Anabolism provides all the substances required for cellular growth and repair. For example, an anabolic process called dehydration synthesis (de"hi-dra'shun sin'the-sis) joins many simple sugar molecules (monosaccharides) to form larger molecules of glycogen. When a runner consumes pasta the night before a race, digestion breaks down the complex carbohydrates to mono-saccharides, which can be absorbed into the bloodstream, which carries these energy-rich molecules to body cells. Here, dehydration synthesis joins the monosaccharides to form glycogen, which stores energy that the runner may not need until later, as the finish line nears. When monosaccharide units join, an —OH (hydroxyl group) from one monosaccharide molecule and an —H (hydrogen atom) from an —OH group of another are removed. As the —H and —OH react to produce a water molecule, the monosaccharides are joined by a shared oxygen atom, as figure 4.1 shows (read from left to right). As the process repeats, the molecular chain extends, forming a polysaccharide.

Similarly, glycerol and fatty acid molecules join by dehydration synthesis in fat (adipose) tissue cells to form fat molecules. In this case, three hydrogen atoms are removed from a glycerol molecule, and an —OH group is removed from each of three fatty acid molecules, as figure 4.2 shows (read from left to right). The result is three water molecules and a single fat molecule, whose glycerol and fatty acid portions are bound by shared oxygen atoms.

In cells, dehydration synthesis also builds protein molecules by joining amino acid molecules. When two amino acid molecules are united, an —OH from one and an —H from the —NH2 group of another are removed. A water molecule forms, and the amino acid molecules join by a bond between a carbon atom and a nitrogen atom (fig. 4.3; read from left to right). This type of bond, called a peptide bond, holds the amino acids together. Two such bound amino acids form a dipeptide, and many joined in a chain form a polypeptide. Generally, a polypeptide consisting of 100 or more amino acid molecules is called a protein, although the boundary between polypeptides and proteins is not precisely defined.

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