Water is one of the most unique molecules known to man and also one of the most important to biological systems. Not only does water exist in nature in all three states of matter (solid, liquid, gas), it also covers 75 percent of the earth and composes roughly 78 percent of the human body.
The uniqueness of water comes from its molecular structure. Because it is a polar covalent molecule, it has a slight positive and slight negative charge on opposite ends. Examine the illustration Water molecule and note two important characteristics. First, notice the location of the slight positive and negative ends. Second, observe that water is a bent molecule, not linear or straight.
Because water is a bent, partially polar molecule, it possesses the following biologically important characteristics of what is formed by the joining of many water molecules—all of them are critical to the creation and support of life on Earth:
Polarity simply means that the molecule has both a positively and negatively charged end. More important, the polarity of water is responsible for effectively dissolving other polar molecules, such as sugars and ionic compounds such as salt. Ionic compounds dissolve in water to form ions. This is important to remember because for most biological reactions to occur, the reactants must be dissolved in water. Because water is able to dissolve so many common substances, it is known as the universal solvent. Substances that cannot be dissolved by water (such as oils) are called fat soluble and are nonpolar, nonionic compounds that are strongly covalently bonded. Insoluble substances make excellent containers of water, such as cell membranes and cell walls.
When water molecules align with each other, a weak bond is established between the negatively charged oxygen atom of one water molecule and the positively charged hydrogen atoms of a neighboring water molecule. The weak bond that often forms between hydrogen atoms and neighboring atoms is the hydrogen bond. Hydrogen bonds are very common in living organisms; for example, hydrogen bonds form between the bases of DNA to help hold the DNA chain together. Hydrogen bonds give water molecules two additional characteristics: cohesion and surface tension.
Because of the extensive hydrogen bonding in water, the molecules tend to stick to each other in a regular pattern. This phenomenon, called cohesion, is easily observed as you carefully overfill a glass with water and observe the water molecules holding together above the rim until gravity overtakes the hydrogen bonding and the water molecules spill down the side of the glass. Likewise, the cohesive property of water allows tall trees to bring water to their highest leaves from sources below ground.
To avoid the pain of a belly-flop, divers enter the water with their hands or feet first to break the surface tension of water.
A special type of cohesion is surface tension. The tension on the surface of water occurs when water molecules on the outside of the system align and are held together by hydrogen bonding to create an effect similar to a net made of atoms. For example, the surface tension of water allows water spiders to literally walk on water.
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.