Difference Between C3 and C4 Plants in Photosynthesis

All plants use light energy from the sun as their ultimate source of energy for survival. This energy is captured by the process known as “Photosynthesis” (“photo” meaning light). Photosynthesis is a complex pathway which is used by plants to fix carbon, present in the atmosphere, into sugar (carbohydrate) molecules. All plant species rely on these products  to produce their source of energy.

What are C3 Plants

Plants have evolved to capture the ultimate source of energy in different ways. From an evolutionary point of view, plants first developed the “C3 pathway”. It is the simplest form of photosynthesis, also known as the Calvin cycle. A typical plant on the earth that uses photosynthesis is a C3 plant. In this process 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.  Approximately 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 hence the name “C3 pathway”.

The Calvin cycle is useful to convert CO2 into carbon. It eliminates greenhouse gas (CO2) from the atmosphere efficiently. The 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, and orchard grass. 

Since C3 pathway is a more primitive pathway than C4, they have no known adaptive features to combat photorespiration. In the Calvin cycle, approximately 25% of the RuBP is oxygenated (addition of O₂ instead of CO₂) by the enzyme RUBISCO, an undesirable feature, causing wastage of energy, as it cannot further undergo the Calvin cycle. 

What are the C4 Plants?

C4 plants are known to have evolved from the C3 plants. Various evolutionary trends suggest that the development of the C4 pathway was in response to the low carbon dioxide levels in the atmosphere. Thus, the C4 pathway confers an evolutionary advantage to these plants. They possess a particular type of leaf anatomy and 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 crop yields. Researchers are focusing on understanding the evolution of the C4 plant’s 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.

Comparison 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 if 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 carbon dioxide directly in the chloroplast.

6. C3 plants have a 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 networks. 

Difference Between C3 and C4 Plants