Activation

: "This article describes the term used in chemistry and biochemistry.": "For activation (installing) of computer software, see product activation.": "For activation (calling) of a subroutine in computing, see Subroutine."

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 required for a chemical reaction to occur. The phrase energy of activation [ [http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html The Activation Energy of Chemical Reactions ] ] 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.

Biochemistry

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 Activation Analysis (PGAA) is a rapid, nondestructive, instrumental, nuclear technique which is used for trace and major component analysis of various elements.

A number of enzymes are synthesised as inactive proenzymes. Only when a polypeptide sequence is cleaved from the end of the protein, in a process called activation, is the enzyme brought to its full level of activity.

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.

Electrophysiology

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

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