficult and the true solution is usually unobtainable. The science of chemistry is historically a recent development but has its roots in alchemy which has been practiced for millennia throughout the world. The word chemistry is directly derived from the word alchemy, however the etymology of alchemy is unclear (see alchemy).

Subdisciplines of chemistry

Chemistry typically is divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry. ; Analytical chemistry : Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdiciplines of chemistry, exluding purely theoretical chemistry. ; Biochemistry : Biochemistry is the study of the chemicals, chemical reactions and chemical interactions that take place in living organisms. Biochemistry and organic chemistry are closely related f.e. in medicinal chemistry. ; Inorganic chemistry : Inorganic chemistry is the study of the properties and reactions of inorganic compounds. The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline of organometallic chemistry. ; Organic chemistry : Organic chemistry is the study of the structure, properties, composition, mechanisms, and reactions of organic compounds. ; Physical chemistry : Physical chemistry or physicochemistry is the study of the physical basis of chemical systems and processes. In particular, the energetics and dynamics of such systems and processes are of interest to physical chemists. Important areas of study include chemical thermodynamics, chemical kinetics, electrochemistry, statistical mechanics, and spectroscopy. Physical chemistry has large overlap with molecular physics. ; Theoretical chemistry : Theoretical chemistry is the study of chemistry via theoretical reasoning (usually within mathematics or physics). In particular the application of quantum mechanics to chemistry is called quantum chemistry. Since the end of the second world war, the development of computers has allowed a systematic development of computational chemistry, which is the art of developing and applying computer programs for solving chemical problems. Theoretical chemistry has large overlap with molecular physics. ; Other fields : Astrochemistry, Atmospheric chemistry, Chemical Engineering, Electrochemistry, Environmental chemistry, Geochemistry, History of chemistry, Materials science, Medicinal chemistry, Molecular Biology, Molecular genetics, Nuclear chemistry, Organometallic chemistry, Petrochemistry, Pharmacology, Photochemistry, Phytochemistry, Polymer chemistry, Supramolecular chemistry, Surface chemistry, and Thermochemistry.

Fundamental concepts

Nomenclature

Nomenclature refers to the system for naming chemical compounds. There are well-defined systems in place for naming chemical species. Organic compounds are named according to the organic nomenclature system. Inorganic compounds are named according to the inorganic nomenclature system. See also: IUPAC nomenclature

Atoms

Main article: Atom. An atom is a collection of matter consisting of a positively charged core (the nucleus) which contains protons and neutrons, and which maintains a number of electrons to balance the positive charge in the nucleus.

Elements

Main article: Chemical element. An element is a class of atoms which have the same number of protons in the nucleus. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together. Lists of the elements by name, by symbol, and by atomic number are also available. See also: isotope

Compounds

Main article: Chemical compound A compound is a substance with a fixed ratio of chemical elements which determines the composition, and a particular organisation which determines chemical properties. For example, water is a compound containing hydrogen and oxygen in the ratio of two to one, with the Oxygen between the hydrogens, and an angle of 104.5° between them. Compounds are formed and interconverted by chemical reactions.

Molecules

Main article: Molecule. A molecule is the smallest indivisible portion of a pure compound that retains a set of unique chemical properties. A molecule consists of two or more atoms covalently bonded together.

Ions

Main article: Ion. An ion is a charged species, or an atom or a molecule that has lost or gained an electron. Positively charged cations (e.g. sodium cation Na⁺) and negatively charged anions (e.g. chloride Cl^-) can form neutral salts (e.g. sodium chloride NaCl). Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide (OH^-), or phosphate (PO₄^3-).

Bonding

Main article: Chemical bond. A chemical bond is an interaction which holds together atoms in molecules or crystals. In many simple compounds, valence bond theory and the concept of oxidation number can be used to predict molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory fails and alternative approaches which are based on quantum chemistry, such as molecular orbital theory, are necessary.

States of matter

Main article: Phase (matter). A phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature. Physical properties, such as density and refractive index tend to fall within values characteristic of the phase. The phase of matter is defined by the phase transition, which is when energy put into or taken out of the system goes into rearranging the structure of the system, instead of changing the bulk conditions. Sometimes the distinction between phases can be continuous instead of having a discrete boundary, in this case the matter is considered to be in a supercritical state. When three states meet based on the conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions. The most familiar examples of phases are solids, liquids, and gases. Less familiar phases include plasmas, Bose-Einstein condensates and fermionic condensates and the paramagnetic and ferromagnetic phases of magnetic materials. Even the familiar ice has many different phases, depending on the pressure and temperature of the system. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which is getting a lot of attention because of its relevance to biology.

Chemical reactions

Main article: Chemical reaction. Chemical reactions are transformations in the fine structure of molecules. Such reactions can result in molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Quantum chemistry

Main article: Quantum chemistry. Quantum chemistry describes the behavior of matter at the molecular scale. It is, in principle, possible to describe all chemical systems using this theory. In practice, only the simplest chemical systems may realistically be investigated in purely quantum mechanical terms, and approximations must be made for most practical purposes (e.g., Hartree-Fock, post Hartree-Fock or Density functional theory, see computational chemistry for more details). Hence a detailed understanding of quantum mechanics is not necessary for most chemistry, as the important implications of the theory (principally the orbital approximation) can be understood and applied in simpler terms.

Laws

The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction. Modern physics shows that it is actually energy that is conserved, and that energy and mass are related; a concept which becomes important in nuclear chemistry. Conservation of energy leads to the important concepts of equilibrium, thermodynamics, and kinetics. Further laws of chemistry elaborate on the law of conservation of mass. Joseph Proust's law of definite composition says that pure chemicals are composed of elements in a definite formulation; we now know that the structural arrangement of these elements is also important. Dalton's law of multiple proportions says that these chemicals will present themselves in proportions that are small whole numbers (i.e. 1:2 O:H in water); although in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction. Such compounds are known as Non-Stoichiometric Compounds More modern laws of chemistry define the relationship between energy and transformations.
- In equilibrium, molecules exist in mixture defined by the transformations possible on the timescale of the equilibrium, and are in a ratio defined by the intrinsic energy of the molecules—the lower the intrinsic energy, the more abundant the molecule.
- Transforming one structure to another requires the input of energy to cross an energy barrier; this can come from the intrinsic energy of the molecules themselves, or from an external source which will generally accelerate transformations. The higher the energy barrier, the slower the transformation occurs.
- There is a hypothetical intermediate, or transition structure, that corresponds to the structure at the top of the energy barrier. The Hammond-Leffler Postulate states that this structure looks most similar to the product or starting material which has intrinsic energy closest to that of the energy barrier. Stabilizing this hypothetical intermediate through chemical interaction is one way to achieve catalysis.
- All chemical processes are reversible (law of microscopic reversibility) although some processes have such an energy bias, they are essentially irreversible.

History of chemistry

- Alchemy
- Discovery of the chemical elements
- History of chemistry
- Nobel Prize in chemistry
- Timeline of chemical element discovery

Etymology

Old French: alkemie; Arab al-kimia: the art of transformation. See also: alchemy

External links

- [http://www.allchemicals.info/ Chemical Glossary]
- [http://chem.sis.nlm.nih.gov/chemidplus/ Chemistry Information Database includes basic information and some toxicity]
- [http://www.chem.qmw.ac.uk/iupac/ IUPAC Nomenclature Home Page], see especially the "Gold Book" containing definitions of standard chemical terms
- [http://www.cci.ethz.ch/index.html Experiments] videos and photos of the techniques and results
- [http://physchem.ox.ac.uk/MSDS/ Material safety data sheets for a variety of chemicals]
- [http://www.flinnsci.com/search_MSDS.asp Material Safety Data Sheets]

Natural environment

The natural environment comprises all living and non-living things that occur naturally on Earth. In its purest sense, it is thus an environment that is not the result of human activity or intervention. The natural environment may be contrasted to "the built environment." For some, there is a difficulty with the term "natural environment" in that nearly all environments have been directly or indirectly influenced by humans at some point in time. In order to address this concern, some level of human influence is thus allowable without the status of any particular landscape ceasing to be "natural." The term's meaning, however, is usually dependent more on context than a set definition. Many natural environments are the product of the interaction between nature and humans. For this reason, the term ecosystem has been used to describe an environment that contains nature, and includes people. It follows then that environmental problems are human or social problems. Some also consider it dangerously misleading to regard "environment" as separate from "people." It is the common understanding of natural environment that underlies environmentalism—a broad political, social, and philosophical movement that advocates various actions and policies in the interest of protecting what nature remains in the natural environment, or restoring or expanding the role of nature in this environment. While wilderness is increasingly rare, wild nature (e.g., unmanaged forests, uncultivated grasslands, wildlife, wildflowers) can be found in many locations previously inhabited by humans. Goals commonly expressed by environmentalists include: reduction and clean up of man-made pollution, with future goals of zero pollution; reducing societal consumption of non-renewable fuels, development of alternative, green, low carbon or renewable energy sources; conservation and sustainable use of scarce resources such as water, land and air; protection of representative or unique or pristine ecosystems; preservation and expansion of threatened or endangered species or ecosystems from extinction; the establishment of nature and biosphere reserves under various types of protection, and, most generally, the protection of biodiversity and ecosystems upon which all human and other life on earth depends. More recently, there has been a strong concern about climatic changes caused by anthroprogenic releases of greenhouse gases, most notably carbon dioxide, and their interactions with human uses and the natural environment. Efforts here have focused on the mitigition of greenhouse gases that are causing climatic changes (i.e., through the Climate Change Convention and the Kyoto Protocol), and ondeveloping adaptative strategies to assist species, ecosystems, humans, nations and regions in adjusting to these climatic changes.

Peroxisome

Peroxisomes are ubiquitous organelles in eukaryotes that function to rid the cell of toxic substances. They have a single membrane that separates their contents from the cytosol (the internal fluid of the cell) and that contains membrane proteins critical for various functions, such as importing proteins into the organelles and aiding in proliferation. Unlike lysosomes, which are formed in the secretory pathway, peroxisomes usually self-replicate by enlarging and then dividing, although there is some indication that new ones may be formed directly. Peroxisomes were discovered by Christian de Duve in 1965.

Occurence and evolution

Peroxisomes are found in all eucaryotic cells. Prokaryotes lack peroxisomes, so they are more vulnerable to toxic substances like hydrogen peroxide. Peroxisomes are thought to be a vestige of an ancient organelle that performed all the oxygen metabolism in the primitive ancestors of eukaryotic cells. When the oxygen produced by photosynthetic bacteria first began to accumulate in the atmosphere, it would have been highly toxic to most cells. Peroxisomes might have served to lower the intracellular concentration of oxygen, while at the same time exploiting its chemical reactivity to perform useful oxidative reactions. Peroxisomes were largely rendered obsolete by the arrival of mitochondria, which coupled the oxidative reactions to ATP formation by means of oxidative phosphorylation. The oxidative reactions performed by the peroxisomes in present-day cells would therefore be those that have important functions not taken over by the mitochondria. Peroxisomes are actually made out of lipids. The peroxisomes are stored in the nucleus and and later used as fuel for the mitochondria. Peroxisomes will actually use other peroxisomes for fuel in times of need.

Function

Peroxisomes contain oxidative enzymes, such as catalase, D-amino acid oxidase and uric acid oxidase. By using molecular oxygen, hydrogen atoms are removed from specific organic substrates (labeled as R), in an oxidative reaction, producing hydrogen peroxide (H₂0₂, a toxic byproduct of cellular metabolism): :RH₂ + O₂ → R + H₂O₂ Catalase uses H₂0₂ generated by other enzymes in the peroxisome to oxidize other substrates, including phenols, formic acid, formaldehyde and alcohol, by means of the peroxidation reaction: :H₂0₂ + R'H₂ → R' + 2H₂O This reaction is important in liver and kidney cells were the peroxisomes detoxifies various toxic substances that enter the blood. About 25% of the ethanol we drink is oxidized to acetalaldehyde in this way. In addition, when excess H₂0₂ accumulates in the cell, catalase converts it to H₂O through this reaction: :2H₂O₂ → 2H₂O + O₂ A major function of the peroxisome is the breakdown of fatty acid molecules, in a process called beta-oxidation. In this process, the fatty acids are broken down two carbons at a time, converted to Acetyl-CoA, which is then transported back to the cytosol for furhter use. In animal cells, beta-oxidation can also occur in the mitochondria. In yeast and plant cells this process is exclusive for the peroxisome. Peroxisomes catalyzes are the first reactions in the formation of plasmalogen, which is the most abundant phospholipid in myelin. Deficiency of plasmalogens causes profund abnormalities in the myelination of nerve cells, which is on reason why many peroximal disorders lead to neurological disease. Peroxisomes also play a role in the production of bile acids.

Protein import

Proteins are selectively imported into peroxisomes. Since the organelles contain no DNA or ribosomes and thus have no means of producing proteins, all of their proteins must be imported across the membrane. A specific protein signal (PTS or peroxisomal targeting signal) of three amino acids at the C-terminus of many peroxisomal proteins signals the membrane of the peroxisome to import them into the organelle. Other peroxisomal proteins contain a signal at the N-terminus. There are at least 23 know peroxisomal proteins, called peroxins, which participate in the process of importing proteins by means of ATP hydrolysis. Proteins do not have to unfold to be imported into the peroxisome. At least one protein receptor, the peroxin Pex5, accompanies its cargo all the way into the peroxisome were it releases the cargo and then returns to the cytosol.

Deficiencies

A deficiency in the protein import can lead to empty peroxisomes, leading to abnormalities in the brain, called Zellweger syndrome. A deficiency in the peroxin Pex2 has shown to be responsible for one form of the syndrome. A milder inherited disease is caused by a defective receptor for the N-terminal import signal. Deficiency of the formation of plasmalogens can also cause severe brain disorders, leading to neurological disease.

Links

- [http://dx.doi.org/10.1016/j.cell.2005.04.025 Contribution of the Endoplasmic Reticulum to Peroxisome Formation] Cell, 2005.
- [http://www.jcb.org/cgi/content/full/169/5/765 Inp1p is a peroxisomal membrane protein required for peroxisome inheritance in Saccharomyces cerevisiae] JCB, 2005. Category:Organelles

Peroxide

Peroxide has three distinct meanings:

Colloquial meaning

In common usage, peroxide is an aqueous solution of hydrogen peroxide (HOOH or H₂O₂) sold for use as a disinfectant or mild bleach. The usual peroxide used in commercial applications is a dilute solution containing traces of stabilisers, and is sold in either brown glass or opaque polyethylene bottles to minimise the rate of decomposition. The concentrations sold are generally either 3% w/v or 6% w/v; these are sometimes described as "10 volume" and "20 volume", respectively. This refers to the relative volume of oxygen gas produced, at STP, or the ideal state of gas, from the complete decomposition of the peroxide. 20 volume peroxide is strong enough to bleach skin it touches, causing unnaturally white blotches. Due to the presence of catalase in blood, peroxide is only marginally effective in disinfecting open wounds, but excellent for bleaching blood stains. It is also often used as a disinfectant in the dairy industry because, after application, it leaves absolutely no harmful residues.

Organic chemistry

In organic chemistry, peroxide is a specific functional group or a molecule containing that functional group. :ROOR' Organic peroxides tend to decompose easily to free radicals of the form: :RO· This makes them useful as catalysts for some types of polymerisation, such as the polyester resins used in glass-reinforced plastics. MEKP (methyl ethyl ketone peroxide) is commonly used for this purpose. However, the same property also means that organic peroxides can accidentally initiate explosive polymerisation in materials with unsaturated chemical bonds. Since peroxides can form spontaneously in some materials, some caution must be exercised with such "peroxide forming materials". In addition, many liquid ethers in the presence of air, light, and metal slowly (over a period of months) form ether peroxides (e.g. diethyl ether peroxide) which are extremely unstable. Consequently it is recommended that ether be stored over potassium hydroxide, which not only destroys peroxides but also acts as a powerful drying agent. Extreme care must be taken with samples showing signs of crystal growth or precipitates.

Inorganic chemistry

In inorganic chemistry, peroxide is the anion O₂^2-, usually formed by burning alkali metals or alkaline earth metals in air or oxygen. Hydrogen peroxide H₂O₂ is a typical example. The peroxide ion contains two electrons more than the oxygen molecule. These two electrons, according to the molecular orbital theory, complete the two π
- antibonding orbitals. This has as result a weakening of the bond strength of the peroxide ion and a greater length for the bond O-O : Li₂O₂ 130 pm to BaO₂ 147 pm. Furthermore, the peroxide ion is diamagnetic. The peroxides of the alkali metals and Ca, Sr and Ba are ionic. The peroxides of a number of electropositive metals such as Mg, the lanthanides and the uranyl-ion show an intermediary character, between ionic and covalent. The peroxides of metals such as Zn, Cd and Hg are mainly covalent. Peroxides are powerful oxidizers, and usually fairly unstable. Ionic peroxides react with water and diluted acids to form hydrogen peroxide. Organic compounds are oxidized to carbonates, even at normal temperatures. Sodium peroxide is a powerful oxidator of metals, such as iron. The oxides, peroxides and superoxides are closely related, forming a chain of oxygen ions of progressively higher oxidation number. Barium peroxide is used in pyrotechnics and tracer ammunition, and was once used in the manufacture of hydrogen peroxide. Sodium peroxide is used as a carbon dioxide absorber and oxygen regenerator (e.g. in some submarines), through the reaction: :2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂

External links

- [http://www.chem-world.com Shangyuchem, a commercial supplier of inorganic peroxides]

Organelle

(2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles]] In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell. Organelles were historically identified through the use of microscopy, and were also identified through the use of cell fractionation. A few large organelles probably originated from endosymbiont bacteria:
- chloroplast
- Other plastids, such as leucoplasts, amyloplasts, Etioplasts, Elaioplast, rhodoplasts, leukoplasts, and chromoplasts.
- mitochondrion Other organelles include:
- acrosome
- centriole
- endoplasmic reticulum
- golgi apparatus
- lysosome
- myofibril
- nucleus
- peroxisome
- ribosome
- vacuole
- vesicle
- melanosome
- cilium/flagellum
- parenthesome Other related structures:
- cytosol
- endomembrane system
- nucleosome
- microtubule
- cell membrane

Charge

Charge is a word with many different meanings.

Science

In science, the concept of charge is derived from the observation of conserved quantum numbers.
Various charge-like quantum numbers have been introduced by theories of particle physics, e.g. electric charge for electromagnetic interaction, magnetic charge (currently purely theoretic), colour charge for gluons, quark charges like strangeness, and isospin for electroweak interactions. In the formalism of particle theories charge-like quantum numbers can sometimes be inverted by means of a charge conjugation operator called C. Chiral fermions often can't.

Others

- to take charge, or being in charge means to take or have authority and responsibility for decisions
- In law: :# A criminal charge is an accusation before a court by a prosecuting authority :# A jury charge is an instruction or set of instructions given by a judge to a jury concerning the law applicable to the case under consideration
- more loosely, charged or loaded language employs emotional overtones.
- In Group dynamics, Charge (group dynamics) is the build up of negative emotions
- In heraldry, a charge means objects on the shield.
- In munitions and explosives, the charge is the explosive material used, for instance, to propel a bullet or shell, or demolish a structure.
- During the European Middle Ages, a charge often meant an underage person placed under the supervision of a nobleman.
- To charge (warfare) is a maneuver in battle where soldiers rush towards their enemy to engage in close combat.
- In context of wartime operations, to charge with certain rights, such as guaranteeing persons held in custody are allowed those rights.
- In money, a charge (finance) is any fee assessed, such as a charge for using an automatic teller machine (ATM), entering a museum, being late with a payment, etc.
- In basketball, a charge is an offensive foul, called when an offensive player with the ball makes illegal contact with a defensive player who has legally established his position.
- Charge!! is a 2005 album by The Aquabats.
- Charge is the name of a legal party pill in New Zealand.

Atoms

:For alternative meanings see atom (disambiguation). An atom (Greek άτομον from ά: non and τομον: divisible) is a submicroscopic structure found in all ordinary matter. It is the smallest unit of an element to retain all the chemical properties of that element. The word atom originally meant a smallest possible particle of matter, not further divisible. Later, the objects that had been called atoms were found to be further divisible into smaller subatomic particles, but the word atom nonetheless continues to refer to them. Most atoms are composed of three types of massive subatomic particles which govern their external properties:
- electrons, which have a negative charge and are the least massive of the three;
- protons, which have a positive charge and are about 1836 times more massive than electrons; and
- neutrons, which have no charge and are about 1838 times more massive than electrons. Together, protons and neutrons form the nucleus of an atom, which is surrounded by the electrons. Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons, although the same element can differ in the number of neutrons which are then called isotopes of that element. Atoms are electrostatically neutral if they have an equal number of protons and electrons. Atoms which have either gained or lost electrons are called ions. Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen and one oxygen atom.

Properties of the atom

Subatomic particles

:see main article subatomic particles Up until 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of varieties of a still smaller particle called the quark, and the electron is considered a type of lepton. Therefore in modern atomic theory, the two basic constituents of matter are the lepton and the quark of which the above three particles of the atom are composed. All particles exhibit a wave-particle duality so that the electron is better understood as a wave when drawn about a nucleus. Unlike planets revolving around the sun, the electron is not held around the nucleus of the atom by gravity, but rather by electromagnetism.
Atom sizes
The atom is many times smaller than the wavelength that human vision can detect in any kind of microscope. However, there are ways of projecting the atom so as to obtain amplified images of it. These include: scanning tunneling microscopy (STM), atomic force microscopy (ATM), and nuclear magnetic resonance (NMR). In measuring an atom, the size of the area that an electron can travel in must be determined. Electrons travel in areas called atomic orbitals. This area forms a cloud where the electron may be situated. In the helium atom above (shown in its ground state), the atomic orbital where the electron may be situated describes a sphere. However, the cloud or atomic orbital in which an electron can travel changes shapes depending on the energy of the electron. So some electrons travel in the shape of a dumbbell with the nucleus in the smallest space in-between. There are other more complicated shapes as well. And the heavier the element, the more electrons there are and the more shapes there are for the orbitals in the atom. It therefore not only becomes more complicated to measure the size of the atom, but it becomes complicated to create models of the atoms of heavier elements. Since the electron orbitals are considered clouds, then the size of an atom is not easily defined since the places where the electron can be just gradually go to zero as the distance from the nucleus increases. For atoms that can form solid crystals, the distance between adjacent nuclei can give an estimate of the atom size. For atoms that do not form solid crystals other techniques are used, including theoretical calculations. As an example, the size of a hydrogen atom is estimated to be approximately 1.0586×10 m. Compare this to the size of the proton which is the only particle in the nucleus of the hydrogen atom which is approximately 10 m. Thus the ratio of the sizes of the hydrogen atom to its nucleus is about 100,000:1. Atoms of different elements do vary in size, but the sizes are roughly the same to within a factor of 2 or so. The reason for this is that elements with a large positive charge on the nucleus attract the electrons to the center of the atom more strongly. To illustrate the size of an atom, one million atoms can fit within the breadth of a strand of hair. An atom is mostly space. A basic analogy for the ratio of space inside an atom is this: if an atom were the size of a baseball stadium, the nucleus would be the size of a marble on second base and the electrons would orbit the perimeter.
Elements, isotopes and ions
Atoms are generally classified by their atomic number, which corresponds to the number of protons in the atom. The atomic number defines which element the atom is. For example, carbon atoms are those atoms containing six protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the periodic table in order of increasing atomic number. The mass number, atomic mass number, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, because each proton or neutron essentially has a mass of 1 amu. The number of neutrons in an atom has no effect on which element it is. Each element can have numerous different atoms with the same number of protons and electrons, but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example, carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14. The simplest atom is the hydrogen atom, which has atomic number 1 and consists of one proton and one electron. The hydrogen isotope which also contains one neutron so is called deuterium or hydrogen-2; the hydrogen isotope with two neutrons is called tritium or hydrogen-3. Tritium is an unstable isotope which causes the atom to lose mass in a process called radioactivity. The elements in the periodic table beginning with number 86, radon, and those that follow have no stable isotopes and are all radioactive. The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, weighted by their abundance. Although most sources state that there are 92 elements that occur naturally on earth from hydrogen up to uranium in the periodic table, it has been recently discovered that plutonium, the 94th element, also occurs naturally. Most of these elements were created through stellar nucleosynthesis and supernova nucleosynthesis. Several elements that do not occur on earth have been found to be present in stars. Elements not normally found in nature have been artificially created by nuclear bombardment, but they are usually unstable and spontaneously change into stable natural chemical elements by the processes of radioactive decay. Atoms that have either lost or gained electrons are called atomic ions (with either positive(+) or negative charge(−), respectively). Atoms are canonically distinguished from ions by their balanced electrical charge.
Atomic spectrum
:see main article Atomic spectroscopy Each element in the periodic table therefore consists of an atom in a unique configuration i.e. with different amounts of protons in the nucleus. Each atom of each element can also be uniquely described by the shapes of its atomic orbitals and the number of electrons within them. There is also another way in which each element with its own configuration is distinctive, that is, by its atomic spectrum. A spectrum is created when light is passed through a prism and the light breaks up into its component colors. Spectroscopy studies the spectrum of each element. Each atom of each element creates its own light pattern unique to itself, its own spectral signature. Scientists can use a spectrometer to study the atoms in stars and other distant objects, and due to the distinctive spectral lines that each element produces, are able to tell the chemical composition of distant planets, stars and galaxies.
Electron configuration
:see main article electron configuration The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. Electrons fall into shells based on their relative energy level. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus. valence electrons of a hydrogen atom. The principal quantum number is at the right of each row and the azimuthal quantum number is denoted by letter at top of each column.]] An electron shell can hold up to 2n² electrons, where n is the number of the shell. Whichever occupied shell is currently most outward is the valence shell, even if it only has one electron. In the most stable state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell, but at some point it will fall back to its previous level, emitting its excess energy as a photon. Electron shells also have distinctive shapes denoted by letters. In the illustration, the letters s, p, and d describe the shape of the atomic orbital. Electrons also have another property that describes their configuration due to the fact that they rotate in space. Thus electrons are said to have spin (physics).
Valence and bonding
:see main article valence electrons and chemical bond The number of electrons in an atom's outermost shell (ie the valence shell) governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. Group (i.e. column) 1 elements contain one electron on their outer shell; Group 2, two electrons; Group 3, three electrons; etc. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 metals are therefore very reactive, with caesium, rubidium, and francium being the most reactive of all metals. Every atom is much more stable (i.e. less energetic) with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms (a covalent bond), or it can remove electrons from other atoms (an ionic bond). Another form of ionic bonding involves an atom giving some of its electrons to another atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules or ionic compounds. Five major types of bonds exist:
- ionic bonds;
- covalent bonds;
- coordinate covalent bonds;
- hydrogen bonds; and
- metallic bonds.
Atoms and antimatter
:see main article antimatter Antimatter can also form atoms, composed of antielectrons (positrons), antiprotons, and antineutrons.

Atoms and the Big Bang

In models of the Big Bang, Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang all the current atomic material in the universe was created producing no heavier element than lithium, but mostly hydrogen and helium. However, although the basic atomic particles of matter were created, atoms themselves could not form in the intense heat. Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000 K which allowed the first atoms to form. It was then cool enough to allow protons to capture one electron each and form neutral atoms of hydrogen. Hydrogen makes up approximately 75% of the atoms in the universe. Helium makes up 24% and all other elements make up 1%. Since the size of the universe is unknown, the total numbers of atoms in the universe is unknown, but the number is not thought to be infinite because current theory suggests we live in a finite universe. One thing we can say about the mass of the baryons in the universe, meaning the mass of the protons and neutrons, is that we can tell what the ratio of their density ought to be from the Big Bang model. Einstein's theory of General Relativity suggests that the universe is the same in all directions and from all viewpoints. Therefore, examining one region of the universe and the density of atoms in that region should tell us how densely atoms are scattered throughout the entire universe, but as said previously, does not tell us how far the universe extends and how many atoms exist in total. Big Bang Nucleosynthesis predicts that 1/20 of the total mass of the Universe is baryonic matter. (The baryon is the category used to describe neutrons and protons which are similar in mass but different in electric charge.) So theoretically we should be able to study a region of space and calculate the amount of matter we see through our telescopes and one-twentieth of the matter should be baryons. However, from the density we can see through telescopes of matter in regions of the visible universe, 99% of the baryons are missing. This has given rise to theories of dark matter (which should also be made of baryons--or if you prefer atoms, since baryons make up the nucleus of atoms) in order to make up the difference in missing matter. What that means is that there are probably more atoms out there than we can see through our usual means of detection. In other words, we cannot see visible light from these atoms nor have we detected electromagnetic radiation, but they exist. In fact, in some cases we have detected, through radio-wave detectors, entire galaxies such as Virgo H121 that do not appear in normal telescopes.

Atomic theory

The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms.

Historical theories

Democritus and Leucippus, Greek philosophers in the 5th century BC, presented the first theory of atoms (see article atomism for more details). They held that each atom had a different shape, like a pebble, that governed the atom's properties. Dalton and Avogadro rediscovered the works of Democritus and Leucippus and suggested in the 19th century that matter was made up of atoms, but they knew nothing of their structure. This theory was conflicting with the theory of infinite divisibility, which states that matter can always be divided into smaller parts. The controversy ended in 1911 when Jean Perrin demonstrated the existence of atoms through experimental validation of Einstein's theory of Brownian motion (which relied on atomic theory). For much of this time, atoms were thought to be the smallest possible piece of matter. However, in 1897, J.J. Thomson published his work proving that cathode rays are made of negatively charged particles (electrons). Since cathode rays are essentially emitted from matter, this proved that atoms are made up of subatomic particles and are therefore divisible, and not the indivisible "atomos" Democritus talked about. Physicists later invented a new term for indivisible units, namely elementary particles since the word atom had already been taken and come into common use. At first, it was believed that the electrons were distributed more or less uniformly in a sea of positive charge (the plum pudding model). However, an experiment conducted a few years later by Rutherford demonstrated that atoms are mostly empty space, with a lot of mass concentrated in a nucleus. In the gold foil experiment, he shot alpha particles (emitted by polonium) through a sheet of gold. He observed that most of the particles passed straight through the sheet without deflection (striking a fluorescent screen on the other side), but that, surprisingly, a small number were bounced right back (having come close to a nucleus). This led to the planetary model of the atom, in which the electrons orbited the nucleus like the planets orbiting the sun. The nucleus was later discovered to contain protons, and further experimentation by Rutherford found that the nuclear mass of most atoms surpassed the number of protons it possessed; this led him to postulate the existence of neutrons, whose existence would be proven in 1932 by James Chadwick. The planetary model of the atom still had shortcomings. Firstly, a moving electrical charge emits electromagnetic waves; according to classical physics, an orbiting charge would steadily lose energy and spiral towards the nucleus, eventually colliding with it. Secondly, the model didn't explain why hydrogen gas, when submitted to an electrical discharge, emitted light only in certain discrete spectra. Experiments by Max Planck and Albert Einstein demonstrated that energy is transferred in tiny fixed amounts known as quanta. In 1913, Niels Bohr used this idea in his Bohr model of the atom, in which the electrons could only orbit the nucleus in fixed circles. They couldn't spiral downwards because they couldn't lose energy in a continuous manner; they could only make quantum leaps between fixed energy levels. The Bohr model would eventually be replaced by a full quantum mechanics model in 1925.

Study of atoms

Because of their ubiquitous nature, atoms have been an important field of study for many centuries. Current research focuses on quantum effects, such as in Bose-Einstein condensate. The study of atoms was done largely by indirect means through the 19th century and early 20th century. In recent years, however, new techniques have made the identification and study of atoms easier and more accurate. The electron microscope, invented in 1931, can image large molecules, however, not the atom itself. Atomic force microscopy is another technique by which individual atoms can be visualized and even arranged into patterns. Methods also exist to identify atoms and compounds. Elemental analysis allows the exact identification of the types and amounts of atoms in a substance.

Practical uses of the atom

Atoms have given us the key to understanding our universe, understanding our earth and life upon it, improving technology, and creating life-saving pharmaceuticals. There does not exist a scientific field that is not affected by the understanding of the atom. Atoms are the basis for chemistry, physics, geology, astronomy and biology. Within the tiny atom are the powers to both create and destroy. Through fusion and fission man has learned to unleash the power of the atom. Our sun and other stars use fusion of the atom to create the heavier elements in the universe that were not created in the Big Bang. Fission of the atom is used to create power in nuclear power plants. Fusion of the atom may one day be used to create safer forms of power than current fuels that are destroying the delicate balance of earth's ecosystem.

External links

- [http://www.howstuffworks.com/atom.htm How Atoms Work] ko:원자 ms:Atom ja:原子 simple:Atom th:อะตอม

Endocytosis

Endocytosis is a process whereby cells absorb material (molecules or other cells) from outside by engulfing it with their cell membranes. It is used by cells (especially protists) because most substances important to them are polar and consist of big molecules, and thus cannot pass through the highly hydrophobic plasma membrane. Endocytosis is the opposite of exocytosis, and always involves the formation of a vesicle from part of the cell membrane. There are three types of endocytosis:
- Phagocytosis (literally, cell-eating) is the process by which cells ingest large objects, such as prey cells or large chunks of dead organic matter. The membrane folds around the material, and vesicles are sealed off into large vacuoles. Lysosomes then merge with the vacuoles, turning them into a digestive chamber. The products of the digestion are then released into the cytosol. Macrophages are cells of the immune system that specialize in the destruction of antigens (bacteria, viruses and other foreign particles) by phagocytosis.
- Pinocytosis (literally, cell-drinking) is the invagination of the cell membrane to form a pocket filled with extracellular fluid (and molecules within it). The pocket then pinches off to form a vesicle, and the vesicle ruptures to release its contents into the cytosol.
- Receptor-mediated endocytosis is similar to pinocytosis, except it is prompted by the binding of a large extracellular molecule - such as a protein - to a receptor on the cell membrane. These receptors are often associated with the cytosolic protein clathrin, which is coating the membrane, forming a pit. When the receptors bind their target molecules, the pit deepens until a clathrin-coated vesicle is released into the cytosol. Category:Cell biology ja:エンドサイトーシス

Phospholipid

Phosphoglycerides

Phosphatidyl choline

Image:Phosphatidyl-Choline.png
Phosphatidyl choline is the major component of lecithin.

Phosphatidyl ethanolamine

Image:Phosphatidyl-Ethanolamine.png
Phosphatidyl ethanolamine is the major component of cephalin.

Phosphatidyl inositol

Image:Phosphatidyl-Inositol.png

Phosphatidyl serine

Image:Phosphatidyl-Serine.png

Diphosphatidyl glycerol

Image:Diphosphatidyl-Glycerol.png

Synthesis

Sphingomyelin

Amphipathic character

References

# Berg, J.M., J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pagings)
- th:ฟอสโฟไลปิด

Glycolipid

Glycolipids are carbohydrate-attached lipids. Their role is to provide energy and also serve as markers for cellular recognition. They occur where a carbohydrate chain is associated with phospholipids in the cell surface membrane. The carbohydrates are found on the outer surface of all eukaryotic cell membranes. They extend from the phospholipid bilayer into the aqueous environment outside the cell where it acts as a recognition site for specific chemicals as well as helping to maintain the stability of the membrane and attaching cells to one another to form tissues. Category:Lipids th:ไกลโคไลปิด

Sphingomyelin

Phosphoglycerides

Phosphatidyl choline

Image:Phosphatidyl-Choline.png
Phosphatidyl choline is the major component of lecithin.

Phosphatidyl ethanolamine

Image:Phosphatidyl-Ethanolamine.png
Phosphatidyl ethanolamine is the major component of cephalin.

Phosphatidyl inositol

Image:Phosphatidyl-Inositol.png

Phosphatidyl serine

Image:Phosphatidyl-Serine.png

Diphosphatidyl glycerol

Image:Diphosphatidyl-Glycerol.png

Synthesis

Sphingomyelin

Amphipathic character

References

# Berg, J.M., J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pagings)
- th:ฟอสโฟไลปิด

Sphingosine

Sphingosine is a compound that forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phospholipid. Sphingosine can be phosphorylated in vivo via two kinases, sphingosine kinase type 1 and sphingosine kinase type 2.

Synthesis

Sphingosine is synthesized from palmitoyl CoA and serine in a condensation required to yield dehydrosphingosine. Dehydrosphingosine is then reduced by NADPH to dihydrosphingosine, and finally oxidized by FAD to sphingosine. Image:Sphingosine-Synthesis.png Category:Biochemicals

Phosphatidate

Phosphatidates are biochemical compounds that consist of a glycerol backbone, with a (usually) saturated fatty acid bonded to carbon-1, a (usually) unsaturated fatty acid bonded to carbon-2 and a phosphate group bonded to carbon-3.

Synthesis

In mammalian cells, phosphotidates are synthesized in the ER and mitochondrial membrane. The first step in phosphatidate synthesis is the acylation of glycerol 3-phosphate by acyl CoA to form lysophosphatidate. Lysophosphatidate is again acylated to yield phosphatidate. Both of these acylations are catalyzed by glycerol phosphate acyltransferase. Phosphatidates are precursors for triacylglycerols and phospholipids.

References

# Berg, J.M., J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pagings) Category:Biochemicals

Choline

Choline is a quaternary saturated amine with the chemical formula :(CH₃)₃N⁺CH₂ CH₂ OHX^-. where X^- is a counterion such as chlorine (see choline chloride), hydroxide or tartrate. Choline was discovered by Strecker in 1862 and chemically synthesized in 1866. In 1998 choline was classified as an essential nutrient by the Food and Nutrition Board of the Institute of Medicine (U.S.A.) and Adequate Intakes (AI) have been established. Choline and its metabolites are needed for three main physiological purposes: structural integrity and signaling roles for cell membranes, cholinergic neurotransmission (acetylcholine synthesis), and as a major source for methyl-groups via its metabolite, betaine that participates in the S-adenosylmethionine synthesis pathways. Category:Amines Category:Alcohols Category:Nutrition

Cerebroside

Cerebrosides are glycosphingolipids which are important components in animal muscle and nerve cell membranes. They consist of a ceramide with a single sugar residue at the 1-hydroxyl moiety. The sugar residue can be either glucose or galactose; the two major types are therefore called glucocerebrosides and galactocerebrosides.

Role in disease

A defect in the degradation of glucocerebrosides is Gaucher's disease. The corresponding defect for galactocerebrosides is Krabbe disease. Category:Lipids

Glucose

Glucose (Glc), a monosaccharide, is one of the most important carbohydrates. The cell uses it as a source of energy and metabolic intermediate. Glc is one of the main products of photosynthesis and starts cellular respiration. The natural form (D-glucose) is also referred to as dextrose, especially in the food industry. This article deals with the D-form of Glc (see Isomers-section bellow)

Structure

cellular respiration cellular respiration Glc contains six carbon atoms and an aldehyde group and is therefore refered to as an aldohexose. Glc molecule can exist in an open-chain (acyclic) and ring (cyclic) form, the latter being the result of a intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an intramolecular hemiacetal. In water solution both forms are in equilibrium, and at pH 7 the cyclic one is the predominant. As the ring contains 5 carbon and one oxygen atoms, which resembles the structure of pyran, the cyclic form of Glc is also refered to as glucopyranose. In this ring, each carbon is linked to hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH₂OH group.

Isomers

Glc has 4 optic centers which means that in theory Glc can have 15 optical stereoisomers. In living organisms only 7 of them are found, of which Gal and Man are the most important. These eight isomers (including Glc) are all diastereoisomers in relation to each other and all belong to the [http://en.wikipedia.org/wiki/Monosaccharide#Isomerism D-series]. An additional asymetric center at C-1 (called the anomeric carbon atom) is created when Glc cyclizes and two ring structures, called anomers, can be formed - α-Glc and β-Glc. They structurally differ in the orientation of the hydroxyl group linked to C-1 in the ring. When D-Glc is drawn as a Haworth_projection, the designation α means that the hydroxyl group attached to C-1 is bellow the plane of the ring, β means - it is above. The α and β forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of α:β 36:64, in a process called mutarotation. mutarotation.]]

Production

Natural

#Glucose is one of the products of photosynthesis in plants and some prokaryotes. #In animals and fungi, glucose is the result of the breakdown of glycogen, a process known as glycogenolysis. In plants - the breakdown substrate is starch. #In animals, glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol, by a process known as gluconeogenesis.

Commercial

Glc is produced commercially via the enzymatic hydrolysis of starch. Many crops can be used as the source of starch Maize, rice, wheat, potato, cassava, arrowroot, and sago are all used in various parts of the world. In the United States, cornstarch (from maize) is used almost exclusively. This enzymatic process has two stages. Over the course of 1-2 hours near 100 °C, these enzymes hydrolyze starch into smaller carbohydrates containing on average 5-10 Glc units each. Some variations on this process briefly heat the starch mixture to 130 °C or hotter one or more times. This heat treatment improves the solubility of starch in water, but deactivates the enzyme, and fresh enzyme must be added to the mixture after each heating. In the second step, saccharification, the partially hydrolyzed starch is completely hydrolyzed to Glc using the glucoamylase enzyme from the fungus Aspergillus niger. Typical reaction conditions are pH 4.0–4.5, 60 °C, and a carbohydrate concentration of 30–35% by weight. Under these conditions, starch can be converted to Glc at 96% yield after 1–4 days. Still higher yields can be obtained using more dilute solutions, but this approach requires larger reactors and processing a greater volume of water, and is not generally economical. The resulting glucose solution is then purified by filtration and concentrated in a multiple-effect evaporator. Solid D-Glc is then produced by repeated crystallizations.

Function

We can speculate on the reasons why Glc, and not another monosaccharide such as Fru, is so widely used. Glc can form from formaldehyde under abiotic conditions, so it may well have been available to primitive biochemical systems. Probably more important to advanced life is the low tendency of Glc, by comparison to other hexose sugars, to nonspecifically react with the amino groups of proteins. This reaction (glycosylation) reduces or destroys the function of many enzymes. The low rate of glycosylation is due to Glc's preference for the less reactive cyclic isomer. Nevertheless, many of the long-term complications of diabetes (e.g., blindness, kidney failure, and peripheral neuropathy) are probably due to the glycosylation of proteins.

As an energy source

Glc is a ubiquitous fuel in biology. Carbohydrates are the human body's key source of energy, providing 4 calories (17 kilojoules) of food energy per gram. Breakdown of carbohydrates (e.g. starch) yields mono- and disaccharides, most of which is Glc. Through glycolysis and later in the reactions of TCAC, Glc is oxidized to eventually form CO₂ and water, yielding energy, mostly in the form of ATP.

As a precursor

Glc is critical in the production of protein and in lipid metabolism.

Glc is used as a precursor for the synthesis of several important substances. Starch, cellulose, and glycogen ("animal starch") are common Glc polymers (polysaccharides). Lactose - the milk sugar, is a Glc-Gal disaccharide. In sucrose, another important disaccharyde, Glc is joined to Fru.

Sources and absorbtion

All major dietary carbohydrates contain Glc, either as their only building block, as in starch and glycogen, or together with another monosaccharide, as in sucrose and lactose. In the lumen of the duodenum and small intestine the oligo- and polysaccharides are broken down to monosaccharides by the pancreatic and intestinal glycosidases. Glc is then transported across the enterocytes and into the bloodstream, first at the apical membrane by Na⁺-dependent transporter protein (GLUT2) and then at the basal membrane by a totally different protein. Some of Glc goes directly to fuel brain cells and erythrocytes, while the rest makes its way to the liver and muscles, where it is stored as glycogen, and to fat cells, where it is stored as fat. Glycogen is the body's auxiliary energy source, tapped and converted back into Glc when there is needs for energy.

External links

- (D-glucose)
- (L-glucose)
- (D-glucose)
- (L-glucose)
- [http://www.evowiki.org/index.php/Glucose More on the chemistry and function of glucose in biology at EvoWiki]
- [http://www.compchemwiki.org/index.php?title=Glucose Computational Chemistry Wiki] Category:Chemical pathology Category:Monosaccharides Category:Nutrition Category:Sweeteners ko:포도당 ja:グルコース

Ganglioside

Ganglioside is a compound composed of a glycosphingolipid (ceramide and oligosaccharide) with one or more sialic acids (AKA n-acetylneuraminic acid) linked on the sugar chain. It is a component the cell plasma membrane which modulates cell signal transduction events. They have recently been found to be highly important in immunology. Natural and semisynthetic gangliosides are considered possible therapeutics for neurodegenerative disorders [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16158191&query_hl=1]. A disease of the accumulation of gangliosides is called "Gangliosidosis" (ICD-10 E75.0-E75.1). GM2 is also known as Tay-Sachs disease.

Common gangliosides

- GD1a
- GD1b
- GD2
- GD3
- GM1
- GM2
- GM3
- GT1b

External links

- [http://www.lipidlibrary.co.uk/Lipids/gang/ Overview of gangliosides]
- [http://www.cyberlipid.org/glycolip/glyl0036.htm Overview of gangliosides]
- [http://www.emedicine.com/ped/topic2891.htm eMedicine on GM1 Gangliosidosis]
- [http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=230500 OMIM on Gangliosidosis] Category:Lipids

Category:Membrane biology

Category:Cell biology

倒A高达

∀ Gundam 又稱 Turn-A Gundam，是於1999年為了慶祝 Gundam 系列誕生 25 週年而製作的一套 TV版作品。由原作富野由悠季（富野孝喜）身任製作人。目前沒有被正式代理，翻譯的名稱有逆A或是倒A都有。

∀的意義

∀的形狀就像是一個倒著的Ａ，在數學上這個符號代表著For All，也就是對於所有的的意思。這代表著本作品試著把之前所有的 Gundam 作品結合在同一個時空下。也因此在此作品裡會出現之前在不同鋼彈作品以及世界觀出現的不同機械。

故事概要

故事舞台是在遙遠的未來，人類在地球上重新建立起文明，而科技再度發展到類似工業革命的時代。而被留在月球上的居民Moonrace則是保存了以前人類文明所發展的高科技。留在月球上的居民想要重回地球居住，而派遣了先鋒部隊Dianna Counter來到了Ameria（原來的美洲），想為月球人的移民做預備。而這個舉動惹怒了Ameria上的各領地。於是在地球跟月球的戰爭就開始了。

特色

本次 Gundam 的美術風格跟以往的完全不同，由安田朗（原街頭霸王人物設定）設定的人物們有很強烈的世界名作劇場風格，而由美國的 Syd Meed 重新設計的 Gundam 還有其他的機械也有很異於傳統 Gundam 的外表。劇情方面整體上則缺少以往富野作品中常常出現的悲劇。主角的個性很乖巧，不同以往富野作品裡主角擁有青少年叛逆行為。

機械設定

由著名美國科幻設定大師 Syd Meed 設定的機械一反日式科幻的美形風，讓原 Gundam 的觀眾大吃一驚。 category:鋼彈 ja:∀ガンダム

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Koss Hybrid
The term Koss Hybrid refers to the modification of Koss' The Plug or Spark Plug in which the OEM black memory foam tips are replaced with better ones. The replacement tips are often Etymotic ER-14's or even regular earplugs. The recent availablity of Koss Hybrids on eBay have made them a popular alternative to the expensive high-end noise isolation earphones offered by Shure and Etymotic.

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