Diffusion in Photosynthesis

Although diffusion is a slow process compared to energy transfer and electron transfer at the shortest distances, it can be an exceptionally effective way to move electrons and protons over long distances. However, unlike the hard-wired cofactor chains that guide electron transfer in protein complexes, diffusion faces the problem of directing where 

The ultimate way to control diffusion and reactivity with redox partners is to restrict diffusion by anchoring a portion of the redox molecule and allowing essentially only one-dimensional diffusion. This is effectively the case for the FeS center of the bc complex, which has a mobile head group with a surface-exposed FeS center, but also a transmembrane anchor secured to the membrane portion of the bc complex. This severely restricts the range of motion ( 16 A) but perfectly controls the problem of guiding electron transfer. Diffusion over this distance should be on the submicrosecond time scale, much faster than the catalytic turnover of the complex. In a certain sense, this restricted diffusion has properties that lie between unrestricted diffusion and fixed redox cofactor chains a sort of chain with moving parts. 

By exploiting the relative distance dependences of electron tunneling and diffusion, it seems possible that this restricted FeS diffusion may play a role in regulating the action of the n = 2 quinone binding 

These various diffusion strategies used by cytochrome c, plastocyanin, and quinone are clearly successful, because electron transfer throughput is not limited by pool action, even in tissues that need to support a great deal of electron transfer activity such as bee flight muscle (Suarez et al., 2000). These bioenergetic systems are diffusion-coupled rather than diffusion-limited. It seems that chemistry at catalytic sites is the ultimate throughput restrictor and constrains the power that can be produced by bioenergetic systems. 

(1964). Proton transfer, acid-base catalysis, and enzymatic hydrolysis. Part I Elementary processes. Angew. Chem. Int. Ed. 3, 1-19. 

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