Photosynthesis is a process that uses water, carbon dioxide (CO2) and solar energy to synthesize sugars. It is carried out by many plants, algae and bacteria. In plants and algae, photosynthesis occurs in special parts of the cell called chloroplasts; located in the leaves and stems. Whereas most plants perform what is known as C3 photosynthesis, plants that have adapted to hot environments perform a modified form known as C4 photosynthesis.
In this type of photosynthesis environmental CO2 is first incorporated into 4-carbon acids in cells known as mesophylls. These acids are transported to other cells known as bundle sheath cells. In these cells, the reaction is reversed, CO2 is released and subsequently used in the normal (C3) photosynthetic pathway. The incorporation of CO2 into 3-carbon compounds is catalyzed by an enzyme known as Rubisco.
Advantages of C4 Photosynthesis
In hot and dry environments C4 photosynthesis is more efficient than C3 photosynthesis. This is due to two reasons. The first one is that the system does not undergo photorespiration, a process that runs counter to photosynthesis (see below). The second one is that plants can keep their pores shut longer periods of time, thus avoiding water loss.
This is a process in which, instead of adding CO2 to the growing sugar, Rubisco adds oxygen. In situations in which photosynthesis is taking place fast (at high temperature, high levels of light or both), there is so much O2 available that this reaction becomes a significant problem. C4 plants solve this problem by maintaining a high concentration of CO2 in the relevant portion of the leaf (the bundle sheath cells).
Plants exchange gases, CO2 and O2, with their environment through pores known as stomata. When the stomata are open CO2 can diffuse in to be used in photosynthesis and O2, a product of photosynthesis can diffuse out. However, when the stomata are open the plant also loses water due to transpiration, and this problem is enhanced in hot and dry climates. Plants that perform C4 photosynthesis can keep their stomata closed more than their C3 equivalents because they are more efficient in incorporation CO2. This minimizes their water loss.
Although C4 photosynthesis is clearly advantageous in hot and dry climates, this is not true in cool and moist ones. This is because C4 photosynthesis is more complex: it has more steps and requires a specialized anatomy. For this reason, unless photorespiration or water loss are significant issues, C3 photosynthesis is more effective. This is why the majority of plants perform C3 photosynthesis.
- Plant Physiology; Lincoln Taiz and Eduardo Zeiger; 2002
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