Turnover in
biochemistry refers to the idea that organisms reuse and recycle their components continually. Everything is being regenerated in a cyclical process, degraded and resynthesized. Though the
equilibrium remains fairly constant, the organism is nowhere near
static. For example, in the average human body every single cell will have been replaced with a new one within seven years.
Turnover can speed up or slow down depending on environmental factors. Turnover will grind to a halt in bacteria if they are exposed to extreme cold temperatures, such as a collection hurtling through space or trapped deep beneath ice. As soon as the surroundings change: food becomes abundant or circumstances allow the initiation of biological processes again, turnover will reinitiate. Like a river, no organism remains exactly the same forever. Change is inherent in all composite things.
Turnover is especially important to metabolism, since a degraded part of a pathway must be replaced to ensure the stable continuation of chemical reactions.