Figure 1
The two ways that cells conserve energy in the form of ATP (note that some energy conservation in anaerobes involves ferredoxin instead of ATP [51–53]): 1) chemiosmosis and 2) substrate-level phosphorylation. The figure shows a schematic representation of chemiosmotic energy harnessing [50]. A redox reaction (left) is used to channel electrons though plasma membrane-associated proteins and cofactors in such a way that protons are depleted in the cytosol, creating a chemical and electrochemical gradient. Return of the protons to the cytosol occurs through an ATP synthase (ATPase), where ATP is synthesized from ADP and Pi. D donor, A acceptor, red reduced, ox oxidized. In rhodopsin-based photosynthesis, the protons are pumped without redox reactions, but in the absence of redox chemistry, no cell can survive. See references [49–53].