<P> All organisms are constantly exposed to compounds that they cannot use as foods and would be harmful if they accumulated in cells, as they have no metabolic function . These potentially damaging compounds are called xenobiotics . Xenobiotics such as synthetic drugs, natural poisons and antibiotics are detoxified by a set of xenobiotic - metabolizing enzymes . In humans, these include cytochrome P450 oxidases, UDP - glucuronosyltransferases, and glutathione S - transferases . This system of enzymes acts in three stages to firstly oxidize the xenobiotic (phase I) and then conjugate water - soluble groups onto the molecule (phase II). The modified water - soluble xenobiotic can then be pumped out of cells and in multicellular organisms may be further metabolized before being excreted (phase III). In ecology, these reactions are particularly important in microbial biodegradation of pollutants and the bioremediation of contaminated land and oil spills . Many of these microbial reactions are shared with multicellular organisms, but due to the incredible diversity of types of microbes these organisms are able to deal with a far wider range of xenobiotics than multicellular organisms, and can degrade even persistent organic pollutants such as organochloride compounds . </P> <P> A related problem for aerobic organisms is oxidative stress . Here, processes including oxidative phosphorylation and the formation of disulfide bonds during protein folding produce reactive oxygen species such as hydrogen peroxide . These damaging oxidants are removed by antioxidant metabolites such as glutathione and enzymes such as catalases and peroxidases . </P> <P> Living organisms must obey the laws of thermodynamics, which describe the transfer of heat and work . The second law of thermodynamics states that in any closed system, the amount of entropy (disorder) cannot decrease . Although living organisms' amazing complexity appears to contradict this law, life is possible as all organisms are open systems that exchange matter and energy with their surroundings . Thus living systems are not in equilibrium, but instead are dissipative systems that maintain their state of high complexity by causing a larger increase in the entropy of their environments . The metabolism of a cell achieves this by coupling the spontaneous processes of catabolism to the non-spontaneous processes of anabolism . In thermodynamic terms, metabolism maintains order by creating disorder . </P> <P> As the environments of most organisms are constantly changing, the reactions of metabolism must be finely regulated to maintain a constant set of conditions within cells, a condition called homeostasis . Metabolic regulation also allows organisms to respond to signals and interact actively with their environments . Two closely linked concepts are important for understanding how metabolic pathways are controlled . Firstly, the regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals . Secondly, the control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway (the flux through the pathway). For example, an enzyme may show large changes in activity (i.e. it is highly regulated) but if these changes have little effect on the flux of a metabolic pathway, then this enzyme is not involved in the control of the pathway . </P>

A food preservation method in which microorganisms metabolize components of a food is