Activation

Activation in (bio-)chemical sciences generally refers to the process whereby something is prepared or excited for a subsequent reaction.

Chemistry

In chemistry, activation of molecules is where the molecules enter a state that avails for a chemical reaction to occur. The phrase energy of activation^[1] refers to the energy the reactants must acquire before they can successfully react with each other to produce the products, that is, to reach the transition state. The energy needed for activation can be quite small and the molecules may have enough energy just from thermal fluctuations the molecules naturally have (i.e. lots of reactions don't have to be heated to proceed). The branch of chemistry that deals with this topic is called chemical kinetics.

Biology

Biochemistry

In biochemistry, activation, specifically called bioactivation, is where enzymes or other biologically active molecules acquire the ability to perform their biological function, such as inactive proenzymes being converted into active enzymes that are able to catalyze their substrates into products. Bioactivation may also refer to the process where inactive prodrugs are converted into their active metabolites, or the toxication of protoxins into actual toxins.

An enzyme may be reversibly or irreversibly bioactivated; A major mechanism of irreversible bioactivation is where a piece of the protein is cut off by protein cleavage, causing the enzyme to stay active. On the other hand, a major mechanism of reversible bioactivation is where a cofactor is placed on the enzyme, causing it to only stay active while the cofactor stays on. However, when the cofactor is removed, the enzyme stops being active.

Immunology

In immunology, activation is the transition of leucocytes and other cell types involved in the immune system. On the other hand, deactivation is the transition in the reverse direction. This balance is tightly regulated, since a too small degree of activation causes susceptibility to infections, while, on the other hand, a too large degree of activation causes autoimmune diseases.

Activation and deactivation results from a variety of factors, including cytokines, soluble receptors, arachidonic acid metabolites, steroids, receptor antagonists, adhesion molecules, bacterial products and viral products.

Physics

Similarly, neutron activation is a standard analytical technique used to analyze for elements, usually, metals. The sample is placed in a high neutron flux and in the nuclear reaction involved, a neutron is captured by a nucleus. If the resulting new isotope is unstable, it will undergo radioactive decay. This decay can be monitored and the element emitting identified by the identity and energy of the emitted particles. Prompt Gamma Neutron Activation Analysis (PGNAA) is a rapid, nondestructive, instrumental, nuclear technique which is used for trace and major component analysis of various elements.

Electrophysiology

Activation refers to the opening of ion channels, i.e. the conformational change that allows ions to pass.

References

1. ^ The Activation Energy of Chemical Reactions

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