Photosynthesis is a complex pathway which is used by plants to fix carbon, present in the atmosphere, into sugar. All plant species rely on this process to produce their source of energy.
A typical plant on the earth that uses photosynthesis is a C3 plant. Photosynthesis is the process when carbon dioxide enters a plant through its stomata, and the enzyme Rubisco fixes carbon into sugar using the Calvin cycle. It fuels plant growth. This fixation of carbon dioxide by rubisco is the first step of the Calvin cycle. The plants that use this mechanism of carbon fixation are called C3 plants. Approx 95% of plants on the earth are C3 plants. They are also known as temperate plants.
The photosynthesis process can take place only when the micropores (stomata) on leaves are open. The leaves of C3 plants do not show kranz anatomy. C3 plants exhibit the C3 pathway. It is the three-carbon compound (3-PGA). Here the first carbon compound produced has three carbon atoms.
The Calvin cycle is useful to convert CO2 into carbon. It eliminates greenhouse gas (CO2) from the atmosphere efficiently. Calvin cycle helps plants to store energy for a more extended period.
C3 plants are highly rich in proteins. They can be annual perennial. Some of the C3 plant examples are wheat, rye, oats, orchard grass.
C4 plants possess a particular type of leaf anatomy. They use Phosphoenolpyruvate carboxylase (PEP enzyme) instead of photorespiration to enter the Calvin cycle. Enzymes of C4 metabolism are regulated by light. PEP enzyme is more attracted to CO2 molecules and reacts less with O2 molecules. PEP carboxylase does not tend to bind oxygen.
This process takes place in the mesophyll cells (spongy cells in the middle of the leaf) instead of the stomata where CO2 and O2 enter the plant. The light-dependent reaction occurs in mesophyll cells, and the Calvin cycle occurs in bundle-sheath cells around the leaf veins. Carbon dioxide present in the atmosphere is fixed in the mesophyll cells to form a pure 4-carbon organic acid (oxaloacetate) by the non-rubisco enzyme.
The 4-carbon organic acid is then converted to a similar molecule, called malate, that can be transported into the bundle-sheath cells. Inside the bundle-sheath cells, malate breaks down and releases a molecule of CO2.
Enzymes of C4 metabolism - PEP enzyme
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Then the rubisco fixes the carbon through the Calvin cycle, the same as by C3 plants in photosynthesis.
C4 plants exhibit the C4 pathway. Examples are maize, sorghum, and sugarcane. The leaves possess kranz anatomy. Approx 5% of plants on earth are C4 plants. C4 plants examples are pineapple, corn, sugar cane, etc.
C4 photosynthesis is capable of increasing the crop yields. Researchers are focusing on understanding the evolution of C4 plants metabolism better, in an attempt to engineer important crops with more energy and water efficiency because they use less water and can grow in conditions of drought too.
A Diagram showing C3 and C4 photosynthesis
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Let’s explain more to understand the similarities and differences between C3 and C4 plants.
1. C4 plants have 50% higher photosynthesis efficiency than C3 plants.
2. Unlike C4 plants, C3 plants consist of 3-phosphoglycerate with three carbon atoms.
3. C4 plants have better robustness no matter the objective function is biomass synthesis or CO2 fixation.
4. C4 plants are more productive in hot and dry climates than C3 products because they use 3-fold less water and can grow in conditions of drought or high temperature.
5. Unlike C4 plants, C3 plants reduce into carbon dioxide directly in the chloroplast.
6. C3 plants have denser topology than C4 plants.
7. C3 Plants have less modularity than C4 plants.
8. C4 plants have more carbon dioxide than C3 plants.
9. C4 has higher radiation use efficiency than C3 plants
10. C3 photosynthesis uses the Calvin cycle only for carbon fixation catalyzed by Rubisco, inside the chloroplast in mesophyll cells. While C4 plants photosynthesis activities are divided between mesophyll and bundle sheath cells where carbon fixation is catalyzed by phosphoenolpyruvate carboxylase (PEPC).
The Systematic Comparison of C3 and C4 Plants can be made through Metabolic Network.
1. Is there any similarity between C3 and C4 plants?
Yes, there are the following similarities in C3 and C4 plants:
All the essential reactions in the C3 network are also critical to C4.
Both the plants fix energy from sunlight.
Both are the type of dark reactions of photosynthesis.
Basic metabolism of C4 plants is similar to C3.
Calvin cycle is correlated in both C3 and C4 networks.
Both follow the concept of dark reactions of photosynthesis.
2. Which type of plants is productive and efficient?
C3 Plant and C4 Plants both are productive, but C4 plants are more productive and efficient. C4 plants examples are corn, sorghum, sugarcane, millet, and Panicum virgatum switchgrass.
C3 plants fix the carbon dioxide with the help of Rubisco through a process called photorespiration. O2 can bind to Rubisco instead of CO2. In this way, O2 reduces C3 plant photosynthetic efficiency and water use efficiency. As a result, C4 plants are more productive than C3 plants, even in high-temperature environments.
So it can be undoubtedly said that the C4 pathway is more efficient than the C3 pathway in the sense of carbon fixation.