9: Cells and Metabolism

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Aloft, floating free beneath the moist, gleaming membrane of bright blue sky, is the rising earth, the only exuberant thing in this part of the cosmos .... It has the organized, self-contained look of a live creature, full of information, marvelously skilled in handling the sun.

It takes a membrane to make sense out of disorder in biology. You have to be able to catch energy and hold it, storing precisely the needed amount and releasing it in measured shares. A cell does this ....

From Lives of a Cell by Lewis Thomas

How does food become you? After it’s digested and absorbed, the nutrients are delivered to cells throughout the body. The nutrients then take part in a series of chemical reactions that are collectively called metabolism—from metaballein (to change) in Greek. The nutrients become you.

Metabolism includes: (1) breaking down energy-providing nutrients (fat, carbohydrate, and protein) into smaller molecules and (2) building big molecules from small ones.* These chemical reactions are interdependent. Breaking down the energy-providing nutrients provides the ATP (chemical energy) needed to build big molecules from small ones.

Only the bare essentials of metabolism are discussed here. In reality, nutrients take part in a multitude of chemical reactions, guided by what the body needs at the time. For example, amino acids from your lunch burrito can be used to make protein—muscle protein, antibodies, enzymes, membrane proteins, insulin, etc. Or used for energy, and broken down completely to carbon dioxide and water. Or made into glucose to fuel the brain. Or made into triglycerides stored as body fat. These are but a few of the “or” s.

The body can make almost everything it needs from a variety of nutrients, e.g., the sex hormones testosterone and estrogen can be made from either fat, carbohydrate, or protein. These chemical conversions—metabolism—take place in our cells.

^{*These “build-up” (synthesis) reactions are called anabolic. Anabolic steroids used by some athletes (Chap. 13) promote the synthesis of muscle protein. Their 4-ring structure—like that of cholesterol (Figure 5.2)—identifies them as sterols/ steroids. Anabolic steroids are variations of testosterone, the male sex hormone that’s made from cholesterol.}