CELL METABOLISM AND ENERGY SOURCES

molecules, and the synthesis of enzymes, hormones, and other macromolecules. Energy metabolism refers to the pro­cesses by which fats, proteins, and carbohydrates from the foods we eat are converted into energy or complex energy sources in the cell. Catabolism and anabolism are the two phases of metabolism. Catabolism consists of breaking down stored nutrients and body tissues to produce energy. Anabolism is a constructive process in which more com­plex molecules are formed from simpler ones.

The special carrier for cellular energy is ATP. ATP mole­cules consist of adenosine, a nitrogenous base; ribose, a five-carbon sugar; and three phosphate groups (see Fig. 4-14). The phosphate groups are attached by two high-energy bonds. Large amounts of free energy are released when ATP is hydrolyzed to form adenosine diphosphate (ADP), an adenosine molecule that contains two phosphate groups.

High-energy bonds

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NH₂

Н-С-О-Р-О~ Р-О~ Р-ОН )\| I I I н~с OH OH OH

г

CC I OH

-сн I он

Adenine

Phosphates

Ribose Adenosine

FIGURE4-14 Structure of the adenosine triphosphate (ATP) molecule

Energy is the ability to do work. Cells use oxygen and the breakdown products of the foods we eat to produce the en­ergy needed for muscle contraction, transport of ions and

The free energy liberated from the hydrolysis of ATP is used to drive reactions that require free energy, such as muscle contraction and active transport mechanisms. Energy from foodstuffs is used to convert ADP back to ATP. ATP is often called the energy currency of the cell; en­ergy can be "saved" or "spent" using ATP as an exchange currency.

Two sites of energy production are present in the cell: the anaerobic (i.e., without oxygen) glycolytic pathway, occurring in the cytoplasm, and the aerobic (i.e., with

oxygen) pathways in the mitochondria. The anaerobic glycolytic pathway serves as an important prelude to the aerobic pathways.