Question

Jamie reads that cellular respiration releases energy, while photosynthesis requires energy input
which process is exergonic, and which is endergonic
explain why each process fits its category based on energy flow

Answer 1

Cellular respiration is exergonic; photosynthesis is endergonic.

Why — definitions first:
- Exergonic: a process that releases free energy overall (ΔG < 0) and is thermodynamically favorable.
- Endergonic: a process that requires a net input of free energy (ΔG > 0) and is not spontaneous without energy input.

Why cellular respiration is exergonic
- Overall reaction (simplified): C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy.
- Organic molecules (e.g., glucose) are oxidized and electrons flow to a more electronegative acceptor (O2). That electron flow releases free energy, which the cell captures as ATP and dissipates as heat. The overall ΔG for glucose oxidation is strongly negative, so the reaction releases energy.

Why photosynthesis is endergonic
- Overall reaction (simplified): 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2.
- Carbon dioxide is reduced to form sugar, which is a higher-energy (less oxidized) state. That reduction requires input of energy — plants use light to raise the energy of electrons (in the photosystems) and produce ATP and NADPH, which drive the Calvin cycle to fix CO2 into carbohydrate. The overall process has a positive ΔG without light input, so it requires energy.

Short summary: respiration releases stored chemical energy (exergonic); photosynthesis stores incoming light energy in chemical bonds (endergonic).