Photorespiration is detrimental to photosynthesis primarily because it reduces the efficiency of carbon fixation and wastes energy.
Here's a breakdown of why:
-
Production of 2-Phosphoglycolate (2PG): The enzyme RuBisCO, which is supposed to fix CO2 during the Calvin cycle, can sometimes bind to O2 instead, especially when CO2 levels are low and O2 levels are high. This reaction produces one molecule of 3-phosphoglycerate (a useful intermediate for photosynthesis) and one molecule of 2-phosphoglycolate (2PG). 2PG is not directly useful and must be processed, requiring energy. More importantly, 2PG inhibits multiple enzymes in photosynthetic metabolism.
-
Energy Expenditure to Recover Carbon: The process of converting 2PG back into a usable form (3-phosphoglycerate) is energetically costly. This recovery pathway consumes ATP and NADH, which are essential energy carriers produced during the light-dependent reactions of photosynthesis. This diversion of energy reduces the overall efficiency of photosynthesis.
-
Loss of Fixed Carbon as CO2: During the recovery process, some of the carbon that was initially fixed during photosynthesis is lost as CO2. This effectively reverses some of the work done in fixing the carbon in the first place, further reducing the net carbon gain.
-
Reduced Net Photosynthetic Rate: Overall, the combined effects of producing 2PG, expending energy on its recovery, and losing fixed carbon as CO2 result in a lower net photosynthetic rate. This means that the plant is able to produce less sugar (energy) than it would have without photorespiration.
In summary, photorespiration acts as a wasteful process that consumes energy and releases CO2, thereby decreasing the overall efficiency of photosynthesis. This is particularly problematic in hot, dry conditions where plants close their stomata to conserve water, leading to a buildup of O2 and a depletion of CO2 within the leaves, favoring photorespiration.
