How Do C3 and C4 Pathways Differ in Plants?
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 of Photosynthesis Explained
1. What is the fundamental difference between the C3 and C4 pathways in photosynthesis?
The fundamental difference lies in the first stable product formed after carbon dioxide (CO₂) fixation. In the C3 pathway, the first stable product is a 3-carbon compound called 3-phosphoglyceric acid (3-PGA). In the C4 pathway, the first stable product is a 4-carbon compound, typically oxaloacetic acid (OAA), which is then converted to other 4-carbon acids like malate or aspartate.
2. Why are the photosynthetic pathways named C3 and C4?
The names are based on the number of carbon atoms in the first stable compound produced during the carbon fixation stage. The C3 pathway is named so because its first product, 3-PGA, contains three carbon atoms. Similarly, the C4 pathway is named for its first product, oxaloacetic acid, which contains four carbon atoms.
3. How does the special leaf anatomy of C4 plants contribute to their higher efficiency?
C4 plants possess a unique leaf structure called Kranz anatomy. This involves two types of photosynthetic cells: the outer mesophyll cells and the inner bundle-sheath cells arranged in a wreath-like manner around the vascular bundles. The C4 pathway spatially separates CO₂ fixation steps. CO₂ is first fixed in the mesophyll cells by the enzyme PEP carboxylase, which has a high affinity for CO₂. The resulting 4-carbon acid is then transported to the bundle-sheath cells, where it releases CO₂, creating a high concentration around the RuBisCO enzyme. This mechanism minimises photorespiration and makes C4 photosynthesis more efficient, especially in hot and dry conditions.
4. What are some common examples of C3 and C4 plants?
Understanding the types of plants helps in connecting these concepts to the real world. Here are some common examples:
- C3 Plants: Most temperate plants are C3. Examples include rice, wheat, soybean, potatoes, and spinach.
- C4 Plants: These plants are typically adapted to tropical or arid climates. Examples include maize (corn), sugarcane, sorghum, and amaranth.
5. What is photorespiration, and why is it a major issue in C3 plants but not in C4 plants?
Photorespiration is a wasteful process where the enzyme RuBisCO binds to oxygen (O₂) instead of carbon dioxide (CO₂), especially at high temperatures and low CO₂ concentrations. In C3 plants, this leads to the loss of a previously fixed carbon atom and consumption of energy (ATP) without producing any sugar, thus reducing photosynthetic output. C4 plants have evolved to virtually eliminate photorespiration. Their CO₂-pumping mechanism, using PEP carboxylase and Kranz anatomy, ensures a high concentration of CO₂ in the bundle-sheath cells where RuBisCO is located, thereby preventing RuBisCO from binding with O₂.
6. What are the primary CO₂-accepting enzymes in the C3 and C4 cycles?
The primary enzyme that accepts atmospheric CO₂ is a key differentiator:
- In the C3 cycle, the primary CO₂ acceptor is a 5-carbon compound, Ribulose-1,5-bisphosphate (RuBP), and the reaction is catalysed by the enzyme RuBisCO.
- In the C4 cycle, the initial CO₂ acceptor in the mesophyll cells is a 3-carbon compound, phosphoenolpyruvate (PEP), and this reaction is catalysed by the enzyme PEP carboxylase. RuBisCO is still used later in the bundle-sheath cells but is not the initial acceptor from the atmosphere.
7. Under what environmental conditions would a C4 plant have a significant advantage over a C3 plant?
A C4 plant has a significant photosynthetic advantage over a C3 plant under conditions of high temperature, intense sunlight, and water stress (dryness). In such environments, plants tend to close their stomata to conserve water, which lowers the CO₂ concentration inside the leaf and increases O₂ concentration. For a C3 plant, this triggers high rates of wasteful photorespiration. A C4 plant, with its highly efficient CO₂ concentrating mechanism, can continue to photosynthesise effectively even with partially closed stomata, making it better adapted to these challenging conditions.
