In C3 plants the conservation of water promotes:
photorespiration
photosynthesis
a shift to C4 photosynthesis
the light reactions
the opening of stomata
The Correct Answer and Explanation is
In C3 plants the conservation of water promotes:
Correct Answer: Photorespiration
Explanation:
In C3 plants, the conservation of water is primarily achieved by closing their stomata, which are small openings on the leaf surface that regulate gas exchange. When water is scarce, plants close these stomata to reduce water loss through transpiration. However, this protective mechanism has a significant consequence: it limits the intake of carbon dioxide (CO₂) while allowing oxygen (O₂) to accumulate inside the leaf.
C3 plants are named for the three-carbon compound (3-phosphoglycerate) produced in the first step of the Calvin cycle. The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is responsible for fixing CO₂ in the Calvin cycle. However, RuBisCO is not entirely specific to CO₂—it can also bind to O₂. When CO₂ levels drop (due to closed stomata) and O₂ levels rise inside the leaf, RuBisCO begins to catalyze a wasteful reaction with O₂ instead of CO₂. This process is called photorespiration.
Photorespiration results in the production of phosphoglycolate, which cannot be used in the Calvin cycle and must be recycled through a complex pathway that consumes ATP and releases CO₂—essentially undoing the work of photosynthesis. Thus, instead of producing sugars, the plant loses fixed carbon and energy, making photorespiration inefficient and detrimental to the plant’s productivity.
Therefore, under conditions where water conservation is necessary (e.g., hot, dry climates), C3 plants suffer from increased rates of photorespiration due to the reduction in internal CO₂ concentration. This explains why some plants have evolved the C4 pathway (like corn and sugarcane), which minimizes photorespiration by spatially separating carbon fixation from the Calvin cycle.
To summarize, in C3 plants, conserving water leads to stomatal closure, which decreases CO₂ availability and increases O₂ concentration inside the leaf, thereby promoting photorespiration.