C3 And C4 Pathways - Steps, Differences And FAQ

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:57 PM IST

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.

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.

Frequently Asked Questions (FAQs)

1. What are C3 plants?

C3 plants are plants wherein 3-phosphoglycerate, or 3-PGA, a 3-carbon compound, is the initial product of photosynthesis.

2. What are C4 plants?

C4 plants are plants that use the C4 photosynthetic pathway to produce a 4-carbon compound, namely oxaloacetate.

3. Where are C3 plants commonly found?

C3 plants are usually found in cool and wet environments such as temperate climates.

4. What is Kranz's anatomy?

C4 plants have a specialized leaf anatomy called Kranz anatomy, which provides higher efficiency in carbon fixation.

5. How does photorespiration affect C3 plants?

In C3 plants, photorespiration is not suppressed; the fixed carbon is wasted in this process, and thus photosynthetic efficiency decreases.

6. What are C3 and C4 pathways in photosynthesis?

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.

7. Why did some plants evolve the C4 pathway?

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.

8. What is the key enzyme in the C3 pathway?

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.

9. How does the C4 pathway differ from the C3 pathway in terms of initial CO2 fixation?

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.

10. What is the significance of bundle sheath cells in C4 plants?

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.

11. What is photorespiration and why is it a problem for C3 plants?

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.

12. What is the primary product of carbon fixation in the C3 pathway?

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.

13. What is the "CO2 compensation point" and how does it differ between C3 and C4 plants?

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.

14. What is the significance of the "C4 rice" project?

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.

15. What is the significance of the "C4 cycle" in C4 photosynthesis?

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.

16. What are some examples of C4 plants?

Some common examples of C4 plants include corn (maize), sugarcane, sorghum, millet, amaranth, and many tropical grasses. These plants are often adapted to hot, sunny environments and are important in agriculture, especially in warmer regions.

17. Why is the C4 pathway considered more energy-intensive than the C3 pathway?

The C4 pathway is more energy-intensive because it requires additional ATP for the regeneration of PEP in the mesophyll cells. This extra energy cost is used to pump CO2 from mesophyll to bundle sheath cells, creating the CO2 concentration mechanism. However, this energy investment is offset by increased efficiency in hot, dry conditions.

18. What role does malate play in C4 photosynthesis?

Malate is an important intermediate in C4 photosynthesis. In many C4 plants, the 4-carbon compound oxaloacetate is converted to malate in the mesophyll cells. Malate then travels to the bundle sheath cells, where it's decarboxylated to release CO2 for the Calvin cycle, along with pyruvate which returns to the mesophyll cells.

19. What is the role of carbonic anhydrase in C4 photosynthesis?

Carbonic anhydrase plays a crucial role in C4 photosynthesis by catalyzing the rapid conversion of CO2 to bicarbonate (HCO3-) in the mesophyll cells. This is important because PEP carboxylase, the initial CO2-fixing enzyme in C4 plants, uses HCO3- rather than CO2 as its substrate.

20. What is the significance of the bundle sheath conductance in C4 plants?

Bundle sheath conductance refers to the permeability of bundle sheath cells to CO2. In C4 plants, low bundle sheath conductance is crucial for maintaining the CO2 concentrating mechanism. If the conductance is too high, CO2 can leak out of the bundle sheath cells, reducing the efficiency of the C4 pathway.

21. What is the role of NADP-malic enzyme in some C4 plants?

NADP-malic enzyme is important in one of the C4 subtypes (NADP-ME type). In these plants, malate transported to the bundle sheath cells is decarboxylated by NADP-malic enzyme, releasing CO2 for the Calvin cycle and producing NADPH. This NADPH can then be used in the Calvin cycle, providing an additional source of reducing power.

22. What is the role of pyruvate orthophosphate dikinase (PPDK) in C4 photosynthesis?

Pyruvate orthophosphate dikinase (PPDK) is a key enzyme in C4 photosynthesis that regenerates phosphoenolpyruvate (PEP) from pyruvate in the mesophyll cells. This step is crucial for maintaining the C4 cycle, as it provides the substrate (PEP) for the initial CO2 fixation by PEP carboxylase.

23. What is the significance of the CO2 leakage from bundle sheath cells in C4 plants?

CO2 leakage from bundle sheath cells in C4 plants refers to the diffusion of concentrated CO2 back to the mesophyll cells. While some leakage is inevitable, excessive leakage can significantly reduce the efficiency of the C4 pathway. The balance between CO2 concentration and leakage is a key factor in the overall efficiency of C4 photosynthesis.

24. What is the role of phosphoenolpyruvate carboxykinase (PEPCK) in some C4 plants?

Phosphoenolpyruvate carboxykinase (PEPCK) is the primary decarboxylating enzyme in one subtype of C4 photosynthesis (the PEPCK type). In these plants, PEPCK decarboxylates oxaloacetate in the bundle sheath cells, releasing CO2 for the Calvin cycle and producing PEP, which returns to the mesophyll cells.

25. How do C3 and C4 plants differ in their response to herbivory?

C4 plants often show greater resistance to herbivory compare

26. How does the C4 pathway help plants conserve water?

The C4 pathway helps plants conserve water by allowing them to keep their stomata partially closed in hot, dry conditions. This is possible because the CO2 concentration mechanism allows efficient photosynthesis even with less CO2 intake. Partially closed stomata reduce water loss through transpiration while maintaining photosynthetic efficiency.

27. How do C4 plants minimize photorespiration?

C4 plants minimize photorespiration by concentrating CO2 around RuBisCO in the bundle sheath cells. This high CO2 concentration outcompetes oxygen for the active site of RuBisCO, greatly reducing the occurrence of photorespiration and increasing photosynthetic efficiency.

28. Can a plant use both C3 and C4 pathways?

While most plants use either C3 or C4 pathways exclusively, some plants, like certain species of chenopods, can switch between C3 and C4 pathways depending on environmental conditions. This adaptation is called C3-C4 intermediate photosynthesis.

29. How does the anatomy of C4 leaves differ from C3 leaves?

C4 leaves have a distinct "Kranz anatomy" characterized by a ring of bundle sheath cells surrounding the vascular bundles, with mesophyll cells arranged radially around them. C3 leaves lack this specialized arrangement and have a more uniform distribution of mesophyll cells.

30. What is the role of PEP carboxylase in C4 photosynthesis?

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.

31. How do C3 and C4 plants differ in their optimal temperature ranges for photosynthesis?

C3 plants generally have optimal photosynthetic rates at moderate temperatures (20-30°C) and decline at higher temperatures due to increased photorespiration. C4 plants maintain high photosynthetic rates at higher temperatures (30-45°C) due to their CO2 concentrating mechanism, which suppresses photorespiration.

32. What is the significance of the first stable compound in each pathway?

The first stable compound in each pathway gives these processes their names. In the C3 pathway, it's 3-phosphoglycerate (a 3-carbon compound), while in the C4 pathway, it's oxaloacetate (a 4-carbon compound). These initial products reflect the fundamental differences in how these pathways fix carbon.

33. How does the efficiency of nitrogen use differ between C3 and C4 plants?

C4 plants generally have higher nitrogen use efficiency than C3 plants. This is because C4 plants can achieve the same or higher photosynthetic rates with less RuBisCO, which is a nitrogen-rich enzyme. The CO2 concentrating mechanism in C4 plants allows RuBisCO to operate more efficiently, requiring less of the enzyme.

34. How do C3 and C4 plants differ in their response to increasing atmospheric CO2 levels?

C3 plants generally show a stronger positive response to increasing atmospheric CO2 levels compared to C4 plants. This is because C3 plants are typically CO2-limited, while C4 plants already concentrate CO2 internally. However, the response can vary depending on other environmental factors like temperature and water availability.

35. How do C3 and C4 pathways affect the carbon isotope ratios in plant tissues?

C3 and C4 plants have different carbon isotope ratios in their tissues due to differences in their CO2 fixation processes. C3 plants discriminate more strongly against the heavier 13C isotope, resulting in more negative δ13C values (around -28‰). C4 plants discriminate less, leading to less negative δ13C values (around -14‰). This difference is used in ecological and paleontological studies.

36. How do C3 and C4 plants differ in their light saturation points?

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.

37. How do C3 and C4 plants differ in their water use efficiency?

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.

38. How do C3 and C4 plants differ in their response to low light conditions?

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.

39. How do C3 and C4 plants differ in their leaf nitrogen content?

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.

40. How do C3 and C4 plants differ in their response to salinity stress?

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.

41. How do C3 and C4 plants differ in their stomatal density?

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.

42. What is the role of aspartate in some C4 photosynthesis subtypes?

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.

43. How do C3 and C4 plants differ in their response to elevated ozone levels?

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.

44. How do C3 and C4 plants differ in their photosynthetic quantum yield?

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.