Difference between C3 and C4 Pathways with Steps and Diagram
Plants use photosynthesis to convert light energy into chemical energy. In this process, plants produce glucose by using atmospheric carbon dioxide and water in the presence of sunlight. As a result, oxygen is released into the atmosphere as a by-product.
The photosynthesis process is divided into two broad phases, photochemical and biosynthetic. C3 and C4 pathways, the topic of this article, are two ways of biosynthetic process. In a biosynthetic process, plants use water and carbon dioxide to produce carbohydrates.
C3 Pathway
Also known as the Calvin cycle, Melvin Kelvin first discovered this process. The majority of the plants on the earth use this process. The first stable product produced by this process is three compound carbon. Hence, the name C3 pathway.
Moreover, this cycle occurs in three steps, these are –
Carboxylation: This step fixates CO2 into a steady organic intermediate. Here CO2 is used to carboxylate RuBP. RuBisCO enzyme catalyses this process.
Reduction: This procedure leads to the creation of glucose. It involves two molecules of NADPH for reduction and two molecules of ATP for phosphorylation.
Regeneration: Regeneration of CO2 acceptor molecule is necessary for a C3 pathway to continue its operations.
C4 Pathway
The C4 pathway of photosynthesis occurs before the C3 pathway. Even though the C3 cycle is found in every plant, the C4 pathway is primarily noticeable in plants of the tropical region.
Furthermore, this process is also known as the Hatch and Slack pathways. The first stable product of this process is a four-carbon compound, hence the name.
Moreover, there are two C4 cycle steps that every plant follow; these are –
At first, when CO2 enters a mesophyll cell, phosphoenolpyruvate (PEP) carboxylase enzymes add it to the three-carbon PEP. It produces a four-carbon compound called oxaloacetate. This substance then turns into an organic acid called malate.
Subsequently, malate is conveyed to the bundle sheath cells deep into the leaf where the oxygen concentration is low. The malate is disintegrated, which releases a molecule of CO2. This CO2 enters the Calvin cycle or C3 pathway afterwards, where the rubisco enzyme transforms it into sugar.
Furthermore, the C4 cycle in plants is useful in hot and arid conditions, as plants close their stomata to save water.
Before moving forward, here are some C4 cycle notes for a quick recap –
C4 cycle is not seen in every plant.
C3 and C4 pathways are not the same. They have different purposes and characteristics.
The C4 cycle is primarily seen in plants in the tropical region.
Moreover, this process helps plants to conserve water.
Examples of C4 pathway plants are crabgrass, corn, sugarcane, etc.
Difference between C3 and C4 Pathway
Some key differences between these two pathways are –
First stable compound
The first compound produced in a C3 cycle is a 3-carbon substance named 3-phosphoglyceric acid. On the other hand, in C4 pathways, the primary stable compound is a 4-carbon compound known as oxaloacetate acid.
Presence in plants
C3 cycle is found in every plant. But the C4 pathway diagram is found primarily in plants of the tropical region.
Fundamental carbon dioxide acceptor
In the case of C3, it is Ribulose Bi Phosphate (RUBP). For C4, it is phosphoenolpyruvate (PEP).
Carboxylase enzyme
In C4 plants it is PEP carboxylase and rubisco. However, in the case of C3, it is only rubisco.
Carbon fixation
Only one carbon fixation occurs in a C3 cycle. In the C4 cycle, double carbon fixation occurs.
Photorespiration
The photorespiration rate in C3 is high. In the case of C4, photorespiration is absent.
C3 and C4 pathways are two essential steps in every photosynthesis process. For more biology study materials like this, you can download the Vedantu app or go through the study materials available on our website. Additionally, you can attend all the live classes with our subject experts and students from across the country.
Install Vedantu’s app today!
Questions
1. The Calvin pathway occurs in which of the following:
Only C3
Only C4
All the photosynthesis plants
Only in C2 plants
Ans: c. All the photosynthesis plants.
2. CO2 fixation happens in the chloroplast of____ during the C4 pathway.
Guard cells
Mesophyll cells
Bundle sheath cells
Spongy parenchyma
Ans: b. Mesophyll cells.
3. The _______ separates the C 4 and C 3 pathways of CAM plants.
Bundle sheath
Mesophyll and bundle sheath cells
Mesophyll and bundle sheath chloroplast
Time
Ans: d. Time
4. The C4 pathway is used by plants that have evolved to dry tropical regions. Is this true or false?
Ans: The statement is true. Plants that use the C4 pathway have unique adaptations that enable them to withstand extreme environmental conditions. Tolerance of high temperatures, responsiveness to high light intensities, and lack of photorespiration are examples of such adaptations.
FAQs on C3 and C4 Pathways in Photosynthesis
1. What are C3 and C4 pathways in photosynthesis?
The C3 and C4 pathways are two different mechanisms of carbon fixation used by plants during photosynthesis to convert carbon dioxide into organic compounds.
- C3 pathway: The most common pathway where the first stable product is a 3-carbon compound called 3-phosphoglycerate (3-PGA).
- C4 pathway: An alternative pathway where the first stable product is a 4-carbon compound called oxaloacetate.
- Both pathways occur in the Calvin cycle but differ in how CO₂ is initially fixed.
2. What is the main difference between C3 and C4 plants?
The main difference between C3 and C4 plants lies in their method of carbon fixation and adaptation to environmental conditions.
- C3 plants fix CO₂ directly using the enzyme RuBisCO in mesophyll cells.
- C4 plants first fix CO₂ using PEP carboxylase in mesophyll cells and then transfer it to bundle sheath cells.
- C4 plants are better adapted to high temperature and low CO₂ conditions.
- C3 plants are more common in cooler and moist climates.
3. Why are C4 plants more efficient than C3 plants?
C4 plants are more efficient than C3 plants because they minimize photorespiration by concentrating CO₂ around RuBisCO.
- PEP carboxylase has a higher affinity for CO₂ and does not bind oxygen.
- CO₂ is transported to bundle sheath cells, increasing its concentration.
- This reduces oxygen competition and energy loss due to photorespiration.
4. What is photorespiration and how is it related to C3 plants?
Photorespiration is a wasteful process in which RuBisCO binds oxygen instead of carbon dioxide, reducing photosynthetic efficiency.
- It commonly occurs in C3 plants under high temperature and low CO₂ conditions.
- It leads to loss of previously fixed carbon and energy.
- C4 plants largely avoid photorespiration due to CO₂ concentration mechanisms.
5. What are examples of C3 and C4 plants?
Common examples of C3 and C4 plants differ based on their photosynthetic adaptation.
- C3 plants: Wheat, rice, barley, and potato.
- C4 plants: Maize, sugarcane, sorghum, and millets.
- C4 plants are typically found in tropical and subtropical regions.
6. Where do C3 and C4 pathways occur in the leaf?
The C3 and C4 pathways occur in different cell types within the leaf.
- In C3 plants, the entire Calvin cycle occurs in mesophyll cells.
- In C4 plants, initial CO₂ fixation occurs in mesophyll cells, and the Calvin cycle occurs in bundle sheath cells.
- C4 plants show a special leaf anatomy called Kranz anatomy.
7. What is Kranz anatomy in C4 plants?
Kranz anatomy is a special leaf structure in C4 plants where bundle sheath cells form a ring around vascular bundles.
- Mesophyll cells surround the bundle sheath cells.
- This arrangement helps concentrate CO₂ in bundle sheath cells.
- It enhances efficiency of the C4 photosynthetic pathway.
8. Which enzyme is responsible for carbon fixation in C3 and C4 pathways?
Different enzymes are responsible for initial carbon fixation in C3 and C4 pathways.
- In C3 plants, carbon fixation is carried out by RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
- In C4 plants, the initial fixation is done by PEP carboxylase.
- RuBisCO functions later in the Calvin cycle in both pathways.
9. How does the C4 pathway reduce photorespiration?
The C4 pathway reduces photorespiration by spatially separating initial CO₂ fixation and the Calvin cycle.
- CO₂ is first fixed into a 4-carbon compound in mesophyll cells.
- This compound is transported to bundle sheath cells and releases CO₂.
- High CO₂ concentration around RuBisCO prevents oxygen binding.
10. Under what environmental conditions are C3 and C4 plants most efficient?
C3 plants are most efficient in cool, moist environments, while C4 plants perform better in hot, dry conditions.
- C3 plants: Optimal in moderate temperature and high CO₂ availability.
- C4 plants: Adapted to high temperature, intense sunlight, and low CO₂ levels.
- C4 plants also show better water-use efficiency.
