C4 And CAM pathway: Meaning, Process In C3 C4 And CAM Plants

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

Overview Of Photosynthesis

Photosynthesis is the biological process through which green plants and other autotrophs synthesize glucose through the conversion of light energy to chemical energy. It is a fundamental biological process/ through which photosynthesis in bacteria manufactures glucose using only carbon dioxide and water, with the gaseous release of molecular oxygen. The general chemical equation for photosynthesis can be summarized as follows:

6CO2+6H2O→C6H12O6+6O2

In this, plants use sunlight as the ultimate energy source in driving this important process, which has two major phases: a photochemical phase and a biosynthetic phase. The former is involved in the production of ATP and NADPH through light, while the latter is concerned with the biosynthesis of glucose through carbon fixation.

Importance Of Carbon Fixation

In the photosynthetic biosynthetic phase, carbon dioxide reacts with water to form carbohydrates, mainly sugar molecules. As this is the carbon-fixation reaction, diversity between the plants resulted in the fundamental division of plants into two major pathways: C3 and C4.

How The C3 Pathway (Calvin Cycle) Functions

The Calvin cycle, or the C3 pathway, is the most widespread carbon fixation pathway in plants. It is a 3-carbon acid, and the first stable product is the 3-phosphoglyceric acid (PGA). The Calvin cycle has three major steps: carboxylation, reduction, and regeneration.

Carboxylation: In carboxylation, the enzyme RuBisCo catalyzes the reaction of carbon dioxide with RuBP to yield two molecules of PGA.
Reduction: The ATP and NADPH produced in the light-dependent reactions are used in the phosphorylation and reduction of PGA to form glyceraldehyde-3-phosphate (G3P).
Regeneration: A regeneration step in which some of the G3P molecules are used in generating more molecules of the acceptor molecule by combining with RuBP, whereas the rest of the G3P molecules are used to generate glucose and other carbohydrates.

Some of the common C3 plants are beans, rice, wheat, and potatoes, which grow in temperate regions.

Understanding The C4 Pathway (Hatch And Slack Pathway)

The C4 pathway is also known as the Hatch and Slack pathway. A photorespiratory escape mechanism is a rescue pathway developed by some plants on a normal mode of distribution and operation found in their tropical and arid regions. In the process, the first stable product of carbon fixation, is a 4-carbon compound, oxaloacetic acid (OAA).

Mechanism Of The C4 Pathway

The main C4 pathway employs a 3-carbon molecule- phosphoenolpyruvate (PEP) as the primary CO2 receptor molecule. This pathway is of the biphasic type and takes place through the subsequent steps.

Carboxylation: The reaction of PEP with CO2 resulting from PEP carboxylase gives rise to oxaloacetic acid (OAA).
Decarboxylation: OAA is further decarboxylated to produce malic acid or aspartic acid and then transferred to the bundle sheath cells from the mesophyll cells.
Decarboxylation: In the bundle sheath cells, malic acid undergoes decarboxylation to release carbon dioxide, which is subsequently released in the Calvin cycle. Then, the 3-carbon molecule is diffused back to the mesophyll cells to regenerate PEP.

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Examples of C4 plants are corn, sugarcane, and some bushes, favouring high light intensity and temperatures.

Main Contrasts Between The C3 And C4 Pathways

Both of these pathways involve the Calvin cycle while having some very important differences:

First Stable Product: The first stable product in C3 plants is PGA while in C4 plants it is oxaloacetic acid (OAA).
Site of Calvin Cycle: The Calvin cycle in C3 plants occurs only in mesophyll cells whereby the C4 plants initially fix the carbon in mesophyll cells and then carry the Calvin cycle in bundle sheath cells.
Efficiency of Enzymes: C4 plants employ PEP carboxylase, an enzyme with greater efficiency in terms of its affinity for carbon dioxide than RuBisCO employed by C3 plants.
Photorespiration: C4 plants possess adaptations for preventing photorespiration, thus they are highly productive in hot and arid climates.
Ecological Adaptation: C3 plants typically exist within cooler and wetter climates, whereas C4 plants are located within hot and bright environmental conditions.

Significance Of C3 And C4 Pathways

Such an understanding of the differences between C3 and C4 pathways is pretty useful for plant physiology, ecology, and agriculture practices. Generally, C4 plants are more efficient in terms of carbon fixation and water use and can even represent a better crop to be cultivated in harsher climates. This knowledge could also drive the crop selection and breeding strategy toward improved yield and resilience against the alteration of climate conditions.

Frequently Asked Questions (FAQs)

1. What is the C3 pathway?

The C3 pathway is also known as the Calvin cycle, a process of carbon fixation with 3-phosphoglyceric acid (PGA) as the first stable product.

2. What is the C4 pathway?

The C4 pathway is a type of carbon dioxide assimilation pathway which produces Oxaloacetic acid as the first stable product besides occurring generally in most plants from the tropics and arid regions of the world.

3. Compare the C3 and C4 pathways of carbon fixation.

C3 plants fix carbon dioxide directly into PGA, whereas C4 plants first fix carbon dioxide into OAA before it enters in Calvin cycle.

4. What kind of plant is mostly the C4 route taken by?

Mostly corn, sugarcane, and some grasses are the kind of C4 plant.

5. Why are C4 plants better than the C3 plants?

C4 plants are relatively more efficient because of reductions in photorespiration and because they possess PEP carboxylase which has a greater affinity for carbon dioxide.

6. What is the difference between obligate and facultative CAM plants?

Obligate CAM plants always use the CAM pathway for carbon fixation, regardless of environmental conditions. Facultative CAM plants, on the other hand, can switch between C3 photosynthesis and CAM depending on environmental stress, particularly water availability.

7. What is the relationship between temperature and the efficiency of C3 vs C4 photosynthesis?

C3 photosynthesis is generally more efficient at lower temperatures, while C4 becomes more efficient as temperature increases. This is because higher temperatures increase photorespiration in C3 plants, while C4 plants can maintain high CO2 levels around RuBisCO regardless of temperature.

8. What is the primary advantage of CAM photosynthesis?

The primary advantage of CAM photosynthesis is extreme water conservation. By opening stomata at night and closing them during the day, CAM plants can survive in very arid environments where water loss during the day would be detrimental.

9. What is the carbon fixation product stored overnight in CAM plants?

In CAM plants, CO2 is fixed into organic acids, primarily malic acid, which is stored in vacuoles overnight. This malic acid is then decarboxylated during the day to release CO2 for use in the Calvin cycle.

10. How does nitrogen use efficiency compare between C3 and C4 plants?

C4 plants generally have higher nitrogen use efficiency than C3 plants. This is because C4 plants can maintain high photosynthetic rates with less RuBisCO, which is a nitrogen-rich enzyme. The CO2-concentrating mechanism allows C4 plants to use RuBisCO more efficiently.

11. What is the significance of vacuoles in CAM photosynthesis?

Vacuoles are crucial in CAM photosynthesis as they store the organic acids (primarily malic acid) produced from nighttime CO2 fixation. These large vacuoles allow CAM plants to accumulate significant amounts of fixed carbon overnight for use in photosynthesis during the day.

12. What are some examples of CAM plants?

CAM plants are typically found in very dry environments. Examples include cacti, pineapples, agaves, and some orchids. These plants are highly adapted to conserve water by opening their stomata at night to fix CO2.

13. What is the role of carbonic anhydrase in C4 and CAM photosynthesis?

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

14. What is the role of phosphoenolpyruvate (PEP) in C4 and CAM photosynthesis?

Phosphoenolpyruvate (PEP) is the CO2 acceptor molecule in the initial carbon fixation step of both C4 and CAM photosynthesis. PEP carboxylase uses PEP to fix CO2 (as bicarbonate) into oxaloacetate, which is then converted to other 4-carbon compounds.

15. What are C3, C4, and CAM pathways in photosynthesis?

C3, C4, and CAM are three different carbon fixation pathways in plants. C3 is the most common pathway, where CO2 is fixed directly into a 3-carbon compound. C4 and CAM are adaptations to hot, dry environments. In C4, CO2 is first fixed into a 4-carbon compound, while in CAM, CO2 is fixed at night and stored for use during the day.

16. What is the primary carbon fixation enzyme in C3, C4, and CAM pathways?

In all three pathways, the primary carbon fixation enzyme for the Calvin cycle is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). However, C4 and CAM plants use PEP carboxylase for the initial CO2 fixation before RuBisCO comes into play.

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

C4 plants have a distinctive "Kranz anatomy" not found in C3 plants. This includes a ring of bundle sheath cells surrounding vascular bundles, with mesophyll cells arranged radially around them. C3 plants have a more uniform distribution of mesophyll cells.

18. How do C4 and CAM plants avoid photorespiration?

Both C4 and CAM plants avoid photorespiration by separating the initial CO2 fixation from the Calvin cycle. C4 plants do this spatially (in different cell types), while CAM plants do it temporally (at different times of day). This ensures a high CO2 concentration around RuBisCO, reducing its oxygenase activity.

19. Can a plant switch between C3 and C4 pathways?

No, plants cannot switch between C3 and C4 pathways as these require different leaf anatomies and enzyme systems. However, some plants can switch between C3 and CAM pathways depending on environmental conditions, a phenomenon known as facultative CAM.

20. How do C4 plants reduce photorespiration?

C4 plants reduce photorespiration by concentrating CO2 around RuBisCO, the enzyme responsible for carbon fixation. They do this by initially fixing CO2 in mesophyll cells and then transporting the resulting 4-carbon compound to bundle sheath cells, where CO2 is released and used in the Calvin cycle.

21. What is the first stable product of carbon fixation in C4 plants?

The first stable product of carbon fixation in C4 plants is oxaloacetate, a 4-carbon compound. This is quickly converted to malate or aspartate for transport to the bundle sheath cells.

22. How do C4 plants concentrate CO2 around RuBisCO?

C4 plants concentrate CO2 around RuBisCO through a process called the CO2 pump. They fix CO2 in mesophyll cells using PEP carboxylase, convert it to a 4-carbon compound, transport this to bundle sheath cells, and then release the CO2 there, where RuBisCO is located.

23. What is the role of PEP carboxylase in C4 and CAM photosynthesis?

PEP carboxylase is the enzyme responsible for the initial fixation of CO2 in both C4 and CAM photosynthesis. It's more efficient than RuBisCO at low CO2 concentrations and doesn't perform oxygenase activity, making it ideal for the first step of carbon fixation in these pathways.

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

Bundle sheath cells are crucial in C4 photosynthesis as they contain RuBisCO and are the site of the Calvin cycle. These cells are arranged in a wreath-like pattern around the vascular bundles, creating a compartment where CO2 can be concentrated.

25. What is the evolutionary relationship between C3, C4, and CAM pathways?

C3 photosynthesis is the most ancient pathway. C4 and CAM evolved independently multiple times from C3 ancestors as adaptations to specific environmental pressures, particularly hot, dry conditions. These pathways represent convergent evolution in response to similar selective pressures.

26. Why did C4 and CAM pathways evolve?

C4 and CAM pathways evolved as adaptations to hot, dry environments. These pathways help plants conserve water and use CO2 more efficiently by reducing photorespiration, which is a process that wastes energy in C3 plants under high temperature and low CO2 conditions.

27. What is the main difference between C4 and CAM pathways?

The main difference is in the timing of CO2 fixation. C4 plants spatially separate the initial CO2 fixation and the Calvin cycle, using different cell types. CAM plants temporally separate these processes, fixing CO2 at night and using it for photosynthesis during the day.

28. What are some examples of C4 plants?

Common C4 plants include corn (maize), sugarcane, sorghum, and many tropical grasses. These plants are often adapted to hot, sunny environments and are more efficient in water and nitrogen use compared to C3 plants.

29. How do CAM plants conserve water?

CAM plants conserve water by opening their stomata at night when temperatures are cooler and humidity is higher. They fix CO2 into organic acids, which are stored in vacuoles. During the day, stomata close to prevent water loss, and the stored CO2 is released for use in photosynthesis.

30. How does the efficiency of C4 photosynthesis compare to C3?

C4 photosynthesis is generally more efficient than C3, especially in hot, dry environments. C4 plants can continue photosynthesis even when stomata are partially closed, use nitrogen and water more efficiently, and have higher rates of photosynthesis under high light intensities.

31. How do environmental factors influence the evolution of C4 and CAM pathways?

Environmental factors such as high temperature, high light intensity, and water scarcity have driven the evolution of C4 and CAM pathways. These conditions increase photorespiration in C3 plants, making the CO2-concentrating mechanisms of C4 and CAM advantageous.

32. What is the significance of decarboxylation in C4 and CAM pathways?

Decarboxylation is crucial in both C4 and CAM pathways as it releases the CO2 that was initially fixed. In C4 plants, this occurs in bundle sheath cells, while in CAM plants, it happens during the day from the stored organic acids. This released CO2 is then used in the Calvin cycle.

33. How do C4 plants maintain a high CO2 to O2 ratio around RuBisCO?

C4 plants maintain a high CO2 to O2 ratio around RuBisCO by concentrating CO2 in the bundle sheath cells. This is achieved through the CO2 pump mechanism, where CO2 is fixed in mesophyll cells, transported as a 4-carbon compound, and then released in the bundle sheath cells.

34. What role does malate play in C4 and CAM photosynthesis?

Malate is a key intermediate in both C4 and CAM photosynthesis. In C4 plants, it's often the 4-carbon compound that transports CO2 from mesophyll to bundle sheath cells. In CAM plants, malate is the primary organic acid used to store fixed CO2 overnight in vacuoles.

35. How does the energy requirement differ between C3, C4, and CAM photosynthesis?

C4 and CAM photosynthesis require more energy (ATP) per CO2 fixed compared to C3 photosynthesis due to the additional steps involved in concentrating CO2. However, this extra energy cost is often offset by reduced losses to photorespiration, especially in hot, dry conditions.

36. How do C4 plants adapt to high light intensities?

C4 plants are well-adapted to high light intensities due to their CO2-concentrating mechanism. This allows them to maintain high photosynthetic rates even when stomata are partially closed to conserve water. They also tend to have higher light saturation points than C3 plants.

37. How do CAM plants regulate their stomatal opening?

CAM plants open their stomata at night and close them during the day, which is opposite to most plants. This is regulated by circadian rhythms and environmental cues. The night opening allows for CO2 uptake when temperatures are cooler and humidity is higher, reducing water loss.

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

The spatial separation of carbon fixation in C4 plants allows for the concentration of CO2 around RuBisCO in bundle sheath cells. This reduces photorespiration, increases photosynthetic efficiency, and allows C4 plants to thrive in conditions that would be stressful for C3 plants.

39. How do C4 and CAM pathways affect water use efficiency in plants?

Both C4 and CAM pathways significantly improve water use efficiency in plants. C4 plants can maintain high photosynthetic rates with partially closed stomata, while CAM plants open stomata at night when water loss is minimized. This allows these plants to thrive in water-limited environments.

40. How do C4 plants maintain a high CO2 concentration in bundle sheath cells?

C4 plants maintain a high CO2 concentration in bundle sheath cells through the CO2 pump mechanism. CO2 is fixed into 4-carbon compounds in mesophyll cells, transported to bundle sheath cells, and then decarboxylated. The released CO2 is trapped within the thick-walled bundle sheath cells.

41. How do C4 plants adapt to low CO2 conditions?

C4 plants are well-adapted to low CO2 conditions due to their CO2-concentrating mechanism. They can maintain high photosynthetic rates even when atmospheric CO2 is low by concentrating CO2 around RuBisCO in bundle sheath cells, effectively creating a high CO2 environment internally.

42. How does the rate of photosynthesis in C4 plants compare to C3 plants under different light intensities?

C4 plants generally have higher rates of photosynthesis than C3 plants, especially at high light intensities. C4 plants have a higher light saturation point and can continue to increase photosynthetic rate at light levels where C3 plants have plateaued.

43. How do C4 plants regulate the balance between C4 and C3 cycles?

C4 plants regulate the balance between C4 and C3 cycles through various mechanisms, including the control of enzyme activities and metabolite transport between mesophyll and bundle sheath cells. This balance ensures efficient CO2 fixation and utilization in the Calvin cycle.

44. What are the energy costs associated with the CO2 concentrating mechanisms in C4 and CAM plants?

The CO2 concentrating mechanisms in C4 and CAM plants require additional ATP compared to C3 photosynthesis. In C4 plants, this is used for regenerating PEP. In CAM plants, energy is needed for acid accumulation at night and decarboxylation during the day. However, these costs are often offset by increased photosynthetic efficiency.

45. How do C4 and CAM plants cope with high temperatures?

C4 and CAM plants are well-adapted to high temperatures. C4 plants maintain high photosynthetic rates by concentrating CO2 around RuBisCO, reducing photorespiration. CAM plants avoid heat stress by fixing CO2 at night when temperatures are cooler and closing stomata during hot days.

46. What is the importance of bundle sheath conductance in C4 photosynthesis?

Bundle sheath conductance is crucial in C4 photosynthesis as it affects the efficiency of the CO2 concentrating mechanism. Low conductance helps maintain high CO2 levels around RuBisCO, but if too low, it can limit the rate of CO2 supply to the Calvin cycle.

47. How do C4 and CAM pathways affect plant growth rates?

C4 plants often have higher growth rates than C3 plants, especially in warm, high-light environments, due to their more efficient photosynthesis. CAM plants typically have slower growth rates due to the energy costs of their CO2 concentrating mechanism and the need to conserve water.

48. 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. This regeneration is crucial for the continuous operation of the C4 cycle, allowing for sustained CO2 fixation in mesophyll cells.

49. How do C4 and CAM plants differ in their response to drought stress?

Both C4 and CAM plants are adapted to drought stress, but in different ways. C4 plants can maintain photosynthesis with partially closed stomata, improving water use efficiency. CAM plants take this further by only opening stomata at night, allowing extreme water conservation.