Let’s consider some conceptual links between the processes of aerobic respiration and photosynthesis.
1. Both processes are built around the same reaction:
2. Both processes are built around membrane proteins.
Now, refer to these figures to spot the other links.
3. Both processes use several membrane protein complexes to couple electron transport along the membrane with proton transport through the membrane.
4. Both processes thus generate a proton gradient that is in turn used to run the ATP synthase.
5. Photosynthesis uses photons to generate high energy electrons while respiration strips high energy electrons from sugars (which were originally formed by reactions tied to photosynthesis).
6. Photosynthesis strips electrons from water to generate oxygen, while respiration donates the electrons to oxygen to form water.
7. Respiration strips electrons from an electron carrier, nicotinamide adenine dinucleotide (NADH):
while photosynthesis donates electrons to a closely related electron carrier, nicotinamide adenine dinucleotide phosphate (NADP):
8. Both processes use three basic protein complexes, with the same one occupying the central position – cytochrome bc1 and b6f.
9. Both process also use Coenzyme Q to transport electron from the first complex to the second.
10. In photosynthesis, the first complex uses photons to generate high energy electrons and couples this to stripping them from water; in respiration the first complex accepts high energy electrons from the electron carrier, NADH.
11. In photosynthesis, the last complex passes electrons to the electron carrier, NADP; in respiration, the last complex passes electrons to oxygen to form water.
12. Both respiration and photosynthesis are the only two established examples whereby endosymbiosis generated an organelle for eukaryotes – mitochondria and chloroplasts.
13. While neither respiration nor photosynthesis is essential for life, mitochondria made it possible to evolve animals and chloroplasts made it possible to evolve plants.