Difference Between C3, C4 and CAM Pathway: Steps, Differences and FAQ

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:58 PM IST

What Is Photosynthesis?

Photosynthesis is the process through which green plants, algae, and some bacteria transform the energy obtained from sunlight into a storable form of chemical energy in the form of glucose. The process involves the fixation of carbon dioxide from the atmosphere, which transforms organic compounds. Carbon fixation can occur through one of three main pathways, namely, C3, C4 and CAM pathways.

This Story also Contains

What Is Photosynthesis?
What Is Carbon Fixation?
Overview Of C3, C4, And Cam Pathways
Detailed Explanation Of The C4 Pathway
Summary Of The Differences Between The C3, C4 And Cam Pathways

What Is Carbon Fixation?

Carbon fixation is the first step of photosynthesis where inorganic carbon in the form of CO2 gets converted into organic compounds. The process is essential in making sugars, products plants and, indirectly, other living organisms require for sources of energy.

Overview Of C3, C4, And Cam Pathways

The key differences between C3, C4, and CAM are how the plants capture carbon dioxide and the nature of photosynthesis products.

C3 pathway: 3-phosphoglyceric acid is the first product to be yielded in carbon fixation.
C4 pathway: The first product to be produced is an oxaloacetic acid 4-carbon molecule, and again it enters into the Calvin cycle.
CAM pathway: Carbon dioxide is fixed in the night and stored as malic acid, which is used in the daytime.

What Is The C3 Pathway?

Another name for the C3 pathway is the Calvin Cycle; it is the most common form of carbon fixation. It takes place in most plants that are mainly found in temperate climates. Some of the key features of the C3 pathway include that the first stable product formed from carbon fixation is 3-phosphoglyceric acid, which is a 3-carbon compound. The process happens in the stroma of chloroplasts. The stroma of chloroplasts is the fluid-filled space inside chloroplasts in which the Calvin Cycle occurs.

Steps Of The C3 Pathway

Carboxylation: CO2 is fixed by the enzyme RuBisCO as the result of the reaction between CO2 and the RuBP (ribulose bisphosphate), which produces PGA.
Reduction: The PGA that is produced is reduced into G3P, a 3-carbon sugar, glyceraldehyde-3- phosphate, with the use of ATP and NADPH that is generated during light-dependent photosynthesis reactions.
Regeneration: Some of the G3P molecules are used in regenerating the RuBP so that it can pick up more CO2. The cycle has to turn six times to produce one molecule of glucose, and through these six turns a total of 6 CO2, 18 ATP, and 12 NADPH are consumed.

Examples Of C3 Plants

Some examples of plants in the C3 category are:

Beans
Spinach
Sunflower
Rice
Cotton

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Detailed Explanation Of The C4 Pathway

It is named for the number of carbons in the first organic compound produced in it, but it is also known as the Hatch-Slack pathway. It is an adaptation by plants to be able to effectively fix carbon dioxide in hot and dry environments.

C4 Pathway Characteristics

Primary Product: A 4-carbon molecule; oxaloacetic acid.

Location: The pathway occurs in the mesophyll cells and bundle sheath cells which are specialized cells found in the leaves.

Steps Of The C4 Pathway

At the mesophyll cells, CO2 is fixed through the activity of PEP carboxylase into OAA. Later, it may be reduced to all other 4-carbon acids, for example, malic acid
The 4-carbon acid is moved into the bundle sheath cells into CO2 is released from the malate.
Now this CO2 diffuses into the chloroplasts of bundle sheath cells, which then undergoes the Calvin cycle and finally ends up as glucose.

Examples Of C4 Plants

The common examples of these plants are :

Maize, or corn
Sorghum
Sugarcane

What Is Cam Pathway?

The CAM pathway is the special adaptation in plants that belong to dry environments.

Key Features Of The Cam Pathway

Fixation at Noon: In CAM plants carbon fixation takes place during the night when the cool climate and humidity prevail.

Storage of Malic Acid: Carbon is fixed by storing it in malic acid and then stored in the vacuoles of the plant overnight.

Steps Involved In The Cam Pathway

Night-time Carbon Fixation: Stomata open at night to let in CO2. The latter is converted into malic acid and then stored.

Daytime Utilization: During the day, the stomata close to save water and stored malic acid is converted back to CO2, which enters into the cycle of Calvin for producing sugar.

Examples Of Cam Plants

Common examples of plants that involve CAM are:

Cacti
Orchids
Euphorbias

Summary Of The Differences Between The C3, C4 And Cam Pathways

Initial Products:

C3: 3-phosphoglycerate PGA)
C4: Oxaloacetic acid (OAA)
CAM: Malic acid (night) and PGA (day)

Timing Of Carbon Fixation:

C3: All processes occur during the day.
C4: Carbon fixation occurs in the day but the initial step takes place in mesophyll cells.
CAM: Carbon fixation at night, Calvin Cycle during the day

Environmental Adaptation:

C3: Cooler, wetter climates.
C4: Specialised in warm sunny environments
CAM: Well suited for arid conditions to cut down water loss.

Photorespiration

C3: Higher rates of photorespiration
C4: Lower rates of photorespiration
CAM: Takes place but reduced due to night-time CO2 fixation

Frequently Asked Questions (FAQs)

1. What is the end product of the C3 pathway?

The end product of the C3 pathway is 3-phosphoglycerate (PGA).

2. How do C4 plants differ from C3 plants?

In the C4 plants, an oxaloacetic acid is formed as the very first product and the carbon fixation takes place in cells of both mesophyll and bundle sheath.

3. What is the chief advantage of the CAM pathway?

The inherent advantage of the CAM pathway of plants is that they can fix the carbon dioxide at night and reduce the loss of water during the day when it has a bigger temperature.

4. What kinds of habitats do C4 plants originate from?

The C4 plants, in particular, are mostly distributed in warm, tropical areas. Please name a few examples of plants that undergo the CAM pathway. Examples include cacti, orchids, and euphorbias.

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

The C4 pathway differs from the C3 pathway by having an additional step before RuBisCO fixes CO2. In C4 plants, CO2 is first fixed into a 4-carbon compound (hence the name C4) in mesophyll cells, then transported to bundle sheath cells where it's released for RuBisCO to use. This spatial separation helps concentrate CO2 around RuBisCO, reducing photorespiration.

6. What advantage does the C4 pathway provide to plants?

The C4 pathway provides several advantages, primarily the ability to concentrate CO2 around RuBisCO, which reduces photorespiration and increases photosynthetic efficiency. This makes C4 plants better adapted to hot, dry environments and allows them to use water and nitrogen more efficiently than C3 plants.

7. What types of plants typically use the C4 pathway?

Plants that typically use the C4 pathway include many tropical grasses and important crop plants such as corn (maize), sugarcane, and sorghum. These plants are often found in hot, dry environments where the C4 pathway provides a significant advantage.

8. What is the primary carbon-fixing enzyme in all three pathways?

The primary carbon-fixing enzyme in all three pathways (C3, C4, and CAM) is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). However, the pathways differ in when and where RuBisCO operates within the process.

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

C4 plants have a distinctive "Kranz" anatomy in their leaves, with a ring of bundle sheath cells surrounding the vascular bundles, and mesophyll cells arranged radially around them. C3 plants lack this specialized arrangement, with mesophyll cells more loosely organized between the upper and lower epidermis.

10. Why are C3, C4, and CAM pathways called "carbon fixation" pathways?

These pathways are called "carbon fixation" pathways because they involve the process of converting inorganic carbon (CO2) from the atmosphere into organic compounds within the plant. This is the first major step of photosynthesis, where carbon is "fixed" or incorporated into the plant's biomass.

11. How does the C3 pathway get its name?

The C3 pathway gets its name from the first stable product formed during carbon fixation, which is a 3-carbon compound called 3-phosphoglycerate (3-PGA). This occurs when RuBisCO adds CO2 to a 5-carbon sugar, ribulose bisphosphate (RuBP).

12. What types of plants typically use the C3 pathway?

The C3 pathway is the most common and ancient photosynthetic pathway. It is used by about 85% of plant species, including most trees, wheat, rice, and many other crops. C3 plants are generally adapted to moderate climates with sufficient water availability.

13. What types of plants typically use the CAM pathway?

Plants that typically use the CAM pathway are adapted to extremely arid conditions. These include many succulents like cacti, pineapples, and some orchids. CAM plants are often found in deserts or other water-stressed environments.

14. How does the CAM pathway help plants conserve water?

The CAM pathway helps plants conserve water by allowing them to keep their stomata closed during the hot, dry daytime and open them only at night when it's cooler and more humid. This significantly reduces water loss through transpiration while still allowing the plant to fix carbon dioxide.

15. What are the three main types of carbon fixation pathways in plants?

The three main types of carbon fixation pathways in plants are C3, C4, and CAM (Crassulacean Acid Metabolism). These pathways differ in how they initially fix carbon dioxide during photosynthesis and how they adapt to different environmental conditions.

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

Photorespiration is a process where RuBisCO fixes oxygen instead of carbon dioxide, producing a 2-carbon compound that needs to be recycled, wasting energy. It's a problem for C3 plants because it reduces photosynthetic efficiency, especially in hot, dry conditions when stomata close and oxygen concentrations increase relative to CO2.

17. What is the CAM pathway, and how does it differ from C3 and C4?

The CAM (Crassulacean Acid Metabolism) pathway is a carbon fixation adaptation where stomata open at night to fix CO2 into organic acids, which are then used for photosynthesis during the day when stomata are closed. This temporal separation of CO2 fixation and its use in the Calvin cycle distinguishes CAM from both C3 and C4 pathways.

18. Why are C4 plants more efficient in hot, dry climates compared to C3 plants?

C4 plants are more efficient in hot, dry climates because their CO2 concentration mechanism reduces photorespiration, which increases in C3 plants under these conditions. This allows C4 plants to maintain high photosynthetic rates even when stomata are partially closed to conserve water, making them more water-use efficient and productive in such environments.

19. How does the efficiency of water use compare among C3, C4, and CAM plants?

In terms of water use efficiency (amount of CO2 fixed per unit of water lost), CAM plants are generally the most efficient, followed by C4 plants, with C3 plants being the least efficient. This is due to the different strategies each pathway uses to manage CO2 uptake and water loss through stomata.

20. What role does the enzyme RuBisCO play in photorespiration?

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) can bind both CO2 (carboxylation) and O2 (oxygenation). When it binds O2, it initiates the process of photorespiration. This occurs more frequently at higher temperatures and when the CO2:O2 ratio is low, leading to decreased photosynthetic efficiency, especially in C3 plants.

21. Can a single plant species use more than one carbon fixation pathway?

Yes, some plant species can use more than one carbon fixation pathway, a phenomenon known as photosynthetic plasticity. For example, some plants can switch between C3 and CAM pathways depending on environmental conditions. However, switching between C3 and C4 pathways is rare due to the significant anatomical differences required.

22. What is the primary carbon-fixing enzyme in the initial step of the C4 pathway?

The primary carbon-fixing enzyme in the initial step of the C4 pathway is PEP carboxylase (phosphoenolpyruvate carboxylase). This enzyme fixes CO2 to PEP (phosphoenolpyruvate) in the mesophyll cells, forming the 4-carbon compound oxaloacetate.

23. 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 O2 for binding to RuBisCO, greatly reducing the oxygenation reaction that leads to photorespiration. This allows C4 plants to maintain high photosynthetic efficiency even in conditions that would increase photorespiration in C3 plants.

24. How do CAM plants manage the accumulation of acids during nighttime CO2 fixation?

CAM plants store the organic acids (primarily malic acid) produced from nighttime CO2 fixation in their large vacuoles. During the day, these acids are broken down, releasing CO2 for use in the Calvin cycle. This temporal separation allows CAM plants to fix CO2 at night when water loss is minimized, and use it for photosynthesis during the day when light is available.

25. What role do carbon fixation pathways play in global carbon cycling?

Carbon fixation pathways play a crucial role in global carbon cycling by determining how efficiently plants can remove CO2 from the atmosphere and incorporate it into biomass. C4 plants, despite being less common, contribute significantly to global primary productivity due to their high efficiency. The distribution and productivity of C3, C4, and CAM plants influence carbon storage in terrestrial ecosystems and can affect global climate patterns.

26. How do the carbon fixation pathways affect the optimal temperature for photosynthesis?

The optimal temperature for photosynthesis varies among the pathways. C3 plants typically have lower optimal temperatures (around 15-25°C) due to increased photorespiration at higher temperatures. C4 plants have higher optimal temperatures (30-45°C) because their CO2 concentrating mechanism reduces photorespiration. CAM plants can have variable optimal temperatures depending on the species and their specific adaptations.

27. What is the relationship between carbon fixation pathways and plant growth rates?

Generally, C4 plants have the potential for higher growth rates than C3 plants, especially in warm, high-light environments. This is due to their higher photosynthetic efficiency and reduced photorespiration. CAM plants typically have slower growth rates due to the energy costs of their water-conserving strategy. However, actual growth rates depend on many factors beyond just the carbon fixation pathway.

28. How do C4 plants concentrate CO2 around RuBisCO?

C4 plants concentrate CO2 around RuBisCO through a spatial separation of initial carbon fixation and the Calvin cycle. CO2 is first fixed in mesophyll cells by PEP carboxylase, then the resulting 4-carbon compounds are transported to bundle sheath cells. Here, CO2 is released near RuBisCO, creating a high CO2 concentration that favors carboxylation over oxygenation.

29. How does temperature affect the relative efficiency of C3 and C4 pathways?

Temperature significantly affects the relative efficiency of C3 and C4 pathways. C3 plants are generally more efficient at lower temperatures (below about 25°C or 77°F), while C4 plants become more efficient at higher temperatures. This is because higher temperatures increase photorespiration in C3 plants, while the C4 pathway minimizes this effect.

30. What are the energy costs associated with the C4 pathway?

The C4 pathway requires more energy than the C3 pathway due to the additional steps involved in concentrating CO2. Specifically, it requires 2 ATP per CO2 molecule for the regeneration of PEP (phosphoenolpyruvate) in the mesophyll cells. However, this extra energy cost is often offset by the increased efficiency and reduced photorespiration, especially in hot, high-light environments.

31. 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 CAM and C3 pathways depending on environmental stress. For example, some facultative CAM plants use C3 photosynthesis when water is plentiful but switch to CAM during drought conditions.

32. How does nitrogen use efficiency compare among C3, C4, and CAM plants?

C4 plants generally have the highest nitrogen use efficiency, followed by CAM plants, with C3 plants having the lowest. This is because C4 plants require less RuBisCO (a nitrogen-rich enzyme) to achieve the same photosynthetic rates as C3 plants, due to their CO2 concentrating mechanism. CAM plants also use nitrogen efficiently due to their water-conserving strategy.

33. What are some evolutionary advantages of the C4 pathway?

The C4 pathway provides several evolutionary advantages, including:

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

Bundle sheath cells are crucial in C4 photosynthesis as they are the site of the Calvin cycle and where CO2 is concentrated. These cells have thick walls that prevent CO2 leakage, contain high levels of RuBisCO, and are the location where the 4-carbon compounds from mesophyll cells release CO2 for fixation by RuBisCO. This spatial separation is key to the C4 pathway's efficiency.

35. How do C4 plants adapt to low-light conditions?

C4 plants are generally less adapted to low-light conditions compared to C3 plants due to their higher energy requirements. However, some C4 plants have adaptations for shade tolerance, including:

36. What are the primary environmental factors that influence the distribution of C3, C4, and CAM plants?

The primary environmental factors influencing the distribution of C3, C4, and CAM plants include:

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

C3 plants generally show the strongest positive response to increasing atmospheric CO2 levels because their photosynthesis is not CO2-saturated under current conditions. C4 plants show a smaller response because they already concentrate CO2 around RuBisCO. CAM plants' response can vary depending on the species and environmental conditions, but is generally intermediate between C3 and C4 plants.

38. How do the carbon fixation pathways affect plant responses to drought stress?

CAM plants are typically the most drought-tolerant due to their ability to fix CO2 at night when water loss is minimized. C4 plants are generally more drought-tolerant than C3 plants because they can maintain photosynthesis with partially closed stomata. C3 plants are usually the least drought-tolerant, as they must keep stomata open for efficient CO2 uptake, leading to greater water loss.

39. How do the products of the initial carbon fixation step differ among C3, C4, and CAM pathways?

In the C3 pathway, the initial product is a 3-carbon compound, 3-phosphoglycerate (3-PGA). In the C4 pathway, the initial product is a 4-carbon compound, oxaloacetate, which is quickly converted to malate or aspartate. In the CAM pathway, the initial nighttime product is also a 4-carbon compound, usually malate, which is stored in vacuoles until daytime.

40. What are some agricultural implications of the different carbon fixation pathways?

The carbon fixation pathways have significant agricultural implications:

41. How do C3, C4, and CAM plants differ in their leaf-level water use efficiency?

Leaf-level water use efficiency (WUE) typically follows the order: CAM > C4 > C3. CAM plants have the highest WUE because they open their stomata at night when evaporative demand is low. C4 plants have higher WUE than C3 plants because they can maintain high photosynthetic rates with partially closed stomata. C3 plants generally have the lowest WUE because they must keep stomata open wider to achieve sufficient CO2 uptake.