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 is the C3 pathway of photosynthesis?
The C3 pathway, also known as the Calvin cycle, is the primary mechanism for carbon fixation in the majority of plants. In this process, atmospheric carbon dioxide is directly fixed by the enzyme RuBisCO. The first stable product formed is a three-carbon compound called 3-phosphoglyceric acid (3-PGA), which is why it is named the C3 pathway. This entire process occurs within the mesophyll cells of the leaves.
2. What is the C4 pathway and how does it work?
The C4 pathway, or Hatch-Slack pathway, is an adaptation found in plants in hot, dry climates. It acts as a CO2-concentrating mechanism before the C3 cycle. Initially, CO2 is fixed in mesophyll cells by the enzyme PEP carboxylase to form a four-carbon compound, oxaloacetate. This compound is then transported to specialised bundle sheath cells, where it releases CO2 for the Calvin cycle to proceed efficiently, minimising water loss.
3. What are the key differences between the C3 and C4 pathways?
The main differences between C3 and C4 pathways relate to their efficiency, anatomy, and enzymes involved. Key distinctions include:
- First Product: The C3 pathway's first stable product is a 3-carbon compound (3-PGA), while the C4 pathway's is a 4-carbon compound (oxaloacetate).
- Primary CO2 Acceptor: In C3 plants, it is RuBP (Ribulose-1,5-bisphosphate). In C4 plants, it is PEP (Phosphoenolpyruvate).
- Leaf Anatomy: C3 plants have standard leaf anatomy, whereas C4 plants exhibit specialised Kranz anatomy.
- Photorespiration: C3 plants experience significant photorespiration, which reduces efficiency. C4 plants have a mechanism to suppress photorespiration.
4. What are some common examples of C3 and C4 plants?
Most plants on Earth are C3 plants. Common examples include rice, wheat, soybeans, and potatoes. C4 plants are typically those adapted to tropical or arid conditions and include economically important crops like maize (corn), sugarcane, sorghum, and millets.
5. Why is the C4 pathway an important adaptation for plants in hot and dry climates?
The C4 pathway is a crucial adaptation because it significantly increases the efficiency of photosynthesis in high-temperature and high-light conditions. The enzyme PEP carboxylase has a high affinity for CO2 and is unaffected by oxygen levels, which prevents the wasteful process of photorespiration. By concentrating CO2 in the bundle sheath cells, C4 plants can keep their stomata partially closed to conserve water while still fixing enough carbon for growth, giving them a competitive advantage in arid environments.
6. What is Kranz anatomy and what is its significance in the C4 pathway?
Kranz anatomy refers to the specialised leaf structure found in C4 plants, characterised by a ring of large bundle sheath cells surrounding the vascular bundles (veins). These cells have thick walls, contain numerous chloroplasts, and are impermeable to gases. Its significance is central to the C4 pathway: it provides a site where CO2 can be concentrated and isolated from atmospheric oxygen, allowing the Calvin cycle to run efficiently without interference from photorespiration.
7. Why is photorespiration a problem for C3 plants but almost absent in C4 plants?
Photorespiration is a wasteful process where the enzyme RuBisCO binds with oxygen instead of carbon dioxide, especially in hot conditions. This reduces the efficiency of the C3 pathway as it consumes energy and releases already-fixed CO2. C4 plants effectively eliminate this problem by using PEP carboxylase for initial CO2 fixation, which only binds to CO2. The subsequent delivery of concentrated CO2 to the bundle sheath cells ensures that RuBisCO is always in a high-CO2 environment, preventing it from binding with oxygen.
8. If the Calvin cycle (C3 pathway) occurs in all photosynthetic plants, what is the specific role of the initial steps in the C4 pathway?
The initial steps of the C4 pathway function as a highly efficient 'CO2 pump'. While the Calvin cycle is the ultimate process that produces sugars, its enzyme RuBisCO is inefficient in hot, dry conditions. The C4 mechanism's specific role is to capture atmospheric CO2 in the mesophyll cells and transport it in a concentrated form to the bundle sheath cells. This ensures the Calvin cycle operates at maximum capacity, even when stomata are closed to conserve water.
