Question
Compare photosynthesis and cellular respiration in terms of inputs, outputs, organelle involved, energy change, and when they occur. Are they reverse reactions of each other?
(CBSE Class 7, Class 10, and NEET)
Solution — Step by Step
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Equation | ||
| Inputs | CO₂ + Water + Light energy | Glucose + O₂ |
| Outputs | Glucose + O₂ | CO₂ + Water + ATP (energy) |
| Organelle | Chloroplast | Mitochondria |
| Energy | Absorbed (light to chemical) | Released (chemical to ATP) |
| Occurs in | Plants, algae (with chlorophyll) | All living cells |
| When | Only in light | All the time (day and night) |
The overall equations look like reverses of each other, and in terms of net reactants and products, they are. But the actual pathways are completely different:
- Photosynthesis uses the Calvin cycle (in stroma) and light reactions (in thylakoids)
- Respiration uses glycolysis (in cytoplasm), Krebs cycle (in matrix), and ETC (on inner membrane)
Different enzymes, different organelles, different mechanisms. Same net equation, opposite directions.
- Photosynthesis produces 1 glucose molecule from 6 CO₂ + 6 H₂O
- Respiration produces 36-38 ATP molecules from 1 glucose (in aerobic conditions)
- Plants do BOTH — photosynthesis during the day, respiration all the time
Process Comparison Flowchart
flowchart LR
subgraph Photosynthesis
A["CO₂ + H₂O"] -->|"Light energy + Chloroplast"| B["Glucose + O₂"]
end
subgraph Respiration
C["Glucose + O₂"] -->|"Mitochondria"| D["CO₂ + H₂O + ATP"]
end
B -->|"Glucose feeds into"| C
D -->|"CO₂ feeds into"| AWhy This Works
Photosynthesis and respiration form a cycle that keeps life going. Plants use sunlight to build glucose (storing energy). Then all organisms — plants and animals alike — break down glucose through respiration to release that stored energy as ATP, which powers every cellular activity.
Without photosynthesis, there would be no glucose and no oxygen. Without respiration, cells could not use the glucose. The two processes are interdependent.
Common Mistake
Students often say “plants do photosynthesis, animals do respiration.” This is wrong. Plants do both — they photosynthesise during the day AND respire all the time (day and night). During the day, photosynthesis rate exceeds respiration rate, so the net effect is O₂ release. At night, only respiration occurs, so plants release CO₂. NEET has tested this distinction multiple times.