Question

Why is cellular respiration sometimes thought of as a similar, but reverse, process compared with photosynthesis?

Answer 1

Because the overall inputs and outputs are essentially swapped and they move energy in opposite directions, cellular respiration looks like the “reverse” of photosynthesis — but the internal chemistry is different.

Key point (overall chemical equations)
- Photosynthesis: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
- Cellular respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)

Why people say “reverse”
- Reactants and products are reversed: photosynthesis fixes CO2 into sugars and releases O2; respiration breaks sugars down and consumes O2 to give back CO2 and H2O.
- Energy direction is reversed: photosynthesis stores light energy in chemical bonds (endergonic); respiration releases that stored energy to make ATP (exergonic).
- Electron flow is opposite: in photosynthesis electrons are taken from H2O and used to reduce NADP+ to NADPH; in respiration electrons come from NADH (and FADH2) and are passed to O2 to form H2O.

Important differences (not a perfect reversal)
- Different pathways and enzymes: photosynthesis uses light reactions + the Calvin cycle; respiration uses glycolysis, pyruvate oxidation, the Krebs cycle and oxidative phosphorylation. Many individual steps are not simply the reverse of each other.
- Different electron carriers and energy carriers (NADP+/NADPH in photosynthesis; NAD+/NADH and FAD/FADH2 in respiration).
- Photosynthesis requires light energy; respiration does not.
- Both use an electron transport chain and chemiosmosis/ATP synthase, but with opposite net electron flow and proton gradients.

Bottom line
They are complementary processes in the global carbon and energy cycles: photosynthesis captures and stores solar energy as organic matter and O2, and respiration releases that stored energy for cellular work, returning CO2 and H2O to the environment.