端木·宇 2008-6-19 22:23
The Chemistry of Life
Of the 92 naturally existing elements on the Earth, only 25 play arole in the chemical processes of life. Of these 25, four elementsconstitute more than 98 percent of all biological matter: carbon (C), oxygen (O), hydrogen (H), and nitrogen (N).Virtually every important organic compound is made up of these fourelements. The Big 4 of organic elements can be cut down even further toa Supreme 1: carbon is the most important biological molecule, both forlife as we know it and on the SAT II.
[b] Carbon[/b]
Carbon is the central element of life. Itsimportant role stems from its ability to form four chemical bonds withother elements at the same time:
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Carbons often attach to other carbon atoms,forming long chains called hydrocarbons. These molecules get their namebecause the central carbons also bond to hydrogen:
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In addition to making a connection to fourother atoms, carbon also has the ability to make two or three separateconnections with the same single partner (and make its remaining one ortwo bonds with other substances). These bonds, which are stronger thansingle bonds, are known as double or triple bonds, respectively.
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[b] Monomers and Polymers[/b]
Many biological molecules consist of basic unitsthat are strung together to form long chains, much like beads areplaced on a string to make a necklace. There can be some variation inthese basic units, which are known as monomers. Two monomers connectedto each other are known as a dimer; a chain of monomers is called apolymer.
Polymers can be formed by many different typesof chemical reactions. One special reaction, however, is particularlyimportant in producing the polymers found in the chemistry of life.This reaction involves a carbon that has a hydrogen atom attached and acarbon that has an OH– groupattached. When the carbons bond to each other, they release a watermolecule formed from the oxygen atom and the two hydrogen atoms.
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Because a water molecule is created in order to join the two monomers, this reaction is known as [b]dehydration synthesis[/b]. The reverse of dehydration synthesis, when a water molecule is inserted into a polymer to break off a monomer, is called [b]hydrolysis[/b].
