Efficiency of electron transport processes

Efficiency of electron transport along the linear chain of molecules was investigated from a dynamic viewpoint. It was proposed that two kinds of efficiency are important for electron transport; one is energy efficiency, the other quantum efficiency. In this paper, these two efficiencies are defined for a linear chain system and the correlation between these quantities and the arrangement of various electron transfers is investigated. The optimization of energy and quantum efficiency is found to set different conditions on the arrangement of the rate constants of electron transfer, and there is strong correlation between neighboring electron transfers. In order to maximize both efficiencies, the rate constants of forward and backward transfers of electrons should be bounded by one another in a limited range. In particular, when there are some bypass reactions on the linear chain, as is the case for photosynthesis and respiration, the rate of the backward transfer should be the same order of magnitude as that of the next forward transfer. The present results are applied to some biological processes. In the early stage of photosynthetic electron transfer it seems that quantum efficiency is more important than energy efficiency. The quantum efficiency is close to unity, whereas a considerable part of the free energy is wasted as heat during the primary electron transfers. On the other hand, in the slower electron transfer processes in photosynthesis and respiration, which take place mostly near equilibrium, the energy efficiency seems to be more important than the quantum efficiency. The relation of these properties to biological function is discussed.