Lipids are organic compounds that contain the same elements as carbohydrates: carbon, hydrogen, and oxygen. However, the hydrogen-to-oxygen ratio is always greater than 2:1. More important for biological systems, the carbon-to-hydrogen bonds are nonpolar covalent, which means that lipids are fat soluble and will not dissolve in water. There are four biologically important lipids:
Fats are large molecules that are composed of three fatty acid molecules bonded to a glycerol molecule. The fatty acid molecule is a long chain of covalently bonded carbon atoms with nonpolar bonds to hydrogen atoms all along the carbon chain with a carboxyl group attached to one end. Because the carbon-hydrogen bonds are nonpolar, the chain is hydrophobic, meaning they are not water soluble. Glycerol is a three-carbon-chain compound that bonds with the fatty acids to create a fat. Typically, each carbon in the glycerol molecule bonds via dehydration synthesis to the first carbon atom from a fatty acid molecule so that the resulting fat molecule appears to have a glycerol head with three fatty acid chains streaming from it. This resulting molecule is called a triglyceride. Because carbon-hydrogen bonds are considered energy rich, fats store a lot of energy per unit. In fact, a gram of fat stores more than twice as much energy as a gram of a polysaccharide such as starch. Fats are lipids that are used by living organisms for stored energy.
A saturated fatty acid has hydrogen atoms bonded to all available carbon atoms. An unsaturated fatty acid has one or more carbon atoms double-bonded to the neighboring carbon atom so that fewer hydrogen atoms are needed to create a stable electron cloud. With fewer hydrogen atoms attached, the molecule is considered unsaturated with hydrogen atoms. So saturated fatty acids have more hydrogen atoms attached than unsaturated fatty acid chains. Through their metabolism, plants generally produce triglycerides that contain unsaturated fatty acids such as peanut oil or olive oil, whereas animals generally produce triglycerides that contain saturated fatty acids which humans sometimes convert into butter and lard.
Waxes are similar to fats except that waxes are composed of only one long-chain fatty acid bonded to a long-chain alcohol group attached. Because of their long, nonpolar carbon chains, waxes are extremely hydrophobic (meaning they lack an affinity for water). Both plants and animals use this waterproofing characteristic as part of their composition. Plants most noticeably use waxes for a thin protective covering of stems and leaves to prevent water loss. Similarly, animals employ waxes for protective purposes; for instance, earwax in humans prevents foreign material from entering and possibly injuring the ear canal area.
Phospholipids are similar to fats except they have two fatty acid chains bonded to a glycerol plus they contain the element phosphorus. Phospholipids are unique because they have a hydrophobic and a hydrophilic (water-soluble) end. Phospholipids are biologically important because they are the main structural components of cell membranes. The cell membrane is called a phospholipid bilayer because it consists of two phospholipid layers oriented so that the hydrophyllic “head” of both molecules face the exterior and the hydrophobic “tails” of both molecules create the interior of the membrane. Therefore, water and other cellular fluids are contained. The hydrophobic ends for both molecules face each other on the inside and allow for passage of acceptable, and some objectionable, materials through the cell membrane.
Steroids are structurally different from the other lipids. The carbon skeleton of steroids is bent to form four fused rings that do not contain fatty acids. The most common steroid, cholesterol, is needed to make both the male (testosterone) and female (estrogen) sex hormones, and it is a component of cell membranes and is needed for the proper function of nerve cells. Excessive amounts of cholesterol, however, have been linked to heart disease. Another popular steroid group is the anabolic steroids that are man-made and mimic the effect of the male hormone, testosterone. Originally intended as a treatment for anemia and certain diseases that destroy muscle, athletes have recently been using them to increase muscle mass, stamina, and strength—which they will do. However, the performance-enhancement drugs come with a price. The anabolic steroids are linked to increased cholesterol levels, mood swings, reduced sex drive, possible infertility, and possible connections between liver damage and the resulting liver cancer. Certain beneficial fat-soluble hormones, such as cortisol, are also familiar steroids.
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.