Metabolism

Once nutrients have entered the body cells, they are involved in a wide range of biochemical reactions. Metabolism is the sum of the chemical reactions that occur in cells and the reactions breaking them down. Metabolic reactions either

Anabolism

Catabolism

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Figure 3.1 Anabolism and catabolism are both metabolic reactions. Anabolism is the creation of larger molecules from smaller ones, while catabolism is the breaking of large molecules into smaller pieces. Both processes are illustrated here.

make molecules or structures or break them down. Anabolism refers to reactions in which larger molecules are made from smaller ones; for example, the bonding of amino acids to make proteins. Catabolism refers to reactions in which large or complex structures are broken down into smaller ones (Figure 3.1). Anabolic reactions usually need energy added to them to work. Catabolic reactions tend to release energy from the compounds. The energy released from one reaction runs the other reaction.

Energy is extracted from compounds in two ways. When some chemical reactions occur, there is energy left over. This energy can be used to put a third phosphate onto ADP to form ATP, a process called substrate phosphorylation. A substrate is a compound being acted upon in a chemical reaction using an enzyme to facilitate the process. Phosphorylation is the process of adding the third phosphate. This process accounts for relatively little of the ATP produced. The rest of the ATP is made by harnessing the energy of the electrons of hydrogen atoms. These atoms are split, and the electrons are passed through a series of reactions resulting in a large amount of ATP. Oxygen is used in this process, but only at the end, when it receives an electron. The addition of two electrons to oxygen attracts two

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ooo hydrogen ions (protons) from the surrounding medium, and the result is water (H2O). This second method of producing ATP is called oxidative phosphorylation. Because triglycerides hold a large number of hydrogen atoms, storing twice the energy of carbohydrates, fatty acids are much more efficient as energy storage molecules.

The breakdown of glucose for ATP production involves three connected chemical pathways: glycolysis, the Krebs cycle, and the electron transport chain (Figure 3.2). Glucose enters glycolysis as a six-carbon sugar and comes out as a three-carbon molecule called pyruvic acid, resulting in two ATP molecules. Pyruvic acid loses a carbon dioxide and forms an acetyl group that combines with a form of vitamin B6, resulting in a compound called acetyl CoA. This compound enters the second phase of glucose oxidation, the Krebs cycle. Before the pyruvic acid is changed, it can be used to form the amino acid alanine. Alanine can then be transformed into other amino acids by subsequent chemical reactions.

The Krebs cycle removes electrons from hydrogen atoms to send to the third phase, the electron transport chain. The waste product of the Krebs cycle is carbon dioxide. Each time the Krebs cycle turns, it produces a single ATP through substrate phosphorylation. Several chemicals produced during the Krebs cycle can be removed for amino acid synthesis. These amino acids can also be fed into the Krebs cycle through these intermediate chemicals.

The last pathway is the electron transport chain, a series of chemical reactions that pass electrons from one chemical to the next. During this process, 34 ATP molecules can be produced for each glucose molecule that started the process. It is possible to make a total of 38 ATP molecules through the three pathways. Because many of the intermediate compounds are used for other purposes, the maximum number of ATP molecules is seldom produced, except in skeletal muscle, where all of the ATP is needed for contraction.

First Pass Metabolism

Figure 3.2 Glucose is broken apart to form ATP through the processes of glycolysis, the Krebs cycle, and the electron transport chain. First, glucose, a six-carbon sugar, is broken into three-carbon molecules called pyruvic acid. Next, pyruvic acid loses a carbon and becomes acetyl CoA. Finally, the acetyl CoA goes through the electron transport chain, where electrons are passed between chemicals. The result is 34 molecules of ATP, which can be used as energy.

Figure 3.2 Glucose is broken apart to form ATP through the processes of glycolysis, the Krebs cycle, and the electron transport chain. First, glucose, a six-carbon sugar, is broken into three-carbon molecules called pyruvic acid. Next, pyruvic acid loses a carbon and becomes acetyl CoA. Finally, the acetyl CoA goes through the electron transport chain, where electrons are passed between chemicals. The result is 34 molecules of ATP, which can be used as energy.

TABLE 3.2 APPROXIMATE NUMBER OF CALORIES BURNED PER HOUR BY ACTIVITY

Activity

100 1 h person

150 lb person

200 lb person

Bicycling, 6 tnph

160

240

312

Bicycling, 12 mph

270

410

534

Jogging, 5.5 mph

440

660

962

Jogging, 10 mph

850

1,280

1,664

Jumping rope

500

750

1,000

Swimming, 25 yds/min

185

275

358

Swimming, 50 yds/min

325

500

650

Walking, 2 mph

160

240

312

Walking, 4.5 mph

295

440

572

Tennis (singles)

265

400

535

Source: American Heart Association

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