Electron Transport Phosphorylation

Electron transport phosphorylation is more important than substrate level phosphorylation in phototrophs and aerobic and facultatively anaerobic chemotrophs. It occurs during respiration and photosynthesis. During C metabolism by chemoorganotrophs, NADH and FADH are produced from oxidation of organic substrates, for example in the TCA cycle. Electron transport phosphorylation entails transfer of electrons from donors such as NADH (or FADH) with a negative redox potential to acceptors such as O2 with a less negative or positive redox potential. The associated energy change is coupled to the phosphorylation of ADP + P; ^ ATP. Electron transport phosphorylation is associated with membranes.

Given that ATP is used to energize soil microbial biomass formation, how much ATP is produced during oxidation of a fixed amount of substrate? Understanding the relationship between substrate oxidized, O2 consumed or electrons (e~) transferred, and ATP formed helps us understand carbon utilization efficiency (CUE; ratio of C consumed/C converted to biomass) on one hand or quantities of alternate electron acceptors needed on the other. The CUE in turn helps regulate elemental dynamics; for example, N mineralization increases and N immobilization decreases as CUE decreases. Similarly, as the amount of ATP produced per mole of decreases, there is an associated increase in the number of moles of O2 needed to generate a fixed amount of ATP, or alternate electron acceptor (e.g., NO3), needed in the absence of O2.

Under aerobic conditions, each mole of NADH or FADH carries and each reduce one atom of O. Therefore under aerobic conditions ATP production from electron transport phosphorylation with O2 as the electron acceptor can be equated to electrons released or atoms of O consumed. If O is not the terminal electron acceptor, then ATP production from electron transport phosphorylation can be equated only to electrons released. The ratio of ATP production to released or atoms of O consumed may be designated the P/O or P/2e~ ratio, under aerobic conditions, or P/2e~ ratio when O2 is not the terminal electron acceptor.

The P/O or P/2e~ ratio may be estimated using either a general rule or slightly more detailed calculations. We will consider the general rule first, followed by a more detailed example. Generally the oxidation of 1 mol of NADH is considered to generate 3 mol of ATP, and oxidation of 1 mol of FADH generates 2 mol of ATP. Expressed as a ratio of ATP/NADH or ATP/FADH one gets:

3ATP , 2ATP and

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