Q.a. What is photorespiration? Explain the various factors affecting photosynthesis. OR b. Give the differences between C₃ and C₄ plants.
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Start your 14-day free trial to unlock the full solution →Photorespiration is RuBisCO's wasteful oxygenase side-reaction; photosynthesis overall depends on light, CO₂, temperature and water, each of which can become the limiting factor.
a. Photorespiration:
RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase), the key enzyme of the Calvin cycle, has a dual affinity — it can act as a carboxylase (fixing CO₂, the normal photosynthetic pathway) or as an oxygenase (fixing O₂ instead). When the concentration of O₂ is high relative to CO₂ inside the leaf (e.g., on hot, bright, dry days when stomata partially close), RuBisCO oxidises RuBP instead of carboxylating it. This produces one molecule of phosphoglycerate (PGA, usable in the Calvin cycle) and one molecule of phosphoglycolate (2-carbon), which is metabolised through the peroxisome and mitochondrion, ultimately releasing CO₂ without generating any ATP or NADPH. Because it consumes O₂, releases CO₂, and yields no useful energy, this pathway is called photorespiration and it lowers the net photosynthetic efficiency of C₃ plants (it is largely avoided in C₄ plants due to their CO₂-concentrating Kranz anatomy).
Factors affecting photosynthesis:
- Light: Rate increases with light intensity up to a saturation point, beyond which other factors become limiting; both intensity and quality (wavelength) matter.
- Carbon dioxide concentration: CO₂ is often the major limiting factor in nature since its atmospheric concentration is low; increasing CO₂ (up to a point) increases the rate of photosynthesis.
- Temperature: Photosynthesis is enzyme-driven, so it has an optimum temperature range (typically 25–35°C for most plants); very low or very high temperatures reduce the rate.
- Water: Water deficiency causes stomatal closure (reducing CO₂ entry) and directly affects the plant's metabolic machinery, thereby lowering the photosynthetic rate.
These factors interact according to Blackman's law of limiting factors — the factor that is in shortest supply relative to demand determines the actual rate of photosynthesis at any given time.
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