Of the 92 naturally existing elements on the Earth, only 25 play a role in the chemical processes of life. Of these 25, four elements constitute 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 four elements. The Big 4 of organic elements can be cut down even further to a Supreme 1: carbon is the most important biological molecule, both for life as we know it and on the SAT II.
Carbon
Carbon is the central element of life. Its important role stems from its ability to form four chemical bonds with other elements at the same time:
Carbons often attach to other carbon atoms, forming long chains called hydrocarbons. These molecules get their name because the central carbons also bond to hydrogen:
In addition to making a connection to four other atoms, carbon also has the ability to make two or three separate connections with the same single partner (and make its remaining one or two bonds with other substances). These bonds, which are stronger than single bonds, are known as double or triple bonds, respectively.
Monomers and Polymers
Many biological molecules consist of basic units that are strung together to form long chains, much like beads are placed on a string to make a necklace. There can be some variation in these basic units, which are known as monomers. Two monomers connected to each other are known as a dimer; a chain of monomers is called a polymer.
Polymers can be formed by many different types of chemical reactions. One special reaction, however, is particularly important in producing the polymers found in the chemistry of life. This reaction involves a carbon that has a hydrogen atom attached and a carbon that has an OH– group attached. When the carbons bond to each other, they release a water molecule formed from the oxygen atom and the two hydrogen atoms.
Because a water molecule is created in order to join the two monomers, this reaction is known as dehydration synthesis. The reverse of dehydration synthesis, when a water molecule is inserted into a polymer to break off a monomer, is called hydrolysis.
