Overview of Metabolism

Growth, development, movement and response to stimuli are among the "characteristics of living things". All of these, as well as many other cellular processes, require energy in a readily usable form. Adenosine triphosphate, better known as ATP, is a small molecule with a high energy transfer capability. It serves the function so well that it is often referred to as the "energy currency" of the cell. Chemical energy associated with the phosphate bonds of ATP can be transferred to other molecules; for example, to form peptide bonds between amino acids to make a protein. The chemical energy of ATP can also be transferred to other molecules and converted into kinetic energy; for example, to perform active transport or during muscle contraction.

Animal cells use two main mechanisms to produce ATP: anaerobic and aerobic catabolism. When oxygen is present, most cells are able to produce ATP by aerobic respiration. Aerobic respiration involves three major processes: glycolysis, the Kreb's (tricarboxylic acid or TCA) cycle, and oxidative phosphorylation coupling the electron transport chain with chemiosmosis in the mitochondria. Aerobic catabolism of glucose requires oxygen and produces carbon dioxide and metabolic water. When oxygen is absent, some animal cells are able to catabolize glucose anaerobically. Anaerobic glucose catabolism in animal cells involves glycolysis and the reduction of pyruvate to lactate. Anaerobic catabolism does not require oxygen and does not produce carbon dioxide in animal cells. Aerobic catabolism is much more efficient at making ATP, yielding a net production of 38 ATP in some cells, compared to anaerobic respiration, which only yields a net production of 2 ATP. Skeletal muscle can also produce limited amounts of ATP from the breakdown of stored phosphocreatine (phosphogen system). This mechanism can produce 1 ATP for every molecule of phosphocreatine present.

The hexose (6-carbon sugar) known as glucose is the substrate of choice for ATP production and can be "burned" aerobically or anaerobically. Fructose and galactose are also hexoses and can be converted to glucose in cells with the appropriate enzymes. In humans, this conversion is accomplished by the liver. Some cells, such as neurons, are only able to use glucose. Other cells, such as cardiac and skeletal muscle, can also use glycerol and fatty acids from fats or amino acids that have been deaminated.


Written by J. Ellen Lathrop-Davis, M.Sc.
CCBC - Catonsville
Summer Grant, 2002
All Rights Reserved