Although carbohydrates account for only about 1 percent of the body weight, they play a central role in the chemical reactions that provide cells with energy. Carbohydrates are composed of carbon, hydrogen, and oxygen atoms in the proportions represented by the general formula Cn(H2O)n, where n is any whole number. It is from this formula that the class of molecules gets its name, carbohydrate—water-containing (hydrated) carbon atoms. Linked to most of the carbon atoms in a carbohydrate are a hydrogen atom and a hydroxyl group:

The presence of numerous hydroxyl groups makes carbohydrates readily soluble in water.

Most carbohydrates taste sweet, and it is among the carbohydrates that we find the substances known as sugars. The simplest sugars are the monosaccha-rides (single-sweet), the most abundant of which is glucose, a six-carbon molecule (C6H12O6) often called "blood sugar" because it is the major monosaccharide found in the blood.

There are two ways of representing the linkage between the atoms of a monosaccharide, as illustrated in Figure 2-7. The first is the conventional way of drawing the structure of organic molecules, but the second gives a better representation of their three-dimensional shape. Five carbon atoms and an oxygen atom form a ring that lies in an essentially flat plane. The hydrogen and hydroxyl groups on each carbon lie above and below the plane of this ring. If one of the hydroxyl groups below the ring is shifted to a position above the ring, as shown in Figure 2-8, a different monosaccharide is produced.

Most monosaccharides in the body contain five or six carbon atoms and are called pentoses and hexoses, respectively. Larger carbohydrates can be formed by

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

TABLE 2-5 Major Categories of Organic Molecules in the Body


Percent of Body Weight

Majority of Atoms










C, H

Triacylglycerols Phospholipids


3 fatty acids + glycerol 2 fatty acids + glycerol + phosphate + small charged nitrogen molecule



C, H, O, N

Peptides Proteins

Amino acids Amino acids

Nucleic acids


C, H, O, N


Nucleotides containing the bases adenine, cytosine, guanine, thymine, the sugar deoxyribose, and phosphate Nucleotides containing the bases adenine, cytosine, guanine, uracil, the sugar ribose, and phosphate

linking a number of monosaccharides together. Carbohydrates composed of two monosaccharides are known as disaccharides. Sucrose, or table sugar (Figure 2-9), is composed of two monosaccharides, glucose and fructose. The linking together of most mono-saccharides involves the removal of a hydroxyl group from one monosaccharide and a hydrogen atom from the other, giving rise to a molecule of water and linking the two sugars together through an oxygen atom. Conversely, hydrolysis of the disaccharide breaks this linkage by adding back the water and thus uncoupling the two monosaccharides. Additional disaccharides frequently encountered are maltose (glucose-glucose), formed during the digestion of large carbohydrates in the intestinal tract, and lactose (glucose-galactose), present in milk.

When many monosaccharides are linked together to form polymers, the molecules are known as polysaccharides. Starch, found in plant cells, and glycogen (Figure 2-10), present in animal cells and often called "animal starch," are examples of polysaccharides. Both of these polysaccharides are composed of thousands of glucose molecules linked together in long chains, differing only in the degree of branching along the





Essentials of Human Physiology

Essentials of Human Physiology

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