The necessary and the desirable production of radicals in biology

The production of radicals within biological systems can be necessary, as in the coupling of light capture or oxygen-hydride reactions to the generation of proton gradients in thylakoid and mitochondrial membranes; or it can be extremely desirable, to produce an effective catalyst centre in, for example, ribonucleotide reductases. To fit their function the radicals are usually of controlled reactivity, either through their redox properties or through confinement to a limited volume. The more indiscriminate generation of radicals can be of great use too in protective devices, for example, in the production of cross-linked polymers, or in the generation of oxygen radicals. The different distribution and release of initiating enzymes for all these radical processes ensures the spatial and temporal control of their reactions.