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BiochemistryAn introduction

ContentsArticlesCells and water 1

Biochemistry 1Cells 11Water 21

Structural Biochemistry 41

Nucleic acids 42Nucleic acid 42RNA 45DNA 52

Proteins and amino acids 72Protein 72Amino acid 84Properties of the twenty amino acids 97Myoglobin 107Hemoglobin 112

Enzyme mechanisms 129Enzyme catalysis 129

Enzyme kinetics 137Enzyme kinetics 137

Lipids and membranes 151Lipid 151Biological membrane 159Membrane protein 161Cell membrane 165

Carbohydrate structure 172Carbohydrate 172Polysaccharide 178

Intermediary metabolism 184

Metabolism 185Overview of metabolism 185

Carbohydrate metabolism 200Glycolysis 200Gluconeogenesis 214Glycogen 218Pentose phosphate pathway 222

Citric acid cycle 226Citric acid cycle 226

Oxidative phosphorylation 233Oxidative phosphorylation 233

Photosynthesis 245Photosynthesis 245

Lipid metabolism 259Fatty acid synthesis 259Lipogenesis 267Acetyl-CoA carboxylase 269Fatty acid degradation 276Beta oxidation 278

Nitrogen metabolism 283Nitrogen fixation 283Amino acid synthesis 288Nucleotide 295Urea cycle 301

Integration of metabolism 305Hormone 305Signal transduction 309Diabetes mellitus 316

Informational Macromolecules 327

DNA synthesis and repair 328DNA replication 328DNA repair 335Oncogenes 346

RNA synthesis and processing 350Transcription 350Regulation of gene expression 356

Protein synthesis and modifications 362Translation 362Posttranslational modification 366Proteolysis 370Proteasome 375

ReferencesArticle Sources and Contributors 386Image Sources, Licenses and Contributors 398

Article LicensesLicense 406

1Cells and water

BiochemistryBiochemistry, sometimes called biological chemistry, is the study of chemical processes within, and relating to,living organisms.[1] By controlling information flow through biochemical signaling and the flow of chemical energythrough metabolism, biochemical processes give rise to the complexity of life. Over the last 40 years biochemistryhas become so successful at explaining living processes that now almost all areas of the life sciences from botany tomedicine are engaged in biochemical research.[2] Today the main focus of pure biochemistry is in understanding howbiological molecules give rise to the processes that occur within living cells, which in turn relates greatly to the studyand understanding of whole organisms.Biochemistry is closely related to molecular biology, the study of the molecular mechanisms by which geneticinformation encoded in DNA is able to result in the processes of life. Depending on the exact definition of the termsused, molecular biology can be thought of as a branch of biochemistry, or biochemistry as a tool with which toinvestigate and study molecular biology.Much of biochemistry deals with the structures, functions and interactions of biological macromolecules, such asproteins, nucleic acids, carbohydrates and lipids, which provide the structure of cells and perform many of thefunctions associated with life. The chemistry of the cell also depends on the reactions of smaller molecules and ions.These can be inorganic, for example water and metal ions, or organic, for example the amino acids which are used tosynthesise proteins. The mechanisms by which cells harness energy from their environment via chemical reactionsare known as metabolism.

History

Gerty Cori and Carl Cori jointly won theNobel Prize in 1947 for their discovery of

the Cori cycle at RPMI.

It once was generally believed that life and its materials had some essentialproperty or substance distinct from any found in non-living matter, and itwas thought that only living beings could produce the molecules oflife.[citation needed] Then, in 1828, Friedrich Whler published a paper on thesynthesis of urea, proving that organic compounds can be createdartificially.[]

The dawn of biochemistry may have been the discovery of the first enzyme,diastase (today called amylase), in 1833 by Anselme Payen.[3] EduardBuchner contributed the first demonstration of a complex biochemicalprocess outside of a cell in 1896: alcoholic fermentation in cell extracts ofyeast.[4] Although the term "biochemistry" seems to have been first used in1882, it is generally accepted that the formal coinage of biochemistryoccurred in 1903 by Carl Neuberg, a German chemist.[] Since then,biochemistry has advanced, especially since the mid-20th century, with thedevelopment of new techniques such as chromatography, X-ray diffraction,dual polarisation interferometry, NMR spectroscopy, radioisotopic labeling,electron microscopy, and molecular dynamics simulations. These techniques allowed for the discovery and detailedanalysis of many molecules and metabolic pathways of the cell, such as glycolysis and the Krebs cycle (citric acidcycle).

Biochemistry 2

Another significant historic event in biochemistry is the discovery of the gene and its role in the transfer ofinformation in the cell. This part of biochemistry is often called molecular biology.[5] In the 1950s, James D.Watson, Francis Crick, Rosalind Franklin, and Maurice Wilkins were instrumental in solving DNA structure andsuggesting its relationship with genetic transfer of information.[6] In 1958, George Beadle and Edward Tatumreceived the Nobel Prize for work in fungi showing that one gene produces one enzyme.[] In 1988, Colin Pitchforkwas the first person convicted of murder with DNA evidence, which led to growth of forensic science.[] Morerecently, Andrew Z. Fire and Craig C. Mello received the 2006 Nobel Prize for discovering the role of RNAinterference (RNAi), in the silencing of gene expression.[]

Starting materials: the chemical elements of lifeAround two dozen of the 92 naturally occurring chemical elements are essential to various kinds of biological life.Most rare elements on Earth are not needed by life (exceptions being selenium and iodine), while a few commonones (aluminum and titanium) are not used. Most organisms share element needs, but there are a few differencesbetween plants and animals. For example ocean algae use bromine but land plants and animals seem to need none.All animals require sodium, but some plants do not. Plants need boron and silicon, but animals may not (or may needultra-small amounts).Just six elementscarbon, hydrogen, nitrogen, oxygen, calcium, and phosphorusmake up almost 99% of the massof a human body (see composition of the human body for a complete list). In addition to the six major elements thatcompose most of the human body, humans require smaller amounts of possibly 18 more.[7]

BiomoleculesThe four main classes of molecules in biochemistry are carbohydrates, lipids, proteins, and nucleic acids. Manybiological molecules are polymers: in this terminology, monomers are relatively small micromolecules that arelinked together to create large macromolecules, which are known as polymers. When monomers are linked togetherto synthesize a biological polymer, they undergo a process called dehydration synthesis. Different macromoleculescan assemble in larger complexes, often needed for biological activity.

Carbohydrates

A molecule of sucrose (glucose + fructose), adisaccharide.

Carbohydrates are made from monomers called monosaccharides.Some of these monosaccharides include glucose (C6H12O6), fructose(C6H12O6), and deoxyribose (C5H10O4). When two monosaccharidesundergo dehydration synthesis, water is produced, as two hydrogenatoms and one oxygen atom are lost from the two monosaccharides'hydroxyl group.

Biochemistry 3

Lipids

A triglyceride with a glycerol moleculeon the left and three fatty acids coming

off it.

Lipids are usually made from one molecule of glycerol combined with othermolecules. In triglycerides, the main group of bulk lipids, there is onemolecule of glycerol and three fatty acids. Fatty acids are considered themonomer in that case, and may be saturated (no double bonds in the carbonchain) or unsaturated (one or more double bonds in the carbon chain).

Lipids, especially phospholipids, are also used in various pharmaceuticalproducts, either as co-solubilisers (e.g., in parenteral infusions) or else as drugcarrier components (e.g., in a liposome or transfersome).

Proteins

The general structure of an -amino acid,with the amino group on the left and the

carboxyl group on the right.

Proteins are very large molecules macro-biopolymers made frommonomers called amino acids. There are 20 standard amino acids, eachcontaining a carboxyl group, an amino group, and a side-chain (known asan "R" group). The "R" group is what makes each amino acid different, andthe properties of the side-chains greatly influence the overallthree-dimensional conformation of a protein. When amino acids combine,they form a special bond called a peptide bond through dehydrationsynthesis, and become a polypeptide, or protein.

In order to determine whether two proteins are related, or in other words todecide whether they are homologous or not, scientists usesequence-comparison methods. Methods like Sequence Alignments andStructural Alignments are powerful tools that help scientists identify homologies between related molecules.[]

The relevance of finding homologies among proteins goes beyond forming an evolutionary pattern of proteinfamilies. By finding how similar two protein sequences are, we acquire knowledge about their structure andtherefore their function.

Biochemistry 4

Nucleic acids

The structure of deoxyribonucleic acid (DNA), thepicture shows the monomers being put together.

Nucleic acids are the molecules that make up DNA, an extremelyimportant substance that all cellular organisms use to store theirgenetic information. The most common nucleic acids aredeoxyribonucleic acid and ribonucleic acid. Their monomers arecalled nucleotides. The most common nucleotides are adenine,cytosine, guanine, thymine, and uracil. Adenine binds withthymine and uracil; Thymine binds only with adenine; andcytosine and guanine can bind only with each other.

Carbohydrates

The function of carbohydrates includes energy storage andproviding structure. Sugars are carbohydrates, but not allcarbohydrates are sugars. There are more carbohydrates on Earththan any other known type of biomolecule; they are used to storeenergy and genetic information, as well as play important roles incell to cell interactions and communications.

Monosaccharides

Glucose

The simplest type of carbohydrate is a monosaccharide, which amongother properties contains carbon, hydrogen, and oxygen, mostly in aratio of 1:2:1 (generalized formula CnH2nOn, where n is at least 3).Glucose, one of the most important carbohydrates, is an example of amonosaccharide. So is fructose, the sugar commonly associated withthe sweet taste of fruits.[][a] Some carbohydrates (especially aftercondensation to oligo- and polysaccharides) contain less carbonrelative to H and O, which still are present in 2:1 (H:O) ratio.Monosaccharides can be grouped into aldoses (having an aldehyde group at the end of the chain, e.g. glucose) andketoses (having a keto group in their chain; e.g. fructose). Both aldoses and ketoses occur in an equilibrium (startingwith chain lengths of C4) cyclic forms. These are generated by bond formation between one of the hydroxyl groupsof the sugar chain with the carbon of the aldehyde or keto group to form a hemiacetal bond. This leads to saturatedfive-membered (in furanoses) or six-membered (in pyranoses) heterocyclic rings containing one O as heteroatom.

Disaccharides

Sucrose: ordinary table sugar and probably themost familiar carbohydrate.

Two monosaccharides can be joined together using dehydrationsynthesis, in which a hydrogen atom is removed from the end of onemolecule and a hydroxyl group (OH) is removed from the other; theremaining residues are then attached at the sites from which the atomswere removed. The HOH or H2O is then released as a molecule ofwater, hence the term dehydration. The new molecule, consisting oftwo monosaccharides, is called a disaccharide and is conjoinedtogether by a glycosidic or ether bond. The reverse reaction can also

Biochemistry 5

occur, using a molecule of water to split up a disaccharide and break the glycosidic bond; this is termed hydrolysis.The most well-known disaccharide is sucrose, ordinary sugar (in scientific contexts, called table sugar or cane sugarto differentiate it from other sugars). Sucrose consists of a glucose molecule and a fructose molecule joined together.Another important disaccharide is lactose, consisting of a glucose molecule and a galactose molecule. As mosthumans age, the production of lactase, the enzyme that hydrolyzes lactose back into glucose and galactose, typicallydecreases. This results in lactase deficiency, also called lactose intolerance.Sugar polymers are characterised by having reducing or non-reducing ends. A reducing end of a carbohydrate is acarbon atom that can be in equilibrium with the open-chain aldehyde or keto form. If the joining of monomers takesplace at such a carbon atom, the free hydroxy group of the pyranose or furanose form is exchanged with anOH-side-chain of another sugar, yielding a full acetal. This prevents opening of the chain to the aldehyde or ketoform and renders the modified residue non-reducing. Lactose contains a reducing end at its glucose moiety, whereasthe galactose moiety form a full acetal with the C4-OH group of glucose. Saccharose does not have a reducing endbecause of full acetal formation between the aldehyde carbon of glucose (C1) and the keto carbon of fructose (C2).

Oligosaccharides and polysaccharides

Cellulose as polymer of -D-glucose

When a few (around three to six) monosaccharides are joined together,it is called an oligosaccharide (oligo- meaning "few"). Thesemolecules tend to be used as markers and signals, as well as havingsome other uses. Many monosaccharides joined together make apolysaccharide. They can be joined together in one long linear chain,or they may be branched. Two of the most common polysaccharidesare cellulose and glycogen, both consisting of repeating glucosemonomers.

Cellulose is made by plants and is an important structural component of their cell walls. Humans can neithermanufacture nor digest it.

Glycogen, on the other hand, is an animal carbohydrate; humans and other animals use it as a form of energystorage.

Use of carbohydrates as an energy sourceGlucose is the major energy source in most life forms. For instance, polysaccharides are broken down into theirmonomers (glycogen phosphorylase removes glucose residues from glycogen). Disaccharides like lactose or sucroseare cleaved into their two component monosaccharides.

Glycolysis (anaerobic)

Glucose is mainly metabolized by a very important ten-step pathway called glycolysis, the net result of which is tobreak down one molecule of glucose into two molecules of pyruvate; this also produces a net two molecules of ATP,the energy currency of cells, along with two reducing equivalents in the form of converting NAD+ to NADH. Thisdoes not require oxygen; if no oxygen is available (or the cell cannot use oxygen), the NAD is restored by convertingthe pyruvate to lactate (lactic acid) (e.g., in humans) or to ethanol plus carbon dioxide (e.g., in yeast). Othermonosaccharides like galactose and fructose can be converted into intermediates of the glycolytic pathway.[8]

Biochemistry 6

Aerobic

In aerobic cells with sufficient oxygen, as in most human cells, the pyruvate is further metabolized. It is irreversiblyconverted to acetyl-CoA, giving off one carbon atom as the waste product carbon dioxide, generating anotherreducing equivalent as NADH. The two molecules acetyl-CoA (from one molecule of glucose) then enter the citricacid cycle, producing two more molecules of ATP, six more NADH molecules and two reduced (ubi)quinones (viaFADH2 as enzyme-bound cofactor), and releasing the remaining carbon atoms as carbon dioxide. The producedNADH and quinol molecules then feed into the enzyme complexes of the respiratory chain, an electron transportsystem transferring the electrons ultimately to oxygen and conserving the released energy in the form of a protongradient over a membrane (inner mitochondrial membrane in eukaryotes). Thus, oxygen is reduced to water and theoriginal electron acceptors NAD+ and quinone are regenerated. This is why humans breathe in oxygen and breatheout carbon dioxide. The energy released from transferring the electrons from high-energy states in NADH and quinolis conserved first as proton gradient and converted to ATP via ATP synthase. This generates an additional 28molecules of ATP (24 from the 8 NADH + 4 from the 2 quinols), totaling to 32 molecules of ATP conserved perdegraded glucose (two from glycolysis + two from the citrate cycle). It is clear that using oxygen to completelyoxidize glucose provides an organism with far more energy than any oxygen-independent metabolic feature, and thisis thought to be the reason why complex life appeared only after Earth's atmosphere accumulated large amounts ofoxygen.

Gluconeogenesis

In vertebrates, vigorously contracting skeletal muscles (during weightlifting or sprinting, for example) do not receiveenough oxygen to meet the energy demand, and so they shift to anaerobic metabolism, converting glucose to lactate.The liver regenerates the glucose, using a process called gluconeogenesis. This process is not quite the opposite ofglycolysis, and actually requires three times the amount of energy gained from glycolysis (six molecules of ATP areused, compared to the two gained in glycolysis). Analogous to the above reactions, the glucose produced can thenundergo glycolysis in tissues that need energy, be stored as glycogen (or starch in plants), or be converted to othermonosaccharides or joined into di- or oligosaccharides. The combined pathways of glycolysis during exercise,lactate's crossing via the bloodstream to the liver, subsequent gluconeogenesis and release of glucose into thebloodstream is called the Cori cycle.[9]

Proteins

A schematic of hemoglobin. The red andblue ribbons represent the protein globin;the green structures are the heme groups.

Like carbohydrates, some proteins perform largely structural roles. Forinstance, movements of the proteins actin and myosin ultimately areresponsible for the contraction of skeletal muscle. One property manyproteins have is that they specifically bind to a certain molecule or class ofmoleculesthey may be extremely selective in what they bind. Antibodiesare an example of proteins that attach to one specific type of molecule. Infact, the enzyme-linked immunosorbent assay (ELISA), which usesantibodies, is currently one of the most sensitive tests modern medicineuses to detect various biomolecules. Probably the most important proteins,however, are the enzymes. These molecules recognize specific reactantmolecules called substrates; they then catalyze the reaction between them.By lowering the activation energy, the enzyme speeds up that reaction by arate of 1011 or more: a reaction that would normally take over 3,000 yearsto complete spontaneously might take less than a second with an enzyme.

The enzyme itself is not used up in the process, and is free to catalyze the same reaction with a new set of substrates. Using various modifiers, the activity of the enzyme can be regulated, enabling control of the biochemistry of the cell

Biochemistry 7

as a whole.In essence, proteins are chains of amino acids. An amino acid consists of a carbon atom bound to four groups. One isan amino group, NH2, and one is a carboxylic acid group, COOH (although these exist as NH3

+ and COO

under physiologic conditions). The third is a simple hydrogen atom. The fourth is commonly denoted "R" and isdifferent for each amino acid. There are twenty standard amino acids. Some of these have functions by themselves orin a modified form; for instance, glutamate functions as an important neurotransmitter.

Generic amino acids (1) in neutral form, (2) as they exist physiologically, and (3) joinedtogether as a dipeptide.

Amino acids can be joined together viaa peptide bond. In this dehydrationsynthesis, a water molecule is removedand the peptide bond connects thenitrogen of one amino acid's aminogroup to the carbon of the other'scarboxylic acid group. The resultingmolecule is called a dipeptide, andshort stretches of amino acids (usually,fewer than thirty) are called peptides or polypeptides. Longer stretches merit the title proteins. As an example, theimportant blood serum protein albumin contains 585 amino acid residues.[]

The structure of proteins is traditionally described in a hierarchy of four levels. The primary structure of a proteinsimply consists of its linear sequence of amino acids; for instance,"alanine-glycine-tryptophan-serine-glutamate-asparagine-glycine-lysine-". Secondary structure is concerned withlocal morphology (morphology being the study of structure). Some combinations of amino acids will tend to curl upin a coil called an -helix or into a sheet called a -sheet; some -helixes can be seen in the hemoglobin schematicabove. Tertiary structure is the entire three-dimensional shape of the protein. This shape is determined by thesequence of amino acids. In fact, a single change can change the entire structure. The alpha chain of hemoglobincontains 146 amino acid residues; substitution of the glutamate residue at position 6 with a valine residue changesthe behavior of hemoglobin so much that it results in sickle-cell disease. Finally, quaternary structure is concernedwith the structure of a protein with multiple peptide subunits, like hemoglobin with its four subunits. Not all proteinshave more than one subunit.[10]

Ingested proteins are usually broken up into single amino acids or dipeptides in the small intestine, and thenabsorbed. They can then be joined together to make new proteins. Intermediate products of glycolysis, the citric acidcycle, and the pentose phosphate pathway can be used to make all twenty amino acids, and most bacteria and plantspossess all the necessary enzymes to synthesize them. Humans and other mammals, however, can synthesize onlyhalf of them. They cannot synthesize isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan,and valine. These are the essential amino acids, since it is essential to ingest them. Mammals do possess the enzymesto synthesize alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine, thenonessential amino acids. While they can synthesize arginine and histidine, they cannot produce it in sufficientamounts for young, growing animals, and so these are often considered essential amino acids.If the amino group is removed from an amino acid, it leaves behind a carbon skeleton called an -keto acid.Enzymes called transaminases can easily transfer the amino group from one amino acid (making it an -keto acid) toanother -keto acid (making it an amino acid). This is important in the biosynthesis of amino acids, as for many ofthe pathways, intermediates from other biochemical pathways are converted to the -keto acid skeleton, and then anamino group is added, often via transamination. The amino acids may then be linked together to make a protein.[11]

A similar process is used to break down proteins. It is first hydrolyzed into its component amino acids. Free ammonia (NH3), existing as the ammonium ion (NH4

+) in blood, is toxic to life forms. A suitable method for excreting it must therefore exist. Different strategies have evolved in different animals, depending on the animals' needs. Unicellular organisms, of course, simply release the ammonia into the environment. Likewise, bony fish can

Biochemistry 8

release the ammonia into the water where it is quickly diluted. In general, mammals convert the ammonia into urea,via the urea cycle.[]

LipidsThe term lipid comprises a diverse range of molecules and to some extent is a catchall for relatively water-insolubleor nonpolar compounds of biological origin, including waxes, fatty acids, fatty-acid derived phospholipids,sphingolipids, glycolipids, and terpenoids (e.g., retinoids and steroids). Some lipids are linear aliphatic molecules,while others have ring structures. Some are aromatic, while others are not. Some are flexible, while others arerigid.[12]

Most lipids have some polar character in addition to being largely nonpolar. In general, the bulk of their structure isnonpolar or hydrophobic ("water-fearing"), meaning that it does not interact well with polar solvents like water.Another part of their structure is polar or hydrophilic ("water-loving") and will tend to associate with polar solventslike water. This makes them amphiphilic molecules (having both hydrophobic and hydrophilic portions). In the caseof cholesterol, the polar group is a mere -OH (hydroxyl or alcohol). In the case of phospholipids, the polar groups areconsiderably larger and more polar, as described below.Lipids are an integral part of our daily diet. Most oils and milk products that we use for cooking and eating likebutter, cheese, ghee etc., are composed of fats. Vegetable oils are rich in various polyunsaturated fatty acids (PUFA).Lipid-containing foods undergo digestion within the body and are broken into fatty acids and glycerol, which are thefinal degradation products of fats and lipids.

Nucleic acidsA nucleic acid is a complex, high-molecular-weight biochemical macromolecule composed of nucleotide chains thatconvey genetic information. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid(RNA). Nucleic acids are found in all living cells and viruses. Aside from the genetic material of the cell, nucleicacids often play a role as second messengers, as well as forming the base molecule for adenosine triphosphate, theprimary energy-carrier molecule found in all living organisms.Nucleic acid, so called because of its prevalence in cellular nuclei, is the generic name of the family of biopolymers.The monomers are called nucleotides, and each consists of three components: a nitrogenous heterocyclic base (eithera purine or a pyrimidine), a pentose sugar, and a phosphate group. Different nucleic acid types differ in the specificsugar found in their chain (e.g., DNA or deoxyribonucleic acid contains 2-deoxyriboses). Also, the nitrogenous basespossible in the two nucleic acids are different: adenine, cytosine, and guanine occur in both RNA and DNA, whilethymine occurs only in DNA and uracil occurs in RNA.[13]

Biochemistry 9

Relationship to other "molecular-scale" biological sciences

Schematic relationship between biochemistry, genetics, andmolecular biology

Researchers in biochemistry use specific techniquesnative to biochemistry, but increasingly combine thesewith techniques and ideas developed in the fields ofgenetics, molecular biology and biophysics. There hasnever been a hard-line between these disciplines interms of content and technique. Today, the termsmolecular biology and biochemistry are nearlyinterchangeable. The following figure is a schematicthat depicts one possible view of the relationshipbetween the fields:

Biochemistry is the study of the chemical substancesand vital processes occurring in living organisms.Biochemists focus heavily on the role, function, andstructure of biomolecules. The study of thechemistry behind biological processes and thesynthesis of biologically active molecules areexamples of biochemistry.

Genetics is the study of the effect of genetic differences on organisms. Often this can be inferred by the absenceof a normal component (e.g., one gene). The study of "mutants" organisms with a changed gene that leads to theorganism being different with respect to the so-called "wild type" or normal phenotype. Genetic interactions(epistasis) can often confound simple interpretations of such "knock-out" or "knock-in" studies.

Molecular biology is the study of molecular underpinnings of the process of replication, transcription andtranslation of the genetic material. The central dogma of molecular biology where genetic material is transcribedinto RNA and then translated into protein, despite being an oversimplified picture of molecular biology, stillprovides a good starting point for understanding the field. This picture, however, is undergoing revision in light ofemerging novel roles for RNA.[]

Chemical Biology seeks to develop new tools based on small molecules that allow minimal perturbation ofbiological systems while providing detailed information about their function. Further, chemical biology employsbiological systems to create non-natural hybrids between biomolecules and synthetic devices (for exampleemptied viral capsids that can deliver gene therapy or drug molecules).

Notesa. ^ Fructose is not the only sugar found in fruits. Glucose and sucrose are also found in varying quantities in variousfruits, and indeed sometimes exceed the fructose present. For example, 32% of the edible portion of date is glucose,compared with 23.70% fructose and 8.20% sucrose. However, peaches contain more sucrose (6.66%) than they dofructose (0.93%) or glucose (1.47%).[14]

References[1] http:/ / portal. acs. org/ portal/ acs/ corg/ content?_nfpb=true& _pageLabel=PP_ARTICLEMAIN& node_id=1188&

content_id=CTP_003379& use_sec=true& sec_url_var=region1& __uuid=aa3f2aa3-8047-4fa2-88b8-32ffcad3a93e[2] http:/ / www. biochemistry. org/ Education/ Careers/ Schoolsandcolleges/ Whatisbiochemistry. aspx[3][3] Hunter (2000), p. 75.[4] Hunter (2000), pp. 9698.[5][5] Tropp (2012), p. 2.[6] Tropp (2012), pp. 1920.

Biochemistry 10

[7] Ultratrace minerals. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... etal.. Baltimore : Williams & Wilkins, c1999., p. 283-303. Issue Date: 1999 URI: (http:/ / hdl. handle. net/ 10113/ 46493)

[8] Fromm and Hargrove (2012), pp. 163180.[9] Fromm and Hargrove (2012), pp. 183194.[10] Fromm and Hargrove (2012), pp. 3551.[11] Fromm and Hargrove (2012), pp. 279292.[12] Fromm and Hargrove (2012), pp. 2227.[13] Tropp (2012), pp. 59.[14][14] Whiting, G.C. (1970), p.5

Cited literature Fromm, Herbert J.; Hargrove, Mark (2012). Essentials of Biochemistry. Springer. ISBN978-3-642-19623-2. Hunter, Graeme K. (2000). Vital Forces: The Discovery of the Molecular Basis of Life. Academic Press.

ISBN978-0-12-361811-5. Tropp, Burton E. (2012). Molecular Biology (4th ed.). Jones & Bartlett Learning. ISBN978-1-4496-0091-4.

External links The Virtual Library of Biochemistry and Cell Biology (http:/ / www. biochemweb. org/ ) Biochemistry, 5th ed. (http:/ / www. ncbi. nlm. nih. gov/ books/ bv. fcgi?call=bv. View. . ShowTOC& rid=stryer.

TOC& depth=2) Full text of Berg, Tymoczko, and Stryer, courtesy of NCBI. Biochemistry, 2nd ed. (http:/ / www. web. virginia. edu/ Heidi/ home. htm) Full text of Garrett and Grisham. Biochemistry Animation (http:/ / www. 1lec. com/ Biochemistry/ ) (Narrated Flash animations.) SystemsX.ch - The Swiss Initiative in Systems Biology (http:/ / www. systemsX. ch/ ) Biochemistry Online Resources (http:/ / www. icademic. org/ 97445/ Biochemistry/ ) Lists of Biochemistry

departments, websites, journals, books and reviews, employment opportunities and events. biochemical families: carbohydrates

alcohols glycoproteins glycosides

lipids eicosanoids fatty acids / intermediates phospholipids sphingolipids steroids

nucleic acids constituents / intermediates

proteins amino acids / intermediates

tetrapyrroles / intermediates

Cells 11

Cells

Allium cells in different phases of the cell cycle

The cells of eukaryotes (left) and prokaryotes (right)

The cell is the basic structural,functional and biological unit of allknown living organisms. It is thesmallest unit of life that is classified asa living thing (except virus, whichconsists only from DNA/RNA coveredby protein and lipids), and is oftencalled the building block of life.

It consists of a protoplasm enclosedwithin a membrane, which containsmany biomolecules such as proteinsand nucleic acids. [1] Organisms can beclassified as unicellular (consisting of asingle cell; including most bacteria) ormulticellular (including plants andanimals).

While the number of cells in plants andanimals varies from species to species,Humans contain about 100 trillion(1014) cells.[2] Most plant and animalcells are between 1 and100micrometres and therefore arevisible only under the microscope.[3]

The cell was discovered by RobertHooke in 1665. The cell theory, firstdeveloped in 1839 by Matthias JakobSchleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells comefrom preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditaryinformation necessary for regulating cell functions and for transmitting information to the next generation of cells.[4]

Cells emerged on planet Earth at least 4.04.3 billion years ago.

The word cell comes from the Latin cella, meaning "small room".[5] The descriptive term for the smallest livingbiological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells hesaw through his microscope to the small rooms monks lived in.[6]

Cells 12

AnatomyThere are two types of cells: Eukaryote and prokaryote. Prokaryotic cells are usually independent, while eukaryoticcells can either exist as a single celled organism or be found in multicellular organisms.

Table 1: Comparison of features of prokaryotic and eukaryotic cells

Prokaryotes Eukaryotes

Typical organisms bacteria, archaea protists, fungi, plants, animals

Typical size ~ 15 m[] ~ 10100 m[] (sperm cells, apart from the tail, are smaller)

Type of nucleus nucleoid region; no real nucleus real nucleus with double membrane

DNA circular (usually) linear molecules (chromosomes) with histone proteins

RNA-/protein-synthesis coupled in cytoplasm RNA-synthesis inside the nucleusprotein synthesis in cytoplasm

Ribosomes 50S+30S 60S+40S

Cytoplasmatic structure very few structures highly structured by endomembranes and a cytoskeleton

Cell movement flagella made of flagellin flagella and cilia containing microtubules; lamellipodia and filopodia containing actin

Mitochondria none one to several thousand (though some lack mitochondria)

Chloroplasts none in algae and plants

Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells

Cell division Binary fission (simple division) Mitosis (fission or budding)Meiosis

Prokaryotic cells

Diagram of a typical prokaryotic cell

The prokaryote cell is simpler, andtherefore smaller, than a eukaryotecell, lacking a nucleus and most of theother organelles of eukaryotes. Thereare two kinds of prokaryotes: bacteriaand archaea; these share a similarstructure.

The nuclear material of a prokaryoticcell consists of a single chromosomethat is in direct contact with thecytoplasm. Here, the undefined nuclearregion in the cytoplasm is called thenucleoid.

A prokaryotic cell has threearchitectural regions: On the outside, flagella and pili

project from the cell's surface.These are structures (not present inall prokaryotes) made of proteins that facilitate movement and communication between cells.

Cells 13

Enclosing the cell is the cell envelope generally consisting of a cell wall covering a plasma membrane thoughsome bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell andseparates the interior of the cell from its environment, serving as a protective filter. Though most prokaryoteshave a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea). The cell wallconsists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents thecell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment.Some eukaryote cells (plant cells and fungal cells) also have a cell wall.

Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts ofinclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacteriumBorrelia burgdorferi, which causes Lyme disease).[7] Though not forming a nucleus, the DNA is condensed in anucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular.Plasmids enable additional functions, such as antibiotic resistance.

Eukaryotic cellsPlants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about 15 times widerthan a typical prokaryote and can be as much as 1000 times greater in volume. The major difference betweenprokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specificmetabolic activities take place. Most important among these is a cell nucleus, a membrane-delineated compartmentthat houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Otherdifferences 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 associated

with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by amembrane. Some eukaryotic organelles such as mitochondria also contain some DNA.

Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation,mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate alarge number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternativelyto cell division and differentiation."[]

Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes.

Structure of a typical animal cell Structure of a typical plant cell

Cells 14

Table 2: Comparison of structures between animal and plant cells

Typical animal cell Typical plant cell

Organelles Nucleus

Nucleolus (within nucleus) Rough endoplasmic reticulum (ER) Smooth ER Ribosomes Cytoskeleton Golgi apparatus Cytoplasm Mitochondria Vesicles Lysosomes Centrosome

Centrioles

Nucleus

Nucleolus (within nucleus) Rough ER Smooth ER Ribosomes Cytoskeleton Golgi apparatus (dictiosomes) Cytoplasm Mitochondria Plastids and its derivatives Vacuole(s) Cell wall

Subcellular componentsAll cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from itsenvironment, regulates what moves in and out (selectively permeable), and maintains the electric potential of thecell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells (except red blood cells whichlack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) possess DNA, thehereditary material of genes, and RNA, containing the information necessary to build various proteins such asenzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists theseprimary components of the cell, then briefly describe their function.

MembraneThe cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. The plasma membrane in plants andprokaryotes is usually covered by a cell wall. This membrane serves to separate and protect a cell from itssurrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules) andhydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaicmembrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps thatmove different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can eitherlet a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cellsurface membranes also contain receptor proteins that allow cells to detect external signaling molecules such ashormones.

Cells 15

Cytoskeleton

Bovine Pulmonary Artery Endothelial cell: nuclei stained blue,mitochondria stained red, and F-actin, an important component in

microfilaments, stained green. Cell imaged on a fluorescentmicroscope.

The cytoskeleton acts to organize and maintain thecell's shape; anchors organelles in place; helps duringendocytosis, the uptake of external materials by a cell,and cytokinesis, the separation of daughter cells aftercell division; and moves parts of the cell in processes ofgrowth and mobility. The eukaryotic cytoskeleton iscomposed of microfilaments, intermediate filamentsand microtubules. There are a great number of proteinsassociated with them, each controlling a cell's structureby directing, bundling, and aligning filaments. Theprokaryotic cytoskeleton is less well-studied but isinvolved in the maintenance of cell shape, polarity andcytokinesis.[8]

Genetic material

Two different kinds of genetic material exist:deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most cells use DNA for their long-term informationstorage. The biological information contained in an organism is encoded in its DNA sequence. RNA is used forinformation transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA)molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in thenucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules calledchromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondriaand chloroplasts (see endosymbiotic theory).A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (themitochondrial genome). In humans the nuclear genome is divided into 46 linear DNA molecules calledchromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrialgenome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very smallcompared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specifictRNAs.Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process calledtransfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain virusesalso insert their genetic material into the genome.

OrganellesThe human body contains many different organs, such as the heart, lung, and kidney, with each organ performing adifferent function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized forcarrying out one or more vital functions. Both eukaryotic and prokaryotic cells have organelles but organelles ineukaryotes are generally more complex and may be membrane bound.There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary,while others (such as mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands). Thecytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

Cells 16

Diagram of a cell nucleus

Cell nucleus eukaryotes only - A cell's information center, thecell nucleus is the most conspicuous organelle found in a eukaryoticcell. It houses the cell's chromosomes, and is the place where almostall DNA replication and RNA synthesis (transcription) occur. Thenucleus is spherical and separated from the cytoplasm by a doublemembrane called the nuclear envelope. The nuclear envelopeisolates and protects a cell's DNA from various molecules that couldaccidentally damage its structure or interfere with its processing.During processing, DNA is transcribed, or copied into a specialRNA, called messenger RNA (mRNA). This mRNA is thentransported out of the nucleus, where it is translated into a specificprotein molecule. The nucleolus is a specialized region within thenucleus where ribosome subunits are assembled. In prokaryotes,DNA processing takes place in the cytoplasm.

Mitochondria and Chloroplasts eukaryotes only - the power generators: Mitochondria are self-replicatingorganelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondriaplay a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy byoxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining toglucose) to generate ATP. Mitochondria multiply by splitting in two. Respiration occurs in the cell mitochondria.Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make ATP.

Diagram of an endomembrane system

Endoplasmic reticulum eukaryotes only: The endoplasmicreticulum (ER) is the transport network for molecules targeted forcertain modifications and specific destinations, as compared tomolecules that float freely in the cytoplasm. The ER has two forms:the rough ER, which has ribosomes on its surface and secretesproteins into the cytoplasm, and the smooth ER, which lacks them.Smooth ER plays a role in calcium sequestration and release.[citationneeded]

Golgi apparatus eukaryotes only : The primary function of theGolgi apparatus is to process and package the macromolecules suchas proteins and lipids that are synthesized by the cell.[citation needed]

Ribosomes: The ribosome is a large complex of RNA and proteinmolecules. They each consist of two subunits, and act as anassembly line where RNA from the nucleus is used to synthesiseproteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the roughendoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).[9]

Lysosomes and Peroxisomes eukaryotes only: Lysosomes contain digestive enzymes (acid hydrolases). Theydigest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymesthat rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not containedin a membrane-bound system.[citation needed]

Centrosome the cytoskeleton organiser: The centrosome produces the microtubules of a cell a keycomponent of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes arecomposed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle.A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.[citationneeded]

Cells 17

Vacuoles: Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquidfilled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles,which can pump water out of the cell if there is too much water. The vacuoles of eukaryotic cells are usuallylarger in those of plants than animals.[citation needed]

Structures outside the cell membraneMany cells also have structures which exist wholly or partially outside the cell membrane. These structures arenotable because they are not protected from the external environment by the impermeable cell membrane. In order toassemble these structures export processes to carry macromolecules across the cell membrane must be used.

Cell wallMany types of prokaryotic and eukaryotic cell have a cell wall. The cell wall acts to protect the cell mechanicallyand chemically from its environment, and is an additional layer of protection to the cell membrane. Different typesof cell have cell walls made up of different materials; plant cell walls are primarily made up of pectin, fungi cellwalls are made up of chitin and bacteria cell walls are made up of peptidoglycan.

Prokaryotic

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may bepolysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as instreptococci.[citation needed] Capsules are not marked by normal staining protocols and can be detected by specialstain.[citation needed]

Flagella

Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cellmembrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature.Are most commonly found in bacteria cells but are found in animal cells as well.

Fimbriae (pili)

They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible forattachment of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili)involved in conjunction.[citation needed]

Growth and metabolismBetween successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is theprocess by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, inwhich the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which thecell 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 moleculecalled glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form ofenergy, through two different pathways.The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction isdesigned 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.

Cells 18

The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and can generateenough ATP to run all the cell functions.[citation needed]

An overview of proteinsynthesis.Within the

nucleus of the cell (lightblue), genes (DNA, darkblue) are transcribed intoRNA. This RNA is then

subject topost-transcriptional

modification and control,resulting in a mature

mRNA (red) that is thentransported out of thenucleus and into the

cytoplasm (peach), whereit undergoes translationinto a protein. mRNA istranslated by ribosomes(purple) that match the

three-base codons of themRNA to the three-base

anti-codons of theappropriate tRNA. Newly

synthesized proteins(black) are often further

modified, such as bybinding to an effectormolecule (orange), tobecome fully active.

Self-replication

Cell division involves a single cell (called a mother cell) dividing into two daughtercells. This leads to growth in multicellular organisms (the growth of tissue) and toprocreation (vegetative reproduction) in unicellular organisms.

Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process ofnuclear division, called mitosis, followed by division of the cell, called cytokinesis. Adiploid cell may also undergo meiosis to produce haploid cells, usually four. Haploidcells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, always happens when acell divides through mitosis or binary fission.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNAreplication only occurs before meiosis I. DNA replication does not occur when the cellsdivide the second time, in meiosis II.[10] Replication, like all cellular activities, requiresspecialized proteins for carrying out the job.

Protein synthesis

Cells are capable of synthesizing new proteins, which are essential for the modulationand maintenance of cellular activities. This process involves the formation of newprotein molecules from amino acid building blocks based on information encoded inDNA/RNA. Protein synthesis generally consists of two major steps: transcription andtranslation.

Transcription is the process where genetic information in DNA is used to produce acomplementary RNA strand. This RNA strand is then processed to give messenger RNA(mRNA), which is free to migrate through the cell. mRNA molecules bind toprotein-RNA complexes called ribosomes located in the cytosol, where they aretranslated into polypeptide sequences. The ribosome mediates the formation of apolypeptide sequence based on the mRNA sequence. The mRNA sequence directlyrelates to the polypeptide sequence by binding to transfer RNA (tRNA) adaptermolecules in binding pockets within the ribosome. The new polypeptide then folds into afunctional three-dimensional protein molecule.

Movement or motility

Cells can move during many processes: such as wound healing, the immune response and cancer metastasis. Forwound healing to occur, white blood cells and cells that ingest bacteria move to the wound site to kill themicroorganisms that cause infection. At the same time fibroblasts (connective tissue cells) move there to remodel damaged structures. In the case of tumordevelopment, cells from a primary tumor move away and spread to other parts of the body. Cell motility involvesmany receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.[11] The process is divided intothree steps protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body

Cells 19

and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated byunique segments of the cytoskeleton.[12][13]

OriginsThe origin of cells has to do with the origin of life, which began the history of life on Earth.

Origin of the first cellThere are several theories about the origin of small molecules that could lead to life in an early Earth. One is thatthey came from meteorites (see Murchison meteorite). Another is that they were created at deep-sea vents. A third isthat they were synthesized by lightning in a reducing atmosphere (see MillerUrey experiment); although it is notclear if Earth had such an atmosphere. There are essentially no experimental data defining what the firstself-replicating forms were. RNA is generally assumed the earliest self-replicating molecule, as it is capable of bothstoring genetic information and catalyzing chemical reactions (see RNA world hypothesis). But some other entitywith the potential to self-replicate could have preceded RNA, like clay or peptide nucleic acid.[]

Cells emerged at least 4.04.3 billion years ago. The current belief is that these cells were heterotrophs. Animportant characteristic of cells is the cell membrane, composed of a bilayer of lipids. The early cell membraneswere probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids areknown to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cellmembranes could also have been produced by catalytic RNA, or even have required structural proteins before theycould form.[14]

Origin of eukaryotic cellsThe eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearingorganelles like the mitochondria and the chloroplasts are almost certainly what remains of ancient symbioticoxygen-breathing proteobacteria and cyanobacteria, respectively, where the rest of the cell appears derived from anancestral archaean prokaryote cellan idea called the endosymbiotic theory.There is still considerable debate about whether organelles like the hydrogenosome predated the origin ofmitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells.Sex, as the stereotyped choreography of meiosis and syngamy that persists in nearly all extant eukaryotes, may haveplayed a role in the transition from prokaryotes to eukaryotes. An 'origin of sex as vaccination' theory suggests thatthe eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer.Sex-as-syngamy (fusion sex) arose when infected hosts began swapping nuclearized genomes containing co-evolved,vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.[]

History of research 16321723: Antonie van Leeuwenhoek teaches himself to make lenses, constructs simple microscopes 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 compound microscope.[6]

1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made ofcells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.

1855: Rudolf Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula). 1859: The belief that life forms can occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur

(18221895) (although Francesco Redi had performed an experiment in 1668 that suggested the sameconclusion).

Cells 20

1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935, hehas 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[1] Cell Movements and the Shaping of the Vertebrate Body (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Search& db=books&

doptcmdl=GenBookHL& term=Cell+ Movements+ and+ the+ Shaping+ of+ the+ Vertebrate+ Body+ AND+ mboc4[book]+ AND+374635[uid]& rid=mboc4. section. 3919) in Chapter 21 of Molecular Biology of the Cell (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query.fcgi?cmd=Search& db=books& doptcmdl=GenBookHL& term=cell+ biology+ AND+ mboc4[book]+ AND+ 373693[uid]& rid=mboc4)fourth edition, edited by Bruce Alberts (2002) published by Garland Science.The Alberts text discusses how the "cellular building blocks" move to shape developing embryos. It is also common to describe smallmolecules such as amino acids as " molecular building blocks (http:/ / www. ncbi. nlm. nih. gov/ entrez/ query. fcgi?cmd=Search&db=books& doptcmdl=GenBookHL& term="all+ cells"+ AND+ mboc4[book]+ AND+ 372023[uid]& rid=mboc4. section. 4#23)".

[6] "... I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular[..] these pores, or cells, [..] were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with anyWriter or Person, that had made any mention of them before this. . ." Hooke describing his observations on a thin slice of cork. RobertHooke (http:/ / www. ucmp. berkeley. edu/ history/ hooke. html)

[7] European Bioinformatics Institute, Karyn's Genomes: Borrelia burgdorferi (http:/ / www. ebi. ac. uk/ 2can/ genomes/ bacteria/Borrelia_burgdorferi. html), part of 2can on the EBI-EMBL database. Retrieved 5 August 2012

[12][12] Alberts B, Johnson A, Lewis J. et al. Molecular Biology of the Cell, 4e. Garland Science. 2002[13] Ananthakrishnan R, Ehrlicher A. The Forces Behind Cell Movement. Int J Biol Sci 2007; 3:303317. http:/ / www. biolsci. org/ v03p0303.

htm

This article incorporatespublic domain material from the NCBI document "Science Primer" (http:/ / www.ncbi. nlm. nih. gov/ About/ primer/ index. html).

External links Inside the Cell (http:/ / publications. nigms. nih. gov/ insidethecell/ ) - a science education booklet by National

Institutes of Health, in PDF and ePub. Cells Alive! (http:/ / www. cellsalive. com/ ) Cell Biology (http:/ / www. biology. arizona. edu/ cell_bio/ cell_bio. html) in "The Biology Project" of University

of Arizona. Centre of the Cell online (http:/ / www. centreofthecell. org/ ) The Image & Video Library of The American Society for Cell Biology (http:/ / cellimages. ascb. org/ ), a

collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function andbiology of the cell.

HighMag Blog (http:/ / highmagblog. blogspot. com/ ), still images of cells from recent research articles. New Microscope Produces Dazzling 3D Movies of Live Cells (http:/ / www. hhmi. org/ news/ betzig20110304.

html), March 4, 2011 - Howard Hughes Medical Institute. WormWeb.org: Interactive Visualization of the C. elegans Cell lineage (http:/ / wormweb. org/ celllineage) -

Visualize the entire cell lineage tree of the nematode C. elegans

Cells 21

Textbooks Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). Molecular Biology of the Cell (http:/ / www.

ncbi. nlm. nih. gov/ books/ bv. fcgi?rid=mboc4. TOC& depth=2) (4th ed.). Garland. ISBN0-8153-3218-1. Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipurksy SL, Darnell J (2004). Molecular

Cell Biology (http:/ / www. ncbi. nlm. nih. gov/ books/ bv. fcgi?rid=mcb. TOC) (5th ed.). WH Freeman: NewYork, NY. ISBN978-0-7167-4366-8.

Cooper GM (2000). The cell: a molecular approach (http:/ / www. ncbi. nlm. nih. gov/ books/ bv.fcgi?rid=cooper. TOC& depth=2) (2nd ed.). Washington, D.C: ASM Press. ISBN0-87893-102-3.

Water

Water in three states: liquid, solid (ice), and (invisible) water vapor in the air.Clouds are accumulations of water droplets, condensed from vapor-saturated air.

Water is a chemical compound with thechemical formula H2O. A water moleculecontains one oxygen and two hydrogenatoms connected by covalent bonds. Wateris a liquid at standard ambient temperatureand pressure, but it often co-exists on Earthwith its solid state, ice, and gaseous state(water vapor or steam). Water also exists ina liquid crystal state near hydrophilicsurfaces.[1][2]

Water covers 71% of the Earth's surface,[3]

and is vital for all known forms of life.[4] OnEarth, 96.5% of the planet's water is foundin oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction inother large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particlessuspended in air), and precipitation.[][5] Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is inice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smalleramount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products.[]

Water on Earth moves continually through the hydrological cycle of evaporation and transpiration(evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. Evaporation and transpirationcontribute to the precipitation over land.Safe drinking water is essential to humans and other lifeforms even though it provides no calories or organicnutrients. Access to safe drinking water has improved over the last decades in almost every part of the world, butapproximately one billion people still lack access to safe water and over 2.5 billion lack access to adequatesanitation.[] There is a clear correlation between access to safe water and GDP per capita.[6] However, someobservers have estimated that by 2025 more than half of the world population will be facing water-basedvulnerability.[7] A recent report (November 2009) suggests that by 2030, in some developing regions of the world,water demand will exceed supply by 50%.[8] Water plays an important role in the world economy, as it functions as asolvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately70% of the fresh water used by humans goes to agriculture.[]

Water 22

Chemical and physical properties

Model of hydrogen bonds (1) between moleculesof water

Impact from a water drop causes an upward"rebound" jet surrounded by circular capillary

waves.

Snowflakes by Wilson Bentley, 1902

Water is the chemical substance with chemical formula H2O: onemolecule of water has two hydrogen atoms covalently bonded to asingle oxygen atom.

Water appears in nature in all three common states of matter (solid,liquid, and gas) and may take many different forms on Earth: watervapor and clouds in the sky; seawater in the oceans; icebergs in thepolar oceans; glaciers and rivers in the mountains; and the liquid inaquifers in the ground.

The major chemical and physical properties of water are: Water is a liquid at standard temperature and pressure. It is tasteless

and odorless. The intrinsic colour of water and ice is a very slightblue hue, although both appear colorless in small quantities. Watervapour is essentially invisible as a gas.[9]

Water is transparent in the visible electromagnetic spectrum. Thusaquatic plants can live in water because sunlight can reach them.Infrared light is strongly absorbed by the hydrogen-oxygen or OHbonds.

Since the water molecule is not linear and the oxygen atom has ahigher electronegativity than hydrogen atoms, it carries a slightnegative charge, whereas the hydrogen atoms are slightly positive.As a result, water is a polar molecule with an electrical dipolemoment. Water also can form an unusually large number ofintermolecular hydrogen bonds (four) for a molecule of its size.These factors lead to strong attractive forces between molecules ofwater, giving rise to water's high surface tension[10] and capillaryforces. The capillary action refers to the tendency of water to moveup a narrow tube against the force of gravity. This property is reliedupon by all vascular plants, such as trees.[11]

Water is a good polar solvent and is often referred to as theuniversal solvent. Substances that dissolve in water, e.g., salts,sugars, acids, alkalis, and some gases especially oxygen, carbondioxide (carbonation) are known as hydrophilic (water-loving)substances, while those that are immiscible with water (e.g., fats andoils), are known as hydrophobic (water-fearing) substances.

Most of the major components in cells (proteins, DNA andpolysaccharides) are also dissolved in water.

Pure water has a low electrical conductivity, but this increases withthe dissolution of a small amount of ionic material such as sodiumchloride.

The boiling point of water (and all other liquids) is dependent on thebarometric pressure. For example, on the top of Mt. Everest water

Water 23

Dew drops adhering to a spider web

Capillary action of water comparedto mercury

boils at 68 C (154F), compared to 100 C (212F) at sea level.Conversely, water deep in the ocean near geothermal vents canreach temperatures of hundreds of degrees and remain liquid.

At 4181.3 J/(kgK), water has a high specific heat capacity, as wellas a high heat of vaporization (40.65 kJmol1), both of which are aresult of the extensive hydrogen bonding between its molecules.These two unusual properties allow water to moderate Earth'sclimate by buffering large fluctuations in temperature.

The maximum density of water occurs at 3.98 C (39.16F).[12] Ithas the anomalous property of becoming less dense, not more, whenit is cooled to its solid form, ice. During freezing, the 'openstructure' of ice is gradually broken and molecules enter cavities inice-like structure of low temperature water. There are twocompeting effects: 1) Increasing volume of normal liquid and 2)Decrease overall volume of the liquid. Between 0 and 3.98C, thesecond effect will cancel off the first effect so the net effect isshrinkage of volume with increasing temperature.[13] It expands tooccupy 9% greater volume in this solid state, which accounts for thefact of ice floating on liquid water, as in icebergs.

The density of liquid water is 1,000kg/m3 (62.43lb/cu ft) at 4C.Ice has a density of 917kg/m3 (57.25lb/cu ft).

ADR label for transporting goods dangerouslyreactive with water

Water is miscible with many liquids, such as ethanol, in allproportions, forming a single homogeneous liquid. On the otherhand, water and most oils are immiscible, usually forming layersaccording to increasing density from the top. As a gas, water vaporis completely miscible with air.

Water forms an azeotrope with many other solvents. Water can be split by electrolysis into hydrogen and oxygen. As an oxide of hydrogen, water is formed when hydrogen or

hydrogen-containing compounds burn or react with oxygen oroxygen-containing compounds. Water is not a fuel, it is anend-product of the combustion of hydrogen. The energy required tosplit water into hydrogen and oxygen by electrolysis or any othermeans is greater than the energy that can be collected when thehydrogen and oxygen recombine.[14]

Elements which are more electropositive than hydrogen such as lithium, sodium, calcium, potassium and caesiumdisplace hydrogen from water, forming hydroxides. Being a flammable gas, the hydrogen given off is dangerousand the reaction of water with the more electropositive of these elements may be violently explosive.

Water 24

Taste and odorWater can dissolve many different substances, giving it varying tastes and odors. Humans and other animals havedeveloped senses that enable them to evaluate the potability of water by avoiding water that is too salty or putrid.The taste of spring water and mineral water, often advertised in marketing of consumer products, derives from theminerals dissolved in it. However, pure H2O is tasteless and odorless. The advertised purity of spring and mineralwater refers to absence of toxins, pollutants and microbes, not the absence of naturally occurring minerals.

Distribution in nature

In the universeMuch of the universe's water is produced as a byproduct of star formation. When stars are born, their birth isaccompanied by a strong outward wind of gas and dust. When this outflow of material eventually impacts thesurrounding gas, the shock waves that are created compress and heat the gas. The water observed is quicklyproduced in this warm dense gas.[15]

On 22 July 2011 a report described the discovery of a gigantic cloud of water vapor containing "140 trillion timesmore water than all of Earth's oceans combined" around a quasar located 12 billion light years from Earth.According to the researchers, the "discovery shows that water has been prevalent in the universe for nearly its entireexistence".[][]

Water has been detected in interstellar clouds within our galaxy, the Milky Way. Water probably exists in abundancein other galaxies, too, because its components, hydrogen and oxygen, are among the most abundant elements in theuniverse. Interstellar clouds eventually condense into solar nebulae and solar systems such as ours.Water vapor is present in Atmosphere of Mercury: 3.4%, and large amounts of water in Mercury's exosphere[]

Atmosphere of Venus: 0.002% Earth's atmosphere: ~0.40% over full atmosphere, typically 14% at surface Atmosphere of Mars: 0.03% Atmosphere of Jupiter: 0.0004% Atmosphere of Saturn in ices only Enceladus (moon of Saturn): 91% exoplanets known as HD 189733 b[16] and HD 209458 b.[17]

Liquid water is present on Earth: 71% of surface Europa: 100km deep subsurface oceanStrong evidence suggests that liquid water is present just under the surface of Saturn's moon Enceladus.Water ice is present on Earth mainly as ice sheets polar ice caps on Mars Moon Titan Europa Saturn's rings[]

Enceladus Pluto and Charon[]

Comets and comet source populations (Kuiper belt and Oort cloud objects).

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Recent evidence points to the existence of water ice at the poles of Mercury.[18] Water ice may also be present onCeres and Tethys. Water and other volatiles probably comprise much of the internal structures of Uranus andNeptune and the water in the deeper layers may be in the form of ionic water in which the molecules break downinto a soup of hydrogen and oxygen ions, and deeper down as superionic water in which the oxygen crystallises butthe hydrogen ions float around freely within the oxygen lattice.[19]

Some of the Moon's minerals contain water molecules. For instance, in 2008 a laboratory device which ejects andidentifies particles found small amounts of the compound in the inside of volcanic rock brought from Moon to Earthby the Apollo 15 crew in 1971.[20] NASA reported the detection of water molecules by NASA's Moon MineralogyMapper aboard the Indian Space Research Organization's Chandrayaan-1 spacecraft in September 2009.[21]

Water and habitable zoneThe existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence oflife on Earth as we know it. The Earth is located in the habitable zone of the solar system; if it were slightly closer toor farther from the Sun (about 5%, or about 8 million kilometers), the conditions which allow the three forms to bepresent simultaneously would be far less likely to exist.[22][23]

Earth's gravity allows it to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide atemperature buffer (greenhouse effect) which helps maintain a relatively steady surface temperature. If Earth weresmaller, a thinner atmosphere would allow temperature extremes, thus preventing the accumulation of water exceptin polar ice caps (as on Mars).The surface temperature of Earth has been relatively constant through geologic time despite varying levels ofincoming solar radiation (insolation), indicating that a dynamic process governs Earth's temperature via acombination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.The state of water on a planet depends on ambient pressure, which is determined by the planet's gravity. If a planet issufficiently massive, the water on it may be solid even at high temperatures, because of the high pressure caused bygravity, as it was observed on exoplanets Gliese 436 b[24] and GJ 1214 b.[25]

There are various theories about origin of water on Earth.

On Earth

A graphical distribution of the locations of water on Earth.

Hydrology is the study of themovement, distribution, and quality ofwater throughout the Earth. The studyof the distribution of water ishydrography. The study of thedistribution and movement ofgroundwater is hydrogeology, ofglaciers is glaciology, of inland watersis limnology and distribution of oceansis oceanography. Ecological processeswith hydrology are in focus ofecohydrology.

The collective mass of water found on,under, and over the surface of a planetis called the hydrosphere. Earth's

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Water covers 71% of the Earth's surface; theoceans contain 96.5% of the Earth's water. TheAntarctic ice sheet, which contains 61% of allfresh water on Earth, is visible at the bottom.Condensed atmospheric water can be seen as

clouds, contributing to the Earth's albedo.

approximate water volume (the total water supply of the world) is1,338,000,000km3 (321,000,000mi3).[]

Liquid water is found in bodies of water, such as an ocean, sea, lake,river, stream, canal, pond, or puddle. The majority of water on Earth issea water. Water is also present in the atmosphere in solid, liquid, andvapor states. It also exists as groundwater in aquifers.

Water is important in many geological processes. Groundwater ispresent in most rocks, and the pressure of this groundwater affectspatterns of faulting. Water in the mantle is responsible for the melt thatproduces volcanoes at subduction zones. On the surface of the Earth,water is important in both chemical and physical weathering processes.Water and, to a lesser but still significant extent, ice, are alsoresponsible for a large amount of sediment transport that occurs on thesurface of the earth. Deposition of transported sediment forms manytypes of sedimentary rocks, which make up the geologic record ofEarth history.

Water cycle

Water cycle

The water cycle (known scientificallyas the hydrologic cycle) refers to thecontinuous exchange of water withinthe hydrosphere, between theatmosphere, soil water, surface water,groundwater, and plants.

Water moves perpetually through eachof these regions in the water cycleconsisting of following transferprocesses:

evaporation from oceans and otherwater bodies into the air andtranspiration from land plants andanimals into air.

precipitation, from water vaporcondensing from the air and falling to earth or ocean.

runoff from the land usually reaching the sea.Most water vapor over the oceans returns to the oceans, but winds carry water vapor over land at the same rate asrunoff into the sea, about 47Tt per year. Over land, evaporation and transpiration contribute another 72Tt per year.Precipitation, at a rate of 119 Tt per year over land, has several forms: most commonly rain, snow, and hail, withsome contribution from fog and dew.[26] Dew is small drops of water that are condensed when a high density ofwater vapor meets a cool surface. Dew usually form in the morning when the temperature is the lowest, just beforesunrise and when the temperature of the earth's surface starts to increase.[27] Condensed water in the air may alsorefract sunlight to produce rainbows.

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Water runoff often collects over watersheds flowing into rivers. A mathematical model used to simulate river orstream flow and calculate water quality parameters is hydrological transport model. Some of water is diverted toirrigation for agriculture. Rivers and seas offer opportunity for travel and commerce. Through erosion, runoff shapesthe environment creating river valleys and deltas which provide rich soil and level ground for the establishment ofpopulation centers. A flood occurs when an area of land, usually low-lying, is covered with water. It is when a riveroverflows its banks or flood from the sea. A drought is an extended period of months or years when a region notes adeficiency in its water supply. This occurs when a region receives consistently below average precipitation.

Fresh water storage

The Bay of Fundy at high tide (left) and low tide (right)Some runoff water is trapped for periods of time, for example in lakes. At high altitude, during winter, and in the farnorth and south, snow collects in ice caps, snow pack and glaciers. Water also infiltrates the ground and goes intoaquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs andgeysers. Groundwater is also extracted artificially in wells. This water storage is important, since clean, fresh wateris essential to human and other land-based life. In many parts of the world, it is in short supply.

Sea waterSea water contains about 3.5% salt on average, plus smaller amounts of other substances. The physical properties ofsea water differ from fresh water in some important respects. It freezes at a lower temperature (about 1.9 C) and itsdensity increases with decreasing temperature to the freezing point, instead of reaching maximum density at atemperature above freezing. The salinity of water in major seas varies from about 0.7% in the Baltic Sea to 4.0% inthe Red Sea.

TidesTides are the cyclic rising and falling of local sea levels caused by the tidal forces of the Moon and the Sun acting onthe oceans. Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillatingcurrents known as tidal streams. The changing tide produced at a given location is the result of the changingpositions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the localbathymetry. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is animportant ecological product of ocean tides.

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Effects on life

Overview of photosynthesis and respiration.Water (at right), together with carbon dioxide

(CO2), form oxygen and organic compounds (atleft), which can be respired to water and (CO2).

From a biological standpoint, water has many distinct properties thatare critical for the proliferation of life that set it apart from othersubstances. It carries out this role by allowing organic compounds toreact in ways that ultimately allow replication. All known forms of lifedepend on water. Water is vital both as a solvent in which many of thebody's solutes dissolve and as an essential part of many metabolicprocesses within the body. Metabolism is the sum total of anabolismand catabolism. In anabolism, water is removed from molecules(through energy requiring enzymatic chemical reactions) in order togrow larger molecules (e.g. starches, triglycerides and proteins forstorage of fuels and information). In catabolism, water is used to breakbonds in order to generate smaller molecules (e.g. glucose, fatty acidsand amino acids to be used for fuels for energy use or other purposes).Without water, these particular metabolic processes could not exist.

Water is fundamental to photosynthesis and respiration. Photosyntheticcells use the sun's energy to split off water's hydrogen from oxygen.Hydrogen is combined with CO2 (absorbed from air or water) to formglucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun'senergy and reform water and CO2 in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H+, that is, a proton)donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH) to form water. Water isconsidered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH valuesless than 7 while bases have values greater than 7.

Aquatic life forms

Some of the biodiversity of a coralreef

Earth surface waters are filled with life. The earliest life forms appeared in water;nearly all fish live exclusively in water, and there are many types of marinemammals, such as dolphins and whales. Some kinds of animals, such asamphibians, spend portions of their lives in water and portions on land. Plantssuch as kelp and algae grow in the water and are the basis for some underwaterecosystems. Plankton is generally the foundation of the ocean food chain.

Aquatic vertebrates must obtain oxygen to survive, and they do so in variousways. Fish have gills instead of lungs, although some species of fish, such as thelungfish, have both. Marine mammals, such as dolphins, whales, otters, and sealsneed to surface periodically to breathe air. Some amphibians are able to absorboxygen through their skin. Invertebrates exhibit a wide range of modifications tosurvive in poorly oxygenated waters including breathing tubes (see insect andmollusc siphons) and gills (Carcinus). However as invertebrate life evolved in anaquatic habitat most have little or no specialisation for respiration in water.

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Some marine diatoms a key phytoplanktongroup

Effects on human civilization

Water fountain

Civilization has historically flourished around rivers and majorwaterways; Mesopotamia, the so-called cradle of civilization, wassituated between the major rivers Tigris and Euphrates; the ancientsociety of the Egyptians depended entirely upon the Nile. Largemetropolises like Rotterdam, London, Montreal, Paris, New York City,Buenos Aires, Shanghai, Tokyo, Chicago, and Hong Kong owe theirsuccess in part to their easy accessibility via water and the resultantexpansion of trade. Islands with safe water ports, like Singapore, haveflourished for the same reason. In places such as North Africa and theMiddle East, where water is more scarce, access to clean drinkingwater was and is a major factor in human development.

Health and pollution

An environmental science program - a studentfrom Iowa State University sampling water

Water fit for human consumption is called drinking water or potablewater. Water that is not potable may be made potable by filtration ordistillation, or by a range of other methods.

Water that is not fit for drinking but is not harmful for humans whenused for swimming or bathing is called by various names other thanpotable or drinking water, and is sometimes called safe water, or "safefor bathing". Chlorine is a skin and mucous membrane irritant that isused to make water safe for bathing or drinking. Its use is highlytechnical and is usually monitored by government regulations(typically 1 part per million (ppm) for drinking water, and 12 ppm ofchlorine not yet reacted with impurities for bathing water). Water forbathing may be maintained in satisfactory microbiological condition using chemical disinfectants such as chlorine orozone or by the use of ultraviolet light.

In the USA, non-potable forms of wastewater generated by humans may be referred to as greywater, which istreatable and thus easily able to be made potable again, and blackwater, which generally contains sewage and otherforms of waste which require further treatment in order to be made reusable. Greywater composes 5080% ofresidential wastewater generated by a household's sanitation equipment (sinks, showers and kitchen runoff, but nottoilets, which generate blackwater.) These terms may have different meanings in other countries and cultures.

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This natural resource is becoming scarcer in certain places, and its availability is a major social and economicconcern. Currently, about a billion people around the world routinely drink unhealthy water. Most countries acceptedthe goal of halving by 2015 the number of people worldwide who do not have access to safe water and sanitationduring the 2003 G8 Evian summit.[28] Even if this difficult goal is met, it will still leave more than an estimated halfa billion people without access to safe drinking water and over a billion without access to adequate sanitation. Poorwater quality and bad sanitation are deadly; some five million deaths a year are caused by polluted drinking water.The World Health Organization estimates that safe water could prevent 1.4 million child deaths from diarrhea eachyear.[29] Water, however, is not a finite resource, but rather re-circulated as potable water in precipitation inquantities many degrees of magnitude higher than human consumption. Therefore, it is the relatively small quantityof water in reserve in the earth (about 1% of our drinking water supply, which is replenished in aquifers aroundevery 1 to 10 years), that is a non-renewable resource, and it is, rather, the distribution of potable and irrigation waterwhich is scarce, rather than the actual amount of it that exists on the earth. Water-poor countries use importation ofgoods as the primary method of importing water (to leave enough for local human consumption), since themanufacturing process uses around 10 to 100 times products' masses in water.In the developing world, 90% of all wastewater still goes untreated into local rivers and streams.[30] Some 50countries, with roughly a third of the world's population, also suffer from medium or high water stress, and 17 ofthese extract more water annually than is recharged through their natural water cycles.[31] The strain not only affectssurfac