Difference between C3 and C4 pathways of photosynthesis with steps and diagram
C3 And C4 Cycle
High crop yield plants are essential for meeting the demands of the ever-growing population and, therefore, need to be ensured that photosynthesis occurs in an optimal way for such plants. As most of the crop plants are C3 in nature (where the first carbon intermediate compounds formed by such plants have three carbon atoms), they tend to bond with the photosynthetic enzyme Rubisco, instead of Carbon dioxide, and thus, wasting energy. These plants then start closing up their stomata to lower water loss, especially in arid regions or hot climates. This is where the evolved C4 pathway comes in as it keeps a high concentration of Carbon dioxide gas in the process, preventing the Rubisco - Oxygen bonding.
However, before going into details about these pathways, here's a detailed version of photosynthesis - the crux to the C3 and C4 pathways.
Photosynthesis is popularly perceived as the formation of energy with sunlight, atmospheric carbon dioxide, and water. This energy then flows throughout the plant, ensuring its nutrition and health. The chemical equation for the process is:
6CO2+ 6H2O ---> C6H12O6 + 6H2O
Generally, The Photosynthesis Process Occurs in Two Main Phases
Photosynthetic: In this phase, the plants utilize the light energy to form energy in ATP and NADPH, which serve as the intermediate for the biosynthetic step.
Biosynthetic: In this phase, the CO2and H2O can combine to yield carbohydrates, and therefore popularly called Carbon Fixation. Different plants obey distinct methods for carbon fixation, and these are known as the C3 and C4 pathways.
C3 Pathway Plants
The C3 pathway plants follow the Calvin cycle in the dark reaction of photosynthesis. Here, the photosynthetic efficiency is lesser because of excess photorespiration. The first step causes the fixation of carbon dioxide by rubisco and, therefore, needs correction. As many as 85% of crop plants like rice, wheat, and trees follow the C3 pathways. These plants also yield a 3-carbon atom acid known as the Phosphoglyceric Acid as its initial product, and the overall process occurs in the following stages:
Carboxylation - Here, the PGA gets generated with the help of the Rubisco enzyme carboxylase.
Reduction - The PGA gets reduced to Adenosine Tri Phosphate, and Nicotinamide Adenine Dinucleotide Phosphate phosphorylate in this stage.
Regeneration - Once it yields glucose, the cycle goes on a loop with the restoration of the RuBP enzyme.
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C4 Pathway Plants
These plants follow an additional step to the usual dark reaction followed by the C4 plants. Here, the first compound that gets formed for these plants has four carbon atoms (also known as the OAA) in it, and therefore, has high photosynthetic abilities. Here is a C4 and pathway circle diagram:
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Found in only 5% of the plants worldwide, the C4 pathway is an ability that comes with tropical desert plants. As soon as the formation of the 3-Carbon ‘ferry’ molecule phosphoenolpyruvate is witnessed, the enzyme works with a PEP carboxylase to form the Oxaloacetic Acid (OAA).
In the next stage, the OAA is further converted into a 4-Carbon compound called Malic acid. The OAA gets reduced into Carbon dioxide and a 3-Carbon molecule that helps in the regeneration of the PEP.
C4 Pathway of Photosynthesis
The C4 plants work on enlarged physiological functions that are a direct connection with the CO2 concentration of these plants, therefore influencing the plant's metabolism. As the cells in the C4 species are enlarged, there is always a close contact between the mesophyll and bundle sheath cells and are interconnected via plasmodesmata. For the direct connection between the bundle sheath and the mesophyll cells, the 5% plants that follow the C4 pathway, have their distinct anatomy of leaves where they have a high vein density, making the ratio of mesophyll and bundle sheath tissues as 1:1. Also termed as the Kranz anatomy, these plants use two-cells mode for the C4 photosynthesis.
Compared to the C3 photosynthesis process, the C4 process takes one or two additional molecules of ATP per fixed particle, without requiring any other reduction equivalents. Such an increase leads to the ATP and NADPH ratio enhancement for the C4 plants, which positively affects ATP production.
Difference Between C3 and C4 Plants
FAQs on C3 and C4 Pathways in Photosynthesis
1. What are C3 and C4 pathways of photosynthesis?
The C3 and C4 pathways of photosynthesis are two different mechanisms by which plants fix carbon dioxide during the light-independent reactions.
- In the C3 pathway, carbon dioxide is fixed directly by the enzyme RuBisCO into a 3-carbon compound called 3-phosphoglycerate (3-PGA).
- In the C4 pathway, carbon dioxide is first fixed into a 4-carbon compound, oxaloacetate, by the enzyme PEP carboxylase.
- C4 plants have a special leaf anatomy called Kranz anatomy, which helps reduce photorespiration.
2. What is the main difference between C3 and C4 plants?
The main difference between C3 and C4 plants lies in how they fix carbon dioxide and manage photorespiration.
- C3 plants use only the Calvin cycle and form a 3-carbon compound as the first stable product.
- C4 plants first form a 4-carbon compound before entering the Calvin cycle.
- C4 plants have Kranz anatomy, while C3 plants do not.
- C4 plants show minimal photorespiration, whereas C3 plants experience higher photorespiration, especially in hot conditions.
3. How does the C3 pathway work step by step?
The C3 pathway works through the Calvin cycle, where carbon dioxide is fixed and converted into glucose.
- Carboxylation: CO2 combines with ribulose-1,5-bisphosphate (RuBP) using RuBisCO to form 3-PGA.
- Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
- Regeneration: RuBP is regenerated to continue the cycle.
4. How does the C4 pathway reduce photorespiration?
The C4 pathway reduces photorespiration by concentrating carbon dioxide around the enzyme RuBisCO.
- CO2 is first fixed by PEP carboxylase in mesophyll cells to form oxaloacetate.
- The 4-carbon compound is transported to bundle sheath cells.
- CO2 is released near RuBisCO, increasing its concentration and preventing oxygen binding.
5. What is Kranz anatomy in C4 plants?
Kranz anatomy is a special leaf structure in C4 plants where bundle sheath cells are arranged in a ring around vascular bundles.
- Mesophyll cells surround the bundle sheath cells.
- Initial CO2 fixation occurs in mesophyll cells.
- The Calvin cycle takes place in bundle sheath cells.
6. Why are C4 plants more efficient in hot climates?
C4 plants are more efficient in hot climates because they minimize photorespiration and conserve water.
- PEP carboxylase has a higher affinity for CO2 and does not bind oxygen.
- CO2 is concentrated in bundle sheath cells, improving carbon fixation.
- Stomata can remain partially closed, reducing water loss.
7. What are examples of C3 and C4 plants?
Examples of C3 and C4 plants differ based on their carbon fixation pathway.
- C3 plants: wheat, rice, barley, and most trees.
- C4 plants: maize, sugarcane, sorghum, and millets.
8. What is photorespiration and how is it related to C3 and C4 pathways?
Photorespiration is a process in which RuBisCO binds oxygen instead of carbon dioxide, reducing photosynthetic efficiency.
- In C3 plants, RuBisCO frequently reacts with oxygen, especially at high temperatures.
- This leads to loss of fixed carbon and energy.
- C4 plants minimize photorespiration by concentrating CO2 around RuBisCO.
9. Do C4 plants require more energy than C3 plants?
Yes, C4 plants require more ATP than C3 plants for carbon fixation.
- The C4 pathway uses additional ATP to regenerate phosphoenolpyruvate (PEP).
- However, this extra energy cost is offset by reduced photorespiration.
- As a result, C4 plants are more efficient under high light intensity and temperature.
10. In which part of the chloroplast do C3 and C4 pathways occur?
In both C3 and C4 plants, the Calvin cycle occurs in the stroma of chloroplasts, but their cellular locations differ.
- In C3 plants, all steps occur in the chloroplasts of mesophyll cells.
- In C4 plants, initial CO2 fixation occurs in mesophyll cells, while the Calvin cycle occurs in chloroplasts of bundle sheath cells.
