Biochemistry

TLO: Define metabolism.

Metabolism (me-TAB-oˉ-lizm; metabol- = change) refers to all of the chemical reactions that occur in the body. There are two types of metabolism: catabolism and anabolism. Those chemical reactions that break down complex organic molecules into simpler ones are collectively known as catabolism (ka-TAB-oˉ-lizm; cata- = downward). Overall, catabolic (decomposition) reactions are exergonic; they produce more energy than they consume, releasing the chemical energy stored in organic molecules. Important sets of catabolic reactions occur in glycolysis, the Krebs cycle, and the electron transport chain, each of which will be discussed later in the chapter.

Chemical reactions that combine simple molecules and monomers to form the body’s complex structural and functional components are collectively known as anabolism (a-NAB-oˉ-lizm; ana- = upward). Examples of anabolic reactions are the formation of peptide bonds between amino acids during protein synthesis, the building of fatty acids into phospholipids that form the plasma membrane bilayer, and the linkage of glucose monomers to form glycogen. Anabolic reactions are endergonic; they consume more energy than they produce.

Metabolism is an energy-balancing act between catabolic (decomposition) reactions and anabolic (synthesis) reactions. The molecule that participates most oft en in energy exchanges in living cells is ATP (adenosine triphosphate), which couples energy-releasing catabolic reactions to energy-requiring anabolic reactions.

Watch this video about carbohydrate metabolism.