Flux

:This article is about the concept of flux in science and mathematics. For other uses of the word, see flux (disambiguation).

Flux in biology

In general, 'flux' in biology relates to movement of a substance between compartments. There are several cases where the concept of 'flux' is important.

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• The movement of molecules across a membrane: in this case, flux is defined by the rate of diffusion or transport of a substance across a permeable membrane. Except in the case of active transport, net flux is directly proportional to the concentration difference across the membrane, the surface area of the membrane, and the membrane permeability constant.
• In ecology, flux is often considered at the ecosystem level - for instance, accurate determination of carbon fluxes (at a regional and global level) is essential for modelling the causes and consequences of global warming.
• Metabolic flux refers to the rate of flow of metabolites along a metabolic pathway, or even through a single enzyme. A calculation may also be made of carbon (or other elements, e.g. nitrogen) flux. It is dependent on a number of factors, including: enzyme concentration; the concentration of precursor, product, and intermediate metabolites; post-translational modification of enzymes; and the presence of metabolic activators or repressors. Metabolic control analysis provides a framework for understanding metabolic fluxes and their constraints.