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Unicellular Organisms


Fatty Acids Fat Lipids are a heterogeneous collection of compounds that share only one property: they are easily dissolved in organic solvents but can only hardly or not at all be dissolved in water. They include fats and oils, phospholipids, steroids, glycolipids, and waxes. The basic units for fat are fatty acids either saturated (in solid

Figure 11-09 Fatty Acids
[view large image]

Figure 11-10 Fat [view large image]

form) or unsaturated (in liquid form, the good one to prevent the deposits of cholesterol and fat on the lining of blood vessels; unsaturated compounds can undergo
addition reactions with various reagents that cause the double or triple bonds to be replaced with single bonds). Each fatty acid has a long chain of carbon atoms with hydrogens attached, and it ends in an acid group (COOH) as shown in Figure 11-09. A fat (or an oil and sometimes also called a triglyceride) is formed when one molecule of glycerol reacts with 3 fatty acids as shown in Figure 11-10. Glycerol is a compound with 3 hydrates of carbon. A fat is nonpolar, i.e., the molecule has no groups that can be ionized and become charged. It is the long-term energy source. Since it contains more C-H bonds and less oxygen than carbohydrates, lipids can store twice as much energy. This is why all animals (and some plants) use them for energy storage and respited after supplies of carbohydrates are exhausted.


Phospholipids, as their name implies, contain a phosphate group PO4-. Essentially, phospholipids are constructed as fats are, except that in place of the third fatty acid, there is a phosphate group or a grouping that contains both phosphate and nitrogen (Figure 11-11). These molecules are not electrically neutral as are the fats because the phosphate group can be ionized.

Figure 11-11 Phospholipids [view large image]

Therefore, the phospholipids have a nonpolar region that is not soluble in water and a polar region that is soluble. Most of the lipids in the cell membrane are phospholipids. Each phospholipid molecule has a polar head and 2 nonpolar tails. When surrounded by water, phospholipid molecules form a bilayer naturally. The heads, being polar, are attracted to the water (hydrophillic), which is also polar; therefore, the heads face outward. The nonpolar tails face inside, away from the water (hydrophobic). Some of the lipids in the cell membrane are glycolipids. Glycolipids are constructed similarly to phospholipids except the polar head consists of a chain of sugar molecules. Glycolipids only occur in the outer half of the bilayer, where they function in cell-to-cell recognition. Different types of cells have different glycolipids.

Steroids Steroids are lipids that have entirely different structures than fats (see Figure 11-12). Molecules such as hormones, vitamin D, bile acids, and cholesterol are examples of steroids in the body. Steroids are found in plant and animal food sources; however, cholesterol is derived only from animal sources. Hormones are used to regulate chemical in body, vitamin D is important for bone and teeth formation, bile acids is digestive fluid for the absorption of fats, and cholesterol is important to the body as a constituent of cell membranes, and is involved in the formation of bile acid and some hormones. Cholesterol is associated with heart and blood vessel diseases because it collects on the inside of vessel walls and restricts blood flow.

Figure 11-12 Steroids

Waxes are found in many plants and animals. Coatings of carnauba wax on fruits and the leaves and stems of plants help to prevent loss of water and damage from pests. Waxes on the skin, fur, and feathers of animals and birds provide a water-proof coating. Properties of some waxes are listed in Table 11-01 below.

Table 11-01 Properties of Waxes

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