Question

Explain what is the advantage of the C4 plants(in 250 words)

Answer 1

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.

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.

C4 carbon fixation or the Hatch–Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide (CO2) to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. "C4" refers to the four-carbon molecule that is the first product of this type of carbon fixation.

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.

Photorespiration

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).

Water Loss

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.

C4 plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or CO2 limitation. When grown in the same environment, at 30 °C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277. This increased water use efficiency of C4 grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.

Today, C4 plants represent about 5% of Earth's plant biomass and 3% of its known plant species.Despite this scarcity, they account for about 23% of terrestrial carbon fixation. Increasing the proportion of C4 plants on earth could assist biosequestration of CO2 and represent an important climate change avoidance strategy

Present-day C4 plants are concentrated in the tropics and subtropics (below latitudes of 45 degrees) where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in C3 plants.

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