What are Lipids ? How to Classify?
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. Lipids are composed of three fatty acids (usually) covalently bonded to a 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 hydrogens bonded to their carbons as possible) or unsaturated (with one or more double bonds connecting their carbons, hence fewer hydrogens). A fat is solid at room temperature, while an oil is a liquid under the same conditions. The fatty acids in oils are mostly unsaturated, while those in fats are mostly saturated.
|Figure 1. Saturated (top and middle) and unsaturated (bottom) fatty acids. The term saturated refers to the “saturation” of the molecule by hydrogen atoms. The presence of a double C=C covalent bond reduces the number of hydrogens that can bond to the carbon chain, hence the application of term “unsaturated”.|
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 makeup 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 behaviour modification.
Another use of fats is as insulators and cushions. The human body naturally accumulates some fats in the “posterior” area. Sub dermal (“under the skin”) fat plays a role in insulation.
Lipids – Phospholipids :
Phospholipids and glycolipids 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.
|Figure 2. Structure of a phospholipid, space-filling model (left) and chain model (right|
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.
|Figure 3. Structure of four steroids.|