An Introduction to C3 and C4 Pathways
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.
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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
1. What are the C3 and C4 cycle?
C3 and C4 cycles are two parts of the overall photosynthesis process. More preciously, these are two sections of the biosynthesis process. In this process, plants use water and carbon dioxide to make carbohydrates.
2. What is C4 pathway cycle?
The C4 pathway cycle is a part of the biosynthesis process, and it occurs before the C3 period. This process is also known as the Hatch and Slack pathways. The first stable product of this process is a four-carbon compound (oxaloacetate acid), hence the name. Plants that belong to the tropical desert regions follow this pathway.
3. Which plants use the C4 cycle?
Notably, plants from the tropical region use this process. Owing to the lack of water in such harsh conditions, these plants use this process to conserve water. C4 plants produce a lot of energy and are productive in hot and dry climatic conditions. Examples are sugarcane, sorghum, maize, millet, etc.
4. What are the similarities between C3 and C4 plants?
The similarities between C3 and C4 plants are:
Both plants are examples of dark reactions to photosynthesis.
These plants store energy from the sunlight.
The light reactions in both are the same.
Carbohydrates are synthesized by both plants.
For photosynthesis, they require chloroplasts.
RuBP accepts CO2 in both C3 and C4 plants.
5. How does the C3 pathway of photosynthesis differ from the C4 pathway?
The primary acceptor of CO2 in the C3 pathway is RuBP, a six-carbon molecule, whereas the primary acceptor of CO2 in the C4 pathway is a PEP, a three-carbon compound.
The stable product in the C3 pathway is 3-phosphoglycerate, whereas the stable product in the C4 pathway is oxaloacetic acid.
The C3 pathway occurs in the leaves' mesophyll cells, whereas the C4 pathway occurs in the leaves' mesophyll and bundle sheath cells.
When compared to the C4 pathway, the C3 pathway has a slower carbon-fixation process.
3 ATP are consumed in the C3 pathway to repair one CO2, whereas 2 ATP are consumed in the C4 pathway to fix one CO2.
6. Where can we get the previous year’s question papers solutions?
You can get the previous years question papers solutions at Vedantu's website. Go to the site and click on the study material column. Then click on the previous year papers. And the last step is to select the grade for which you are looking.
