Biology: Semester I

Sections:

Introduction | Section 1 | Section 2

Section Two:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6

Biology: Chemistry of Life: Part Five 2. Lipids: Among the many other jobs they do in living things, lipids assist in structure, long-term energy storage, and control of cellular processes. The lipid group includes fats, phospholipids, steroids, and waxes. Lipids are an important part of the structure and function of the cell membrane. Fats are lipids that store energy. You have most likely heard of the term “sweet tooth.” Some scientists say that we are alive because of our ancestors’ “fat tooth.” Early humans probably encountered sugar only in fruits, rather than the refined sugars we have today. Fats were plentiful though. The fats were lightweight and contained a lot of energy. Bone marrow is a wonderful source of such fats. When our ancestors could get this rich energy source, it alone provided enough energy to keep them going a day or two. In a modern society that provides plenty of fats, a “fat tooth” is not such a good thing! Most of us can satisfy our need for fat with milk, cheese, ice cream, nuts, and fatty meats such as salami and bacon. High-fat diets have been linked to increased risks of cancer, obesity, diabetes, and heart disease. Our once essential “fat tooth” clearly needs to be pulled, or at least capped! Lipids are involved mainly with long-term energy storage. They are generally insoluble in water (you may have heard the phrase “mixing like oil and water”). Lipids also have important structural functions (phospholipids are the major building block in cell membranes) and act as "messengers" (hormones) that assist communication within and between cells. Lipids are composed of three fatty acids (usually) covalently bonded to a 3-carbon glycerol. The fatty acids are composed of several CH2 units and are hydrophobic (not water soluble). Fatty acids can be saturated meaning they have as many hydrogen atoms bonded to their carbon atoms as possible. Fatty acids also can be unsaturated, with one or more double bonds connecting their carbon atoms. This results in fewer hydrogen atoms. A fat is solid at room temperature, but an oil is a liquid under the same conditions. The fatty acids in fats are mostly saturated, whereas those in oils are mostly unsaturated. Fats and oils are used for energy storage. Animals convert excess sugars (beyond their glycogen storage capacities) into fats. Most plants store excess sugars as starch, though some seeds and fruits (such as corn, peanut, palm, canola, and sunflower) do store energy as oils. Fats store six times more energy than glycogen does. Dieting is the attempt to reduce the amount of fats present in specialized cells known as adipose cells. These cells accumulate in certain areas of the human body. By restricting the intakes of carbohydrates and fats, the body is forced to draw on its own stores to make up the energy debt. The body responds by lowering its metabolic rate, which often causes a drop of perceived "energy level." Successful dieting usually involves three things: exercise, decreasing the amounts of carbohydrates and fats consumed, and behavior changes. Fats also serve as insulators and cushions. Subdermal (below the skin) fat also plays a role in insulation. The human body naturally accumulates some fats in the "posterior" area. Phospholipidsand glycolipids give structure to cell membranes. Phospholipids are modified so that a phosphate group (PO4-) is added to one of the fatty acids. The addition of this group makes a polar "head" and two nonpolar "tails." Waxes are an important structural component for many organisms. A waxy layer, called the cuticle, covers the leaves and stems of many land plants. Other waxes serve as protective coverings on skin and fur of animals. Cholesterol and steroids are a very distinctive group of lipids. Though these terms often are associated with negative news stories, cholesterol and steroids have many biological uses. For example, cholesterol is a part of cell membranes. Cholesterol also forms a covering around some neurons. Excess cholesterol in the blood has been linked to atherosclerosis, a hardening of the arteries and raised blood pressure that leads to increased risk of heart attack and stroke. Cholesterol plaques in the arteries increase blood pressure in much the same way that plumbing blockages cause burst pipes in old houses. Some athletes, seeking a competitive advantage, have misused steroids as performance-enhancing drugs (anabolic steroids). However, not all steroids are bad. For example, cortisone is a steroid that has many therapeutic uses. It can relieve inflammation in joints and ease severe allergy problems. Testosterone is the male sex hormone produced in the male testes that targets a number of cells throughout the male body. All steroids share a common steroid ring structure but differ in the molecules attached to those rings. Back Next

© 2007 Aventa Learning. All rights reserved.