Catabolism

Catabolism is the breakdown of large molecules into small molecules. Its opposite process is anabolism, the combination of small molecules into large molecules. These two cellular chemical reactions are together called metabolism. Cells use anabolic reactions to synthesize enzymes, hormones, sugars, and other molecules needed to sustain themselves, grow, and reproduce.

Energy released from organic nutrients during catabolism is stored within the molecule adenosine triphosphate (ATP), in the form of the high-energy chemical bonds between the second and third molecules of phosphate. The cell uses ATP for synthesizing cell components from simple precursors, for the mechanical work of contraction and motion, and for transport of substances across its membrane. ATP's energy is released when this bond is broken, turning ATP into adenosine diphosphate (ADP).

The cell uses the energy derived from catabolism to fuel anabolic reactions that synthesize cell components.

Although anabolism and catabolism occur simultaneously in the cell, their rates are controlled independently of each other. Cells separate these pathways because catabolism is a so-called "downhill" process during which energy is released, while anabolism is an energetically "uphill" process which requires the input of energy.

The different pathways also permit the cell to control the anabolic and catabolic pathways of specific molecules independently of each other. Moreover, some opposing anabolic and catabolic pathways occur in different parts of the same cell. For example, in the liver, the fatty acids are broken down to acetyl CoA inside mitochondria, while fatty acids are synthesized from acetyl CoA in the cytoplasm of the cell.

Both catabolism and anabolism share an important common sequence of reactions known collectively as the citric acid cycle, or Krebs cycle, which is part of a larger series of enzymatic reactions known as oxidative phosphorylation. Here, glucose is broken down to release energy, which is stored in the form of ATP (catabolism), while other molecules produced by the Krebs cycle are used as precursor molecules for anabolic reactions that build proteins, fats, and carbohydrates (anabolism).

Cells regulate the rate of catabolic pathways by means of allosteric enzymes, whose activity increases or decreases in response to the presence or absence of the end product of the series of reactions. For example, during the Krebs cycle, the activity of the enzyme citrate synthase is slowed by the buildup of succinyl CoA, a product formed later in the cycle.