A lipid is a molecule that is at least partially composed of a chain of hydrocarbons--carbons in association with hydrogen molecules. Hydrocarbon chains tend to exclude water, because there are no vacant sites along the chain for the water to bind to, and are thus described as being hydrophobic (from Latin meaning, "water hating"). Lipids are important in biological systems for a number of reasons, including the synthesis of important compounds and their use in membranes.
In biological systems, there are four classes of lipids. One class consists of fatty acids, compounds that contain a hydrogen chain that ends in a carboxylate group (COOH). Fatty acids can be saturated (there are no double bonds linking the carbons in the chain) or unsaturated (there are one or more double bonds present). Unsaturated fatty acids, such as those that make up margarine, tend to be softer at lower temperatures than saturated fatty acids (contrast the consistency of margarine and butter at room temperature). Some fatty acids are essential, that is, they cannot be made by the body and must be supplied in the diet. Linoleic and linolenic acids are two examples. Fatty acids also make up biological molecules such as lipopolysaccharide of bacteria and the phospholipids that form biological membranes
Another group of lipids is called glycerides. These are fatty acids that contain a phosphate group. The phosphate group is charged, and so ends to associate with water (hydrophilic, from Latin meaning, "water loving"). The presence of hydrophilic and hydrophobic regions on the same molecule allows phosholipids to arrange themselves in so-called bilayers, with the phosphate portions oriented outward toward the water and the fatty acid tails buried inside away from the water. This allows membranes to form. The presence of proteins in the phospholipid bilayer allows membranes in different organs or areas of the body to function in different ways.
A third group of lipids is the so-called complex lipids, that consist of a lipid portion and another portion made of protein or other constituent. Lipoprotein and cholesterol are two complex lipids. There are several different kinds of lipoproteins, having different physiological functions, and which are beneficial or detrimental to health. The fourth group of lipids is called nonglycerides. This group includes steroids, which can function in keeping membranes pliable (cholesterol, which can also be a health hazard if it accumulates and clogs arteries) and stimulating changes in females (progesterone) and males (testosterone). Another nonglyceride is cortisone, which is useful in the treatment of various maladies including rheumatoid arthritis and asthma.
Lipids such as fatty acids are an important source of energy for humans. They are stored in fat droplets in specialized storage cells called adipose cells. If needed, the lipids are broken down into simpler compounds, converted to small spherical structures called chylomicrons, and routed to the bloodstream. There, the lipids can be carried throughout the body for use as fuel. Conversion of the chylomicrons to energy takes place in a specialized compartment of cells known as the mitochondria.
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