The exchange of materials

Considerations of the flow of energy and its transformations reveal that numerous chemical changes facilitated by enzymes occur in cells. It is thus now possible to advance to the concept that these and all the other chemical processes in cells are defined collectively as metabolism , which is of two chief types. Anabolism involves building up macromolecules from small ones (synthesis) in such a way that the total amount of protoplasm increases and energy is stored as PE. In catabolism , macromolecules are broken down, mostly by hydrolysis, into small molecules and in the respiratory process with a decrease of free energy and release of carbon dioxide and water. If anabolism exceeds catabolism a cell grows, and if catabolism exceeds anabolism substances and energy are dissipated which, if unchecked, will result in death. Plants are anabolic in their capacity to assimilate inorganic substances and synthesise them into foods on which both they themselves and animals depend.