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.
In C3 plants, the conservation of water can lead to a phenomenon known as photorespiration. This process becomes more prominent under conditions of water stress or when stomata are closed to reduce water loss. To understand this, let’s break down the key components of the C3 photosynthesis pathway and how it leads to photorespiration.
C3 Photosynthesis Overview:
In C3 plants, carbon dioxide (CO2) is fixed by the enzyme RuBisCO in the Calvin cycle. RuBisCO typically catalyzes the conversion of CO2 and ribulose bisphosphate (RuBP) into a 3-carbon compound, hence the name C3. However, RuBisCO is not highly specific and can also react with oxygen (O2) instead of CO2, especially under certain conditions. When oxygen is fixed instead of carbon dioxide, photorespiration occurs.
Water Conservation and Stomata:
When C3 plants face drought or water scarcity, they reduce the opening of stomata to minimize water loss through transpiration. This reduction in stomatal opening decreases the amount of CO2 available to the plant and increases the concentration of O2 inside the leaf. The high concentration of O2 and low CO2 in these conditions favors photorespiration over photosynthesis.
Photorespiration:
During photorespiration, RuBisCO fixes O2 rather than CO2, leading to the production of a 2-carbon molecule instead of a 3-carbon molecule. This process consumes energy and releases CO2 without producing sugar, reducing the overall efficiency of photosynthesis. Photorespiration is considered wasteful because it essentially competes with the carbon fixation process.
Why It Happens in C3 Plants:
In C3 plants, the primary mode of carbon fixation is inefficient when CO2 levels are low and O2 levels are high, as is the case when stomata are closed to conserve water. This situation triggers photorespiration, which reduces the plant’s ability to efficiently synthesize glucose.
In contrast, C4 and CAM plants have evolved mechanisms to avoid or minimize photorespiration by separating the processes of carbon fixation and the Calvin cycle spatially or temporally, which helps them conserve water while still maintaining high photosynthetic efficiency.