Activation
3394238
208368340
2008-04-26T18:46:26Z
BOT-Superzerocool
338065
robot Adding: [[ja:活性化]]
:''This article describes the natural science term. For "activation" as it refers to computer software, see [[product activation]]''.
'''Activation''' in [[science]] generally refers to the [[Process (science)|process]] whereby something is prepared or excited for a subsequent [[reaction]].
== Chemistry ==
In [[chemistry]], activation of [[molecule]]s is required for a [[chemical reaction]] to occur. The phrase [[activation energy|energy of activation]]<ref>[http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html The Activation Energy of Chemical Reactions<!-- Bot generated title -->]</ref> refers to the [[Energy (chemistry)|energy]] the [[reactant]]s must acquire before they can successfully react with each other to produce the [[Product (chemistry)|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, [[metal]]s. The sample is placed in a high [[neutron flux]] and in the [[nuclear reaction]] involved, a [[neutron]] is [[Neutron capture|captured]] by a [[Atomic nucleus|nucleus]]. If the resulting new [[isotope]] is unstable, it will undergo [[radioactive decay]]. This decay can be monitored and the [[chemical element|element]] emitting identified by the identity and energy of the emitted particles. [[Prompt gamma neutron activation analysis|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 [[proenzyme]]s. 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===
{{Main|Immunologic activation}}
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 receptor]]s, [[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.
== References ==
<references />
[[Category:Chemical kinetics]]
[[ja:活性化]]
[[uk:Активація]]