Chemical reactions are important to all levels of biology. In the simplest terms, a reaction requires reactants and products. Reactants are the atoms or molecules that are involved with the change, and products are the resulting changed atoms or molecules. In most biological reactions, enzymes act as catalysts to increase the rate of a reaction. A chemical reaction occurs when reactants are joined together to create a product that has different chemical properties than the original reactants. This always involves an energy change and a change in the electron configuration around the original atoms. When electrons redistribute their orbitals to include two or more atomic nuclei, as is the case in a covalent bond, or donate or accept electrons, as is the case in an ionic bond, a chemical reaction has occurred. Two general types of bonds form during chemical reactions: ionic and covalent.
Ionic bonds form when the outermost, or valence, electrons of an atom are donated or received in association with a second atom. Because the electrons are now orbiting around the receiving atom and not their original atom, the receiving atom now has an imbalance between the number of protons and electrons and becomes a negatively charged ion. The donating atom also has a proton-electron imbalance and becomes a positively charged ion because it lost a negatively charged electron and the number of its protons remained the same. The resulting molecule has properties different from the original atoms. It is important to remember that because of the unequal electron distribution around the reacting atoms, the resulting ionic compounds have partial charges. This importance is developed in greater detail in Specialized Cell Structure and Function, but it explains the fact that water can dissolve any substance that has a partial charge on it. A typical example for an ionic bond is the joining of a sodium atom, which donates an electron, to a chlorine atom, which accepts the electron, to form sodium chloride, also known as table salt.
Covalent bonds occur when two or more atoms share their electrons. The electrons are not donated/accepted; instead, they incorporate their orbitals to create an electron cloud around all participating atoms. When the electrons are shared evenly around all reacting nuclei, there is no partial charge on the resulting molecule, as is the case when carbon covalently bonds with itself. However, in some cases, the electrons are not shared evenly and partial charges occur, as in the case of polar covalent bonds.
In reality, many bonds are actually a hybridization of ionic and covalent and have characteristics of both types. Atoms with polar covalent bonds share their electrons (covalent characteristic) unevenly (ionic characteristic), giving a slight positive (+) charge to one end of the molecule and a slight negative (-) charge to the other end. Water is a polar covalent molecule because the electrons spend more of their time around the oxygen atom because the oxygen atom has more protons acting as electron-magnets. Because of this uneven sharing of electrons, the oxygen end of the molecule has a slight negative charge, and the hydrogen end has a partial positive charge because the electrons are spending more time orbiting around the oxygen atom. The overall molecule has a partial positive and a partial negative end. As a result, water molecules tend to align themselves so that the positive end of one molecule aligns with the negative end of another molecule (opposites attract).
Notice also in the ionic model that the electrons are drawn away from the sending atom and accepted by the receiving atom. The covalent model shows the electrons being shared equally around all of the atoms, whereas the polar covalent shows the unequal sharing of the electrons.
In all cases, the driving force for any chemical reaction is a move toward greater stability of the atoms. To increase stability, atoms tend to react so that they lower their energy and increase their entropy (randomness or lack of organization). In chemical terms, this means that they seek to have a stable number of electrons in their outermost orbital. The stable number means that the outermost energy level is either completely full or completely empty. Chemists call this the v because often eight valence electrons are required to reach stability. Atoms react to achieve this electron configuration by donating/accepting electrons (ionic) or sharing them (covalent). Biomolecules are considered organic because they contain the element carbon and are covalently bonded.
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.