Lipids are involved mainly with long-term energy storage. They are generally insoluble in polar substances such as water. Secondary functions of lipids include structural components (as in the case of phospholipids that are the major building block in cell membranes) and “messengers” (hormones) that play roles in communications within and between cells.
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Lipids Definition: Lipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, and others.
What are Lipids?
Lipids are composed of three fatty acids (usually) covalently bonded to 3-carbon glycerol. The fatty acids are composed of CH2 units and are hydrophobic/not water soluble. Some examples of fatty acids are shown in Figure 1.
Fatty acids can be saturated (meaning they have as many hydrogen bonded to their carbons as possible) or unsaturated (with one or more double bonds connecting their carbons, hence fewer hydrogen).
A fat is solid at room temperature while oil is a liquid under the same conditions. The fatty acids in oils are mostly unsaturated while those in fats are mostly saturated.
Fats and oils function in long-term energy storage. Animals convert excess sugars (beyond their glycogen storage capacities) into fats. Most plants store excess sugars as starch, although some seeds and fruits have energy stored as oils (e.g. corn oil, peanut oil, palm oil, canola oil, and sunflower oil). Fats yield 9.3 Kcal/gm, while carbohydrates yield 3.79 Kcal/gm. Fats thus store six times as much energy as glycogen.
Diets are attempts to reduce the amount of fats present in specialized cells known as adipose cells that 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 to this by lowering its metabolic rate, often resulting in a drop of “energy level.” Successful diets usually involve three things: decreasing the amounts of carbohydrates and fats; exercise; and behavior modification.
Another use of fats is as insulators and cushions. The human body naturally accumulates some fats in the “posterior” area. Subdermal (“under the skin”) fat plays a role in insulation.
Phospholipids and Glycolipids
These are important structural components of cell membranes. Phospholipids, shown in Figure 2, 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, such as the cuticle, a waxy layer covering the leaves and stems of many land plants; and protective coverings on skin and fur of animals.
Cholesterol and steroids
Most mention of these two types of lipids in the news is usually negative. Cholesterol, illustrated in Figure 3, has many biological uses, it occurs in cell membranes, and its forms the sheath of some types of nerve cells. However, excess cholesterol in the blood has been linked to atherosclerosis, hardening of the arteries.
Recent studies suggest a link between arterial plaque deposits of cholesterol, antibodies to the pneumonia-causing form of Chlamydia, and heart attacks. The plaque increases blood pressure, much the way blockages in plumbing cause burst pipes in old houses.
Biological Role of Lipids (or) Function of Lipids
The main function of lipids are
- Food material: Lipids provide food, highly rich in calorific value. One gram lipid produces 9.3 kilocalories of heat.
- Food reserve: Lipids provide are insoluble in aqueous solutions and hence can be stored readily in the body as a food reserve.
- Structural component: Lipids are an important constituent of the cell membrane.
- Heat insulation: The fats are characterized for their high insulating capacity. Great quantities of fat are deposited in the subcutaneous layers in aquatic mammals such as whale and in animals living in cold climates.
- Fatty acid absorption: Phospholipids play an important role in the absorption and transportation of fatty acids.
- Hormone synthesis: The sex hormones, adrenocorticoids, cholic acids and also vitamin D are all synthesized from cholesterol, a steroidal lipid.
- Vitamin carriers: Lipids act as carriers of natural fat-soluble vitamins such as vitamin A, D, and E.
- Blood cholesterol lowering: Chocolates and beef, especially the latter one, were believed to cause many heart diseases as they are rich in saturated fatty acids, which boost cholesterol levels in blood and clog the arterial passage. But researches conducted at the University of Texas by Scott Grundy and Andrea Bonanomi (1988) suggest that at least one saturated fatty acid, stearic acid, a major component of cocoa butter and beef fat, does not raise blood cholesterol level at all. The researchers placed 11 men on three cholesterol poor liquid diets for three weeks each in random order. One formula was rich in palmitic acid, a known cholesterol booster; the second in oleic acid; and the third in stearic acid. When compared with the diet rich in palmitic acid, blood cholesterol levels were 14% lower in subjects put on the stearic acid diet and 10% lower in those on the oleic acid diet.
- Antibiotic agent. Squalamine, the steroid from the blood of sharks, has been shown to be an antibiotic and antifungal agent of intense activity. This seems to explain why sharks rarely contract infections and almost never get cancer.