Bioenergetics II: Biological oxidation reduction
- Oxidation-reduction (redox) reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced.
- The flow of electrons in redox reactions is responsible for all work done by living organisms.
- Source of electrons: in non-photosynthetic organisms: reduced compounds (foods)
- In photosynthetic organisms, the initial electron donor: H2O in the presence sunlight.
- Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. The carriers in turn donate electrons to acceptors with higher electron affinities, with the release of energy.
Electron motive force (emf)
- Electrons flow from a reducing agent (low electron affinity) to an oxidizing agent (high electron affinity) due to their difference in electron affinities.
- This difference in electron affinities is called the electromotive force (emf) and it is responsible for electron movement in a circuit. The emf (equivalent to few volts) can perform work if an appropriate energy transducer is present in a circuit.
- Cells contain a variety of molecular energy transducers, which convert the emf into useful work.
- Examples: ATP synthase in inner mitochondrial membrane
- The emf generated due to sequential electron flow in respiratory chain produces proton motive force (pmf analogous to emf) across inner mitochondrial membrane). ATP synthase uses pmf to generate ATP.
- Membrane-localized enzymes in E. coli convert emf to pmf, which is then used to power flagellar motion.