Question

When there is plenty of light and a higher concentration of O₂, what kind of pathway does the plant undergo? Analyse the reasons.

Answer 1

The rate of photosynthesis decreases when there is an increase in oxygen concentration. This inhibitory effect of oxygen was first discovered by Warburg (1920) using green algae, chlorella.

Analysis and Reasons:

1. Photorespiration Pathway: In conditions where there is abundant light and high oxygen concentration, the enzyme RuBisCO, which normally fixes carbon dioxide (CO₂) during the Calvin cycle in photosynthesis, has a higher affinity for oxygen. Instead of binding to CO₂, RuBisCO binds to O₂, initiating photorespiration.
2. Mechanism: During photorespiration, RuBisCO reacts with oxygen, leading to the formation of a two-carbon compound called phosphoglycolate, which is then converted into CO₂ and energy in a series of reactions. However, unlike photosynthesis, photorespiration does not produce glucose or ATP and is considered an energy-wasting process.
3. Reasons for Photorespiration:
   • High O₂ Concentration: When the concentration of oxygen is higher than that of carbon dioxide, RuBisCO is more likely to bind with oxygen.
   • High Light Intensity: Increased light intensity boosts the production of oxygen through photosynthesis, further raising the O₂ concentration in the leaf.
   • Closed Stomata: In hot or dry conditions, plants often close their stomata to reduce water loss, limiting CO₂ entry and increasing the relative concentration of O₂ within the leaf.