C4 Pathway: Steps, Differences and FAQ

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

What Is C4 Pathway?

The C4 pathway is one of the metabolic procedures in photosynthesis that allows high efficiency in photosynthesis in certain plants, mainly of tropical origin and those growing in hot, dry climates. First explained by M. D. Hatch and C. R. Slack in 1966, this process allows plants to fix carbon dioxide better than the plants having the C3 pathway in certain environmental conditions. This paper provides information about the mechanisms, steps and the importance of C4 pathway

This Story also Contains

What Is C4 Pathway?
Hatch And Slack Pathway
C4 Cycle
Hatch And Slack Pathway In C4 Plants
Anatomy Of Kranz
Steps Of The C4 Cycle
Importance Of The C4 Pathway
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Hatch And Slack Pathway

The C4 pathway starts by mainly fixing carbon dioxide with phosphoenolpyruvate in the mesophyll cells. This step is catalyzed by the PEP carboxylase. The first reaction produces a four-carbon compound that is oxaloacetic acid. It is then diffused into the bundle sheath cells for further fixation in the Calvin cycle

C4 Cycle

In the C4 cycle, the first stable product that is formed is the oxaloacetic acid. The wide distribution of the C4 pathway can be noted in some families of plants including chenopodiaceae, Gramineae and Cyperaceae. All these belong to both dicots and monocots.

Hatch And Slack Pathway In C4 Plants

C4 is the alternate route to the C3 cycle in carbon fixation. The process gets its name "C4 cycle" from the first compound formed, which is a four-carbon molecule: oxaloacetic acid. This pathway dominates most grasses, including maize and sugarcane; the characteristic anatomy of the leaves in this pathway is called Kranz anatomy.

Anatomy Of Kranz

In leaves of C4 plants, each vascular bundle or rib is enclosed with a bundle sheath composed of larger parenchymatous cells. The bundle sheath cells have larger chloroplasts, lacking intergranal lamellae, and starch grains, while the mesophyll cells have small chloroplasts with grana. The anatomy is thus especially suited to increase carbon fixation efficiency due to the arrangement, so it is called Kranz anatomy from the German for "wreath".

Steps Of The C4 Cycle

The C4 cycle takes four steps as follows:

1. Carboxylation

Carbon dioxide is captured in the chloroplasts of mesophyll cells by the three-carbon compound, phosphoenolpyruvate, producing oxaloacetic acid. This is made possible through the action of an enzyme called PEP carboxylase.

2. Breakdown

The newly formed oxaloacetic acid is reduced by two enzymes, namely transaminase and malate dehydrogenase, to form malate and aspartate, respectively. These C4 products diffuse out from the mesophyll cells into the bundle sheath cells.

3. Cleavage

This will be further cleaved by the bundle sheath cell enzymes to release free carbon dioxide and three-carbon pyruvate. The released CO₂ here will diffuse in the Calvin cycle, get combined with RuBP and get reduced into the formation of 3-PGA.

4. Phosphorylation

The formed pyruvate molecules diffuse back into the mesophyll cells and in the presence of ATP get phosphorylated to re-generate phosphoenolpyruvate, which is catalyzed by the enzyme pyruvate phosphokinase.

Importance Of The C4 Pathway

The C4 pathway thus confers an increased efficiency in carbon fixation—especially when photorespiration is high. From the energy point of view, it yields high growth rates for C4 plants in certain biomes as a consequence of reduced losses due to RuBisCO's oxygenase reaction. There are broad implications for agriculture also, with scientists field-testing whether key commercial crops can be genetically altered to produce C4 plants.

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Frequently Asked Questions (FAQs)

1. What is the C4 pathway?

The C4 pathway is the process of photosynthesis that helps plants to fix carbon dioxide effectively in hot and dry conditions.

2. Who discovered the C4 cycle?

The process of the C4 cycle was described by M.D. Hatch and C.R. Slack in 1966.

3. What is Kranz's anatomy?

Kranz anatomy refers to the specialized leaf anatomy of the C4 plants that consists of a bundle sheath of larger parenchymatous cells ensheathing the vascular bundles.

4. What are the steps of the C4 cycle?

The steps involve carboxylation, breakdown, splitting and phosphorylation as the major procedures.

5. Why are plants C4 more efficient than C3?

C4 plants are efficient in that they minimize photorespiration and can fix carbon dioxide under low stomatal openings.

6. How does leaf anatomy differ between C3 and C4 plants?

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

7. What is the significance of bundle sheath cells in C4 photosynthesis?

Bundle sheath cells are crucial in C4 photosynthesis as they are the site of CO2 concentration and Calvin cycle activity. Their thick walls help maintain high CO2 levels, and their arrangement around vascular bundles facilitates efficient transport of photosynthetic products.

8. What is the primary location of the Calvin cycle in C4 plants?

In C4 plants, the Calvin cycle primarily occurs in the bundle sheath cells. This is different from C3 plants, where the Calvin cycle takes place in all chloroplast-containing cells.

9. How does the C4 pathway affect nitrogen use efficiency in plants?

The C4 pathway generally improves nitrogen use efficiency in plants. By concentrating CO2 around RuBisCO, C4 plants can achieve higher photosynthetic rates with less RuBisCO enzyme, which is a major nitrogen investment for plants.

10. How does the C4 pathway affect plant productivity in different environments?

The C4 pathway enhances plant productivity in hot, high-light environments by reducing photorespiration and improving water use efficiency. However, in cooler or shadier conditions, C3 plants may be more productive due to the lower energy cost of their photosynthetic pathway.

11. What is the significance of pyruvate orthophosphate dikinase (PPDK) in the C4 pathway?

PPDK is crucial for regenerating phosphoenolpyruvate (PEP) in the mesophyll cells of C4 plants. It catalyzes the conversion of pyruvate back to PEP, allowing the C4 cycle to continue. This step is energy-intensive and contributes to the higher energy requirements of C4 photosynthesis.

12. What role does carbonic anhydrase play in the C4 pathway?

Carbonic anhydrase catalyzes the rapid conversion of CO2 and water to bicarbonate in the mesophyll cells of C4 plants. This helps maintain a concentration gradient that facilitates CO2 diffusion into the cell and provides substrate for PEP carboxylase, supporting efficient carbon fixation.

13. What is the role of aspartate in some C4 pathways?

In some C4 plants, aspartate serves as an alternative carrier molecule to malate. After initial CO2 fixation, oxaloacetate can be converted to aspartate, which is then transported to bundle sheath cells and decarboxylated to release CO2 for the Calvin cycle.

14. What is the significance of the spatial separation of carbon fixation steps in C4 plants?

The spatial separation of initial carbon fixation in mesophyll cells and the Calvin cycle in bundle sheath cells is crucial for the CO2-concentrating mechanism of C4 plants. This arrangement allows for the build-up of high CO2 levels around RuBisCO, reducing photorespiration and improving photosynthetic efficiency.

15. How does temperature affect the efficiency of C3 vs C4 photosynthesis?

C4 photosynthesis maintains higher efficiency at higher temperatures compared to C3 photosynthesis. As temperature increases, photorespiration in C3 plants increases more rapidly, reducing their photosynthetic efficiency, while C4 plants continue to perform well due to their CO2-concentrating mechanism.

16. How does the C4 pathway affect the light saturation point of photosynthesis?

C4 plants typically have a higher light saturation point compared to C3 plants. This means they can continue to increase their photosynthetic rate at higher light intensities, whereas C3 plants reach their maximum rate at lower light levels. This characteristic contributes to the high productivity of C4 plants in bright, open environments.

17. What is the role of plasmodesmata in C4 photosynthesis?

Plasmodesmata play a crucial role in C4 photosynthesis by facilitating the rapid transport of metabolites between mesophyll and bundle sheath cells. They allow for the efficient movement of C4 acids (like malate) from mesophyll to bundle sheath cells, and the return of C3 compounds to complete the C4 cycle.

18. What is the significance of the bundle sheath conductance in C4 photosynthesis?

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 conductance is too high, CO2 leaks out of the bundle sheath, reducing the efficiency of the C4 pathway.

19. What is the role of phosphoenolpyruvate (PEP) in the C4 pathway?

Phosphoenolpyruvate (PEP) serves as the initial CO2 acceptor in the C4 pathway. It is carboxylated by PEP carboxylase in mesophyll cells to form oxaloacetate, initiating the CO2-concentrating process. The regeneration of PEP is also a key energy-requiring step in the C4 cycle.

20. What is the role of the enzyme RuBisCO in C4 plants compared to C3 plants?

In C4 plants, RuBisCO functions primarily in the Calvin cycle within bundle sheath cells, where it operates in a high-CO2 environment. This contrasts with C3 plants, where RuBisCO is present in all photosynthetic cells and often engages in photorespiration due to lower CO2 concentrations.

21. How do C4 plants maintain a high CO2 concentration around RuBisCO?

C4 plants maintain a high CO2 concentration around RuBisCO through spatial separation of initial CO2 fixation and the Calvin cycle. CO2 is first fixed in mesophyll cells, then concentrated and released in bundle sheath cells where RuBisCO operates, effectively pumping CO2 to where it's needed.

22. How does the CO2 compensation point differ between C3 and C4 plants?

The CO2 compensation point is lower in C4 plants compared to C3 plants. This means C4 plants can continue net CO2 fixation at lower CO2 concentrations, where C3 plants would reach a balance between photosynthesis and respiration.

23. What is the role of PEP carboxylase in the C4 pathway?

PEP carboxylase (PEPCase) is the primary CO2-fixing enzyme in the C4 pathway. It catalyzes the addition of CO2 to phosphoenolpyruvate (PEP) in the mesophyll cells, forming oxaloacetate. This initial fixation step is key to the CO2-concentrating mechanism of C4 plants.

24. What are the energy requirements of the C4 pathway compared to the C3 pathway?

The C4 pathway requires more energy than the C3 pathway, typically using 5 ATP and 2 NADPH per CO2 fixed, compared to 3 ATP and 2 NADPH in C3 photosynthesis. This additional energy cost is offset by increased efficiency in high-light, high-temperature conditions.

25. What are the main types of C4 photosynthesis based on decarboxylating enzymes?

There are three main types of C4 photosynthesis based on the decarboxylating enzymes used: NADP-malic enzyme type, NAD-malic enzyme type, and PEP carboxykinase type. Each type has slight differences in biochemistry but achieves the same overall goal of concentrating CO2.

26. What is the C4 pathway in photosynthesis?

The C4 pathway is an adaptation in some plants that enhances photosynthetic efficiency, especially in hot, dry environments. It involves a series of biochemical reactions that concentrate CO2 around the enzyme RuBisCO, reducing photorespiration and improving carbon fixation.

27. How does the C4 pathway reduce photorespiration?

The C4 pathway reduces photorespiration by concentrating CO2 around RuBisCO in the bundle sheath cells. This high CO2 concentration outcompetes O2, reducing the oxygenase activity of RuBisCO and thus minimizing photorespiration.

28. What are some examples of C4 plants?

Some common examples of C4 plants include corn (maize), sugarcane, sorghum, amaranth, and many tropical grasses. These plants are often adapted to hot, dry environments where C3 photosynthesis would be less efficient.

29. What advantage does the C4 pathway provide in hot, dry environments?

In hot, dry environments, the C4 pathway allows plants to maintain high photosynthetic rates while conserving water. By concentrating CO2 around RuBisCO, C4 plants can keep their stomata partially closed, reducing water loss through transpiration while still fixing carbon efficiently.

30. Why don't all plants use the C4 pathway?

The C4 pathway requires additional energy input compared to the C3 pathway, making it less efficient in cooler, moister environments where photorespiration is naturally lower. Additionally, the complex anatomical and biochemical adaptations required for C4 photosynthesis have only evolved in certain plant lineages.

31. How does the C4 pathway differ from the C3 pathway?

The C4 pathway differs from the C3 pathway by having an additional step of carbon fixation in mesophyll cells before the Calvin cycle occurs in bundle sheath cells. This spatial separation allows C4 plants to concentrate CO2 around RuBisCO, increasing photosynthetic efficiency.

32. What are the main steps of the C4 pathway?

The main steps of the C4 pathway are: 1) Initial CO2 fixation in mesophyll cells to form oxaloacetate, 2) Conversion of oxaloacetate to malate, 3) Transport of malate to bundle sheath cells, 4) Decarboxylation of malate to release CO2, and 5) CO2 fixation via the Calvin cycle in bundle sheath cells.

33. Which enzyme is responsible for the initial carbon fixation in the C4 pathway?

The enzyme responsible for the initial carbon fixation in the C4 pathway is phosphoenolpyruvate carboxylase (PEPCase). It catalyzes the addition of CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate.

34. What role does malate play in the C4 pathway?

Malate serves as a carrier molecule in the C4 pathway. It is formed from oxaloacetate in the mesophyll cells, then transported to the bundle sheath cells where it is decarboxylated to release CO2 for the Calvin cycle.

35. How does the C4 pathway affect water use efficiency in plants?

The C4 pathway improves water use efficiency by allowing plants to maintain high photosynthetic rates with partially closed stomata. This reduces water loss through transpiration while still allowing sufficient CO2 uptake for photosynthesis.

36. Why is the C4 pathway called "C4"?

The C4 pathway is named after the first product formed in the carbon fixation process: a 4-carbon compound called oxaloacetate. This is in contrast to the C3 pathway, where the first product is a 3-carbon compound.

37. What evolutionary advantages led to the development of C4 photosynthesis?

C4 photosynthesis evolved as an adaptation to high light intensity, high temperature, and low CO2 environments. It provided advantages in water use efficiency, nitrogen use efficiency, and reduced photorespiration, allowing plants to thrive in conditions where C3 photosynthesis would be less efficient.

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

NADP-malic enzyme is involved in the decarboxylation step in some C4 plants. It catalyzes the conversion of malate to pyruvate in the bundle sheath cells, releasing CO2 for the Calvin cycle and generating NADPH, which can be used in carbon fixation.

39. How does the C4 pathway affect plant responses to elevated atmospheric CO2?

C4 plants generally show less response to elevated atmospheric CO2 compared to C3 plants. This is because C4 plants already concentrate CO2 internally, so they are less limited by atmospheric CO2 levels under normal conditions.

40. How does the C4 pathway affect leaf nitrogen distribution?

In C4 plants, leaf nitrogen is distributed differently compared to C3 plants. More nitrogen is allocated to the bundle sheath cells where the Calvin cycle occurs, and less to mesophyll cells. This distribution reflects the specialized roles of these cell types in C4 photosynthesis.

41. How does the C4 pathway affect plant responses to drought stress?

The C4 pathway enhances plant tolerance to drought stress by improving water use efficiency. C4 plants can maintain photosynthesis with partially closed stomata, reducing water loss while still fixing carbon effectively, which is particularly advantageous in water-limited environments.

42. How does the C4 pathway affect plant growth rate and biomass accumulation?

In suitable environments (hot, high-light), C4 plants often show higher growth rates and biomass accumulation compared to C3 plants. This is due to their higher photosynthetic efficiency, reduced photorespiration, and improved water and nitrogen use efficiency.

43. How does the C4 pathway affect plant competitiveness in different ecosystems?

The C4 pathway enhances plant competitiveness in hot, high-light ecosystems by allowing more efficient use of resources (water, nitrogen) and higher productivity. However, in cooler or shadier environments, C3 plants may be more competitive due to their lower energy requirements for photosynthesis.

44. How does the C4 pathway affect plant responses to salinity stress?

The C4 pathway can enhance plant tolerance to salinity stress. The improved water use efficiency of C4 plants helps maintain favorable water relations under saline conditions, and their efficient use of nitrogen can aid in maintaining growth despite the metabolic costs of salt tolerance.

45. How does the C4 pathway affect plant responses to low nitrogen availability?

C4 plants generally show better performance under low nitrogen conditions compared to C3 plants. Their improved nitrogen use efficiency, due to lower RuBisCO requirements and efficient CO2 fixation, allows them to maintain productivity with less nitrogen investment in photosynthetic machinery.

46. What are the main challenges in engineering C4 photosynthesis into C3 crops?

The main challenges in engineering C4 photosynthesis into C3 crops include: modifying leaf anatomy to develop Kranz structure, introducing and regulating C4 cycle enzymes, altering metabolite transport systems, and coordinating the expression of multiple genes across different cell types. These complex changes require significant genetic and metabolic engineering.

47. How does the C4 pathway affect plant responses to varying light conditions?

C4 plants generally perform better under high light conditions due to their ability to avoid light saturation. However, they may be less efficient than C3 plants under low light conditions due to the higher energy cost of their CO2-concentrating mechanism. This affects their distribution and competitiveness in different light environments.

48. How does the C4 pathway affect plant isotope discrimination?

C4 plants show less discrimination against the heavier carbon isotope (13C) compared to C3 plants during carbon fixation. This results in a distinct carbon isotope ratio in C4 plant tissues, which can be used to identify C4 plants in ecological and paleontological studies.

49. How does the C4 pathway affect plant responses to fluctuating light conditions?

C4 plants generally respond more quickly to fluctuating light conditions compared to C3 plants. Their CO2-concentrating mechanism allows for rapid increases in photosynthetic rate when light increases, and their lower photorespiration helps maintain efficiency when light decreases.

50. How does the C4 pathway affect plant responses to elevated ozone levels?

C4 plants are generally more resistant to ozone damage compared to C3 plants. Their ability to maintain high photosynthetic rates with partially closed stomata reduces ozone uptake, while their efficient carbon fixation helps maintain productivity even under stress conditions.

51. What is the significance of the CO2 leak rate from bundle sheath cells in C4 plants?

The CO2 leak rate from bundle sheath cells is an important factor