When the CO2 concentration in the cells of a C3 plant is low compared with the O2 concentration, an enzyme combines RuBP with O2 rather than with CO2. What effect would this enzymatic change have on photosynthesis? Under what environmental conditions would it be more likely to occur?
The enzymatic change described is called photorespiration, and it has a negative effect on photosynthesis. When the enzyme combines RuBP (ribulose bisphosphate) with O2, a byproduct called phosphoglycolate is formed instead of the usual product, phosphoglycerate. This byproduct needs to go through a series of reactions to be converted into a useful compound, resulting in energy and carbon loss for the plant.
Photorespiration competes with the normal pathway of photosynthesis, where RuBP combines with CO2 to produce phosphoglycerate, which can then be used to produce glucose and other organic compounds. Therefore, photorespiration reduces the efficiency of carbon fixation and overall photosynthetic productivity.
The occurrence of photorespiration is more likely under specific environmental conditions: high temperatures and/or high oxygen concentrations. This is because at higher temperatures, the affinity of the enzyme responsible for photorespiration (rubisco) for CO2 decreases, while its affinity for O2 increases. Similarly, at higher oxygen concentrations, the likelihood of oxygen binding to rubisco instead of CO2 is higher.
Overall, photorespiration is a wasteful process that reduces plant productivity and is more likely to occur in C3 plants under high temperatures and/or high oxygen concentrations.
The enzymatic change you're referring to is called photorespiration, which occurs when the enzyme Rubisco, instead of combining RuBP (ribulose-1,5-bisphosphate) with CO2 during the process of photosynthesis, combines it with oxygen (O2) instead. This is often the case when the CO2 concentration in the cells of a C3 plant is low compared to the O2 concentration.
Photorespiration has a negative effect on photosynthesis because it leads to a reduction in the net production of organic molecules, such as glucose. During photorespiration, the products of the reaction are not useful for the plant's further metabolism and energy production, which means that energy and resources are wasted. Hence, photorespiration reduces the efficiency of photosynthesis and overall plant growth.
Photorespiration is more likely to occur under specific environmental conditions. It is favored when the CO2 concentration is low or the O2 concentration is high, such as in hot and dry conditions, or in certain agricultural practices that involve restricting CO2 availability to the plant. Moreover, photorespiration is more pronounced when light intensity is high, especially in plants adapted to low light conditions.
To summarize, the enzymatic change of Rubisco combining RuBP with oxygen instead of carbon dioxide has a detrimental effect on photosynthesis by reducing the overall efficiency of the process. It is more likely to occur in environmental conditions with low CO2 concentration, high O2 concentration, and high light intensity.