Question
Compare photosynthesis and cellular respiration in terms of their inputs, outputs, organelles involved, and energy transformations. How are these two processes connected?
(CBSE Class 7 and 10 boards, NEET — a perennial favourite)
Solution — Step by Step
Photosynthesis:
Cellular Respiration:
Notice something? One equation is essentially the reverse of the other. The output of photosynthesis is the input of respiration, and vice versa.
Photosynthesis occurs in chloroplasts — specifically, light reactions in the thylakoid membrane and the Calvin cycle in the stroma.
Respiration occurs in mitochondria — glycolysis in the cytoplasm, Krebs cycle in the matrix, and the electron transport chain on the inner membrane.
Photosynthesis: light energy to chemical energy (stored in glucose)
Respiration: chemical energy to usable energy (ATP)
Together, they form a complete energy cycle in the biosphere.
graph LR
subgraph Photosynthesis
A[CO2 + H2O] -->|Light Energy| B[Glucose + O2]
end
subgraph Respiration
C[Glucose + O2] -->|Enzymes| D[CO2 + H2O + ATP]
end
B -->|Glucose feeds| C
D -->|CO2 recycles| AWhy This Works
These two processes are the yin and yang of biological energy flow. Plants do both — photosynthesis during the day (when light is available) and respiration 24 hours a day. Animals only perform respiration.
The compensation point is where the rate of photosynthesis equals the rate of respiration — the plant neither gains nor loses net organic matter. Understanding this connection is what separates a rote answer from a conceptual one in NEET.
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Occurs in | Chloroplasts | Mitochondria |
| Inputs | CO2, H2O, light | Glucose, O2 |
| Outputs | Glucose, O2 | CO2, H2O, ATP |
| Energy | Absorbed | Released |
| When | Day only | All the time |
| Type | Anabolic | Catabolic |
Alternative Method
For CBSE board exams, a tabular comparison scores full marks. For NEET, focus on the energy currency aspect — where exactly ATP is produced in respiration (substrate-level vs oxidative phosphorylation) and where ATP is consumed in photosynthesis (Calvin cycle).
Remember the mnemonic: Photo-BUILDS, Respiro-BREAKS. Photosynthesis builds glucose (anabolic), respiration breaks it down (catabolic). This helps you quickly recall which direction the energy flows.
Common Mistake
Many students write that “plants do photosynthesis, animals do respiration” — this is wrong. Plants perform both processes. During the day, photosynthesis rate exceeds respiration, so there is net O2 release. At night, only respiration occurs, and the plant releases CO2.
In NEET, this distinction has been tested directly — “do plants respire?” is not a trick question, it is a factual one. The answer is always yes.