(a) Molecules of the monosaccharide glucose (C6H12O6) may have a straight chain of carbon atoms. (b) More commonly, glucose molecules form a ring structure. (c) This shape symbolizes the ring structure of a glucose molecule.

(a) Monosaccharide (b) Disaccharide

(c) Polysaccharide Figure 2.11

(a) A monosaccharide molecule consisting of one 6-carbon building block. (b) A disaccharide molecule consisting of two of these building blocks. (c) A polysaccharide molecule consisting of many building blocks, which may form branches.

Like carbohydrates, fat molecules are composed of carbon, hydrogen, and oxygen atoms. However, fats have a much smaller proportion of oxygen than do carbohydrates. The formula for the fat tristearin, C57H11oO0, illustrates these characteristic proportions.

The building blocks of fat molecules are fatty acids and glycerol. Although the glycerol portion of every fat molecule is the same, there are many kinds of fatty acids and, therefore, many kinds of fats. All fatty acid molecules include a carboxyl group (—COOH) at the end of a chain of carbon atoms. Fatty acids differ in the lengths of their carbon atom chains, although such chains usually contain an even number of carbon atoms. The fatty acid chains also may vary in the ways the carbon atoms join. In some cases, the carbon atoms are all linked by single carbon-carbon bonds. This type of fatty acid is saturated; that is, each carbon atom binds as many hydrogen atoms as possible and is thus saturated with hydrogen atoms. Other fatty acid chains do not bind their maximum number of hydrogen atoms. Therefore, they have one or more double bonds between carbon atoms. Fatty acids with one double bond are called monounsaturated, and those with two or more double bonds are polyunsaturated (fig. 2.12).

Fatty acids and glycerol are united so that each glycerol molecule combines with three fatty acid molecules. The result is a single fat molecule or triglyceride (fig. 2.13). Fat molecules that contain only saturated fatty acids are called saturated fats, and those that include unsaturated fatty acids are called unsaturated fats. Each kind of fat molecule has distinct properties.

A phospholipid molecule is similar to a fat molecule in that it contains a glycerol portion and fatty acid chains. The phospholipid, however, has only two fatty acid chains, and in place of the third, has a portion containing a phosphate group. This phosphate-containing portion is soluble in water (hydrophilic) and forms the "head" of the molecule, whereas the fatty acid portion is insoluble in water (hydrophobic) and forms a "tail." Figure 2.14 illustrates the molecular structure of cephalin, a phospho-lipid in blood. Other phospholipids are important in cellular structures.

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Essentials of Human Physiology

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  • swen
    Is the glycerol portion of every fat molecule the same?
    8 years ago

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