Many of the macromolecules of which cells are made are generated from their individual building blocks by the removal of the elements of water. Equally, macromolecules can be broken into their individual building blocks by hydrolysis—breakage by the addition of water. Figure 2.20 shows four examples. Lactose is hydrolyzed to the monosaccharides galactose and glucose by the addition of one water molecule. Next, we show a dipeptide being broken into individual amino acids by hydrolysis. These two hydrolysis reactions occur all the time in our intestines, though some people lose the ability to hydrolyze lactose as they get older: they are lactose intolerant. Third, we show a dimeric (formed of two parts) inorganic phosphate ion called pyrophosphate being hydrolyzed to regular phosphate ions. The hydrolysis of pyrophosphate is catalyzed by enzymes found throughout the body, both inside cells and out. Later in the book we will meet a number of instances where a reaction creates

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