Characteristics of the living state

The living state is characterized by instability and change. The numerous chemical reactions, called collectively metabolism, within a living cell both consume (in the form of foodstuffs) and release energy. Metabolism is indicative of life. Even the apparently inert cells of seeds show some metabolism, but a mere fraction of that which occurs during germination and subsequent growth. Metabolism depends upon the interaction of molecules in an ordered sequence. If this order is destroyed (for example by poisons or heat) metabolism ceases and the cell dies. In some instances it is possible to arrest metabolism without death. With yeast and some tissue cultures, for example, this can be achieved by very rapid freezing at temperatures of −160°C (−265°F) or lower. The cells can then be preserved in liquid nitrogen (−195°C; −319°F), in an apparently genuine state of “suspended animation”, indefinitely. With yeast up to 95 percent of cells of rapidly frozen cultures resume metabolism and growth following careful thawing.

The sources of energy a cell requires to maintain its dynamic state are predominantly compounds of carbon. In addition a cell requires water, since much of the metabolism takes place in the aqueous phase in the cell. Also essential are those materials necessary for the maintenance of its structure which it is unable to make for itself.