C3 And C4 Pathways - Steps, Differences And FAQ
What Is Photosynthesis?
Photosynthesis is the process through which plants convert light energy to chemical energy in the form of glucose. This can happen through different pathways, mainly the C3 and C4 pathways. Knowing what makes the two different plays a key role in the NEET exam, as these adaptations by plants to different environmental conditions are brought out. difference between C3 and C4 Plants
C3 and C4 plants are fundamentally very different in photosynthetic pathways, adaptations, and environmental preferences. Differences between C3 and C4 Plants are explained herein in detail.
Commonly Asked Questions
Q: What is photorespiration and why is it a problem for C3 plants?
A:
Photorespiration is a process where RuBisCO fixes oxygen instead of CO2, producing a 2-carbon compound that needs to be recycled, wasting energy and releasing previously fixed CO2. This is a problem for C3 plants, especially in hot, dry conditions, as it reduces photosynthetic efficiency and increases water loss.
Q: What is the primary product of carbon fixation in the C3 pathway?
A:
The primary product of carbon fixation in the C3 pathway is 3-phosphoglycerate (3-PGA), a 3-carbon compound. This is formed when RuBisCO adds CO2 to ribulose-1,5-bisphosphate, splitting the resulting 6-carbon compound into two 3-carbon molecules of 3-PGA.
Q: What is the "CO2 compensation point" and how does it differ between C3 and C4 plants?
A:
The CO2 compensation point is the CO2 concentration at which the rate of photosynthesis equals the rate of respiration, resulting in no net CO2 fixation. C3 plants have a higher CO2 compensation point (around 50-100 ppm) compared to C4 plants (0-10 ppm). This difference reflects the greater efficiency of C4 plants in fixing CO2 at low concentrations.
Q: What is the significance of the "C4 rice" project?
A:
The "C4 rice" project is an ambitious effort to engineer the more efficient C4 photosynthetic pathway into rice, which is naturally a C3 plant. The goal is to increase rice productivity and resource use efficiency, particularly in the face of climate change and growing global food demand. This involves significant changes to rice leaf anatomy and biochemistry.
Q: What is the significance of the "C4 cycle" in C4 photosynthesis?
A:
The "C4 cycle" refers to the additional steps in C4 photosynthesis that concentrate CO2 in the bundle sheath cells. This cycle involves the initial fixation of CO2 in mesophyll cells, transport of 4-carbon compounds to bundle sheath cells, decarboxylation to release CO2, and return of the 3-carbon compound to mesophyll cells. This cycle effectively pumps CO2 into bundle sheath cells, creating a CO2-rich environment for RuBisCO.
C3 Vs C4 Plants
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| Photosynthesis Pathway | C3 pathway (Calvin cycle) | C4 pathway (Hatch-Slack pathway) |
| First Stable Product | 3-phosphoglycerate (3-PGA) | Oxaloacetate (4-carbon compound) |
| Cell Types for Photosynthesis | Only mesophyll cells | Mesophyll and bundle sheath cells |
| Environmental Adaptation | Prefer cool, wet climates | Adapted for hot, dry conditions |
| Abundance | About 95% of green plants | About 5% of green plants |
| Anatomy | No Kranz anatomy | Has Kranz anatomy |
| Chloroplast Types | Only granal chloroplasts | Both granal and agranal chloroplasts |
| Carbon Fixation Frequency | Fixes CO2 once | Fixes CO2 twice |
| Optimal Temperature | Lower optimum temperature for photosynthesis | Higher optimum temperature for photosynthesis |
| Photorespiration | Not suppressed | Suppressed |
| Stomatal Behavior | Photosynthesis occurs only when stomata are open | Can photosynthesize even when stomata are closed |
Conclusion
The differences between the C3 and C4 pathways also bring out various adaptations of plants to their environment. Most plants of the C3 type are confined to temperate regions, while C4 plants dominate areas that are hotter and drier. These pathways are particularly important in Plant physiology and ecology for change studies in climate and agricultural practices.
Commonly Asked Questions
Q: What are C3 and C4 pathways in photosynthesis?
A:
C3 and C4 pathways are two different carbon fixation processes in plants during photosynthesis. The C3 pathway is the most common and occurs in most plants, while the C4 pathway is an adaptation found in some plants to improve efficiency in hot, dry environments. The names come from the number of carbon atoms in the first stable compound formed during carbon fixation.
Q: Why did some plants evolve the C4 pathway?
A:
Plants evolved the C4 pathway as an adaptation to hot, dry environments. This pathway helps reduce photorespiration, a process that wastes energy and water in C3 plants when temperatures are high and water is scarce. C4 plants can concentrate CO2 around the enzyme RuBisCO, making photosynthesis more efficient in these conditions.
Q: What is the key enzyme in the C3 pathway?
A:
The key enzyme in the C3 pathway is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enzyme catalyzes the first major step of carbon fixation, adding CO2 to a 5-carbon sugar (ribulose bisphosphate) to form two 3-carbon compounds.
Q: How does the C4 pathway differ from the C3 pathway in terms of initial CO2 fixation?
A:
In the C4 pathway, the initial CO2 fixation occurs in mesophyll cells using the enzyme PEP carboxylase, which adds CO2 to phosphoenolpyruvate (PEP) to form a 4-carbon compound. In contrast, the C3 pathway fixes CO2 directly using RuBisCO to form a 3-carbon compound in a single cell type.
Q: What is the significance of bundle sheath cells in C4 plants?
A:
Bundle sheath cells are crucial in C4 plants as they are the site of the Calvin cycle (carbon fixation). These cells have a high concentration of RuBisCO and receive the 4-carbon compounds from mesophyll cells. The CO2 is released here, creating a CO2-rich environment that increases the efficiency of RuBisCO and reduces photorespiration.
Frequently Asked Questions (FAQs)
Q: How do C3 and C4 plants differ in their photosynthetic quantum yield?
A:
Photosynthetic quantum yield refers to the amount of CO2 fixed per quantum of light absorbed. C3 plants generally have a higher quantum yield than C4 plants under low light and moderate temperatures. However, C4 plants maintain a more stable quantum yield under high light and temperature conditions where C3 plants suffer from increased photorespiration.
Q: How do C3 and C4 plants differ in their response to elevated ozone levels?
A:
C4 plants are generally more resistant to ozone damage compared to C3 plants. This is partly due to their lower stomatal conductance, which reduces ozone entry into the leaves. Additionally, the compartmentalization in C4 leaves may provide some protection to the sensitive photosynthetic apparatus in the bundle sheath cells.
Q: What is the role of aspartate in some C4 photosynthesis subtypes?
A:
In some C4 subtypes (NAD-ME and PEP-CK types), aspartate serves as the 4-carbon compound that transports CO2 from mesophyll to bundle sheath cells, instead of malate. Aspartate is formed from oxaloacetate in the mesophyll cells and is then transported to the bundle sheath cells where it's decarboxylated to release CO2.
Q: How do C3 and C4 plants differ in their stomatal density?
A:
C4 plants generally have lower stomatal density (number of stomata per unit leaf area) compared to C3 plants. This is because the CO2 concentrating mechanism in C4 plants allows them to maintain high photosynthetic rates even with lower CO2 intake. The lower stomatal density contributes to the higher water use efficiency of C4 plants.
Q: How do C3 and C4 plants differ in their response to salinity stress?
A:
C4 plants generally show better tolerance to salinity stress compared to C3 plants. This is partly due to their higher water use efficiency and ability to maintain photosynthesis with partially closed stomata, which helps in reducing salt uptake. Additionally, the compartmentalization in C4 leaves may provide more options for managing ion balance.
Q: How do C3 and C4 plants differ in their leaf nitrogen content?
A:
C4 plants typically have lower leaf nitrogen content compared to C3 plants when operating at similar photosynthetic rates. This is because C4 plants require less RuBisCO, a nitrogen-rich enzyme, due to their CO2 concentrating mechanism. The lower nitrogen requirement contributes to the higher nitrogen use efficiency of C4 plants.
Q: How do C3 and C4 plants differ in their response to low light conditions?
A:
C3 plants generally perform better than C4 plants under low light conditions. This is because the extra energy required for the CO2 concentrating mechanism in C4 plants becomes a disadvantage when light is limiting. C3 plants, with their simpler and less energy-intensive carbon fixation process, can maintain higher photosynthetic rates in shade or cloudy conditions.
Q: What is the role of PEP carboxylase in C4 photosynthesis?
A:
PEP carboxylase is the primary CO2-fixing enzyme in the mesophyll cells of C4 plants. It catalyzes the addition of CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate, a 4-carbon compound. This step is crucial for concentrating CO2 before it's delivered to RuBisCO in the bundle sheath cells.
Q: How do C3 and C4 plants differ in their water use efficiency?
A:
C4 plants generally have higher water use efficiency than C3 plants. This means they can fix more carbon per unit of water lost through transpiration. This increased efficiency is due to the CO2 concentrating mechanism, which allows C4 plants to maintain high photosynthetic rates even with partially closed stomata, reducing water loss.
Q: How do C3 and C4 plants differ in their light saturation points?
A:
C4 plants generally have higher light saturation points than C3 plants. This means C4 plants can continue to increase their photosynthetic rate at higher light intensities, while C3 plants reach their maximum rate at lower light levels. This difference is due to the CO2 concentrating mechanism in C4 plants, which allows them to utilize more light energy efficiently.
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