The Chemistry of Biology: Organic Chemistry

Organic Chemistry

Carbon is an especially noteworthy element in living systems. The chemistry of carbon, organic chemistry, is a complete study unto itself. We will touch on several highlights that will be useful in succeeding sections.

The Uniqueness of Carbon

The carbon atom has four valence (outermost) electrons. Because of this unique configuration, it is easier for the carbon atom to share its four electrons with another atom or atoms than to lose or gain four electrons. Because each carbon is identical, they all have four valence electrons, so they can easily bond with other carbon atoms to form long chains or rings. In fact, a carbon atom can bond with another carbon atom two or three times to make double and triple covalent bonds between two carbon atoms. Long chains of carbon atoms with double and triple bonds are quite common in biology.

Carbon's tendency toward covalent bonding with itself generates three unique characteristics that create a vast array of compounds, including those necessary to construct and support life:


Carbon to carbon (C-C) bonds form the backbone of all biomolecules and can include thousands of C-C bonds.

  • The single bond that connects carbon atoms to carbon atoms is quite strong, so the subsequent long chains and ring structures are not fragile.
  • The carbon-carbon covalent bonding pattern satisfies the Octet rule, making carbon compounds unwilling to react.
  • Because carbon has four valence electrons and needs eight to satisfy the Octet rule, it can bond with up to four additional atoms, creating countless compound possibilities.

Functional Groups

In the simplest terms, the reactive part of any compound is called the functional group. Normally a functional group is a collection of atoms that operates as one reactive unit and is also the part of the molecule involved in a chemical reaction. Whereas carbon-to-carbon bonds are nonreactive, the instability of the functional groups drives chemical reactions that involve stable carbon-based compounds. For simplicity and reference, three functional groups are presented:

  • Amine is identified by a central nitrogen atom that has three bonds, usually to hydrogen atoms. Amine groups form the basis for amino acids, which when bonded together form proteins.
  • A Carboxylic group (COOH) is attached to the long carbon chains that form fatty acid molecules, which are a type of lipid lipids.
  • Hydroxyl groups (OH) are very reactive. They are a component of alcohols, such as ethanol, the alcoholic component of adult beverages. The oxygen-hydrogen association is unique to this functional group and easily identifies it as an alcohol.

Dehydration Synthesis and Polymer Formation


A hydration reaction is the reverse of a dehydration reaction in that water serves as a reactant to split apart large poly-molecules.

Polymers are small molecules that can be bonded together to create larger molecules. Complex carbohydrates are made from small simple sugars joined together, and giant protein molecules are simply a series of smaller amino acid molecules bonded together. The prefix poly identifies this type of molecular addition. For instance, polysaccharides are large carbohydrates composed of multiple saccharide (sugar) units.

The chemical reaction that powers polymer formation is known by several names, including dehydration synthesis and condensation reaction. Regardless of the name, the molecules are joined by bonding sites created when a positively charged hydrogen ion (H+) is lost from one molecule and a negatively charged hydroxide (OH-) ion is lost from a neighboring molecule. The H+ and OH- combine to form water. So a dehydration synthesis joins two smaller units together with the loss of one water molecule.

Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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