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 :
The correct answer is photorespiration.
Explanation:
C3 plants are characterized by a specific pathway for photosynthesis in which the enzyme RuBisCO fixes carbon dioxide (CO₂) into a 3-carbon compound during the Calvin cycle. However, a significant issue with C3 photosynthesis is the enzyme’s tendency to also fix oxygen (O₂) instead of CO₂, particularly when stomata are closed to conserve water. This phenomenon is known as photorespiration.
What is Photorespiration?
Photorespiration occurs when the enzyme RuBisCO binds to oxygen instead of carbon dioxide. This happens more frequently under conditions where stomata close to prevent water loss, limiting the availability of CO₂ inside the leaf. When the concentration of oxygen rises and CO₂ concentration decreases inside the leaf, RuBisCO starts catalyzing the reaction with oxygen, leading to the production of a compound that eventually has to be metabolized in a process that consumes energy and releases CO₂. This process is inefficient because it leads to a net loss of carbon, which could otherwise be used in the production of sugars through photosynthesis.
Why Does Water Conservation Promote Photorespiration?
Water conservation is crucial for plants, especially under drought or high-temperature conditions. In these situations, C3 plants close their stomata to minimize water loss through transpiration. However, closing the stomata also reduces the influx of CO₂ into the leaf, which lowers the concentration of CO₂ available for the Calvin cycle. When this happens, the likelihood of photorespiration increases because RuBisCO, in the absence of sufficient CO₂, is more likely to react with oxygen instead.
Comparison to C4 and CAM Plants:
C4 and CAM plants have evolved specialized mechanisms to minimize photorespiration. C4 plants, for example, concentrate CO₂ in bundle sheath cells, which reduces RuBisCO’s exposure to oxygen, while CAM plants fix CO₂ at night when stomata are open and can store it as malate. These adaptations help avoid photorespiration and are more water-efficient in dry environments.
In summary, in C3 plants, water conservation (through stomatal closure) can lead to a greater occurrence of photorespiration, a less efficient process compared to regular photosynthesis.