Photorespiration In C3 And C4 plants: Overview, Examples, Materials

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

What Is Photorespiration?

Photorespiration is a process whereby the enzyme RuBP carboxylase/oxygenase of chloroplasts of plant cells oxygenates RuBP (ribulose bisphosphate) instead of carboxylating it, as in photosynthesis. One molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate are formed as by-products. In sharp contrast to photosynthesis, which produces glucose and oxygen, photorespiration consumes energy and releases fixed carbon in the form of CO₂; this makes it a wasteful pathway.

Photorespiration generally occurs when the concentration of oxygen gas is high and that of carbon dioxide is low, such as on hot and dry days when plants close their stomata to prevent the loss of water. This closes the stomata and limits the intake of CO₂ while increasing the level of O₂ inside the leaf, thus favouring oxygenation activity by RuBisCO. This is more common in C3 plants because they lack the mechanism required to concentrate CO₂ around RuBisCo.

Photorespiration lowers photosynthetic efficiency since it consumes ATP and NADPH that would otherwise be involved in carbon fixation and releases CO₂ that could have been fixed in the Calvin cycle. This lowered energy and carbon supply brings down the general productivity and growth of the plant, hence making it less competitive compared to plants which can suppress photorespiration.

Overview Of Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria transform the light energy from the sun into chemical energy, stored as glucose. There are two major stages through which photosynthesis takes place: the light-dependent one in which light energy is absorbed and converted in the thylakoid membranes of chloroplasts into ATP and NADPH, and the Calvin cycle, otherwise called the light-independent reactions, in which carbon dioxide is fixed into glucose in chloroplasts' stroma using the above-mentioned ATP and NADPH.

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Differences Between C3 And C4 Photosynthesis

C3 photosynthesis is the most common pathway: In this case, CO2 is directly fixed by Rubisco in the Calvin cycle and forms a three-carbon compound, 3-PGA. On the other hand, the processes of C4 photosynthesis include a set of reactions wherein, firstly, CO2 is fixed into a four-carbon compound called oxaloacetate in the mesophyll cells. Then, diffusion of this compound in the bundle-sheath cells releases CO2 and refixes it with Rubisco, effectively concentrating CO2 around Rubisco and minimizing photorespiration.

Photorespiration In C3 Plants

Photorespiration is especially important in C3 plants. They get their name from the fact that the first stable compound formed from carbon fixation is a three-carbon molecule, 3-PGA.

Description Of C3 Plants

Most common types of plants (e.g., wheat, rice, soybean)
Fix carbon directly through the Calvin cycle.
Lacking specialised anatomy to minimise photorespiration.

Mechanism Of Photorespiration In C3 Plants

Rubisco oxygenates RuBP instead of carboxylating it.
Produces 3-PGA and 2-phosphoglycolate
2-phosphoglycolate is regenerated by a suite of reactions that require the consumption of ATP and the release of CO2

Impact Of Photorespiration On Productivity Of C3 Plants

Reduces net carbon fixed
Low overall photosynthetic efficiency.
Low growth and yield of the plant.

Photorespiration In C4 Plants

C4 plants have evolved a way to minimise photorespiration, which makes them more productive than most other plants in certain environments.

Description Of C4 Plants

Examples include maize, sugarcane and sorghum.
Leaves have a distinctive anatomy with mesophyll and bundle sheath cells.
Have a two-step carbon fixation.

Photorespiration Mechanism In C4 Plants

PEP carboxylase fixes CO₂in mesophyll cells
First formed four-carbon compound—oxaloacetate
Oxaloacetate transported to bundle sheath cells
CO₂ release and refixation by Rubisco in the Calvin cycle.

How C4 Plants Avoid Photorespiration

PEP carboxylase binds CO₂ more tightly and does not bind O₂
High concentration of CO₂ around Rubisco, inhibiting its oxygenation activity.
The spatial separation of initial CO₂ fixation and the Calvin cycle.

Advantages Of C4 Plants Over High-Temperature Environments

More productive under high light intensity and temperatures
High water-use efficiency because stomata do not open widely
Higher productivity and yield in the tropics and subtropics

Comparative Analysis: C3 Vs. C4 Plants

The differences between C3 and C4 plants explain their efficiencies and adaptations to different environments.

Differences In Leaf Anatomy Between C3 And C4 Plants

C3 plants: Uniform mesophyll cells, all with chloroplasts
C4 plants: Specialised bundle-sheath cells with packed chloroplasts, tightly ensheathing the vascular bundles.

Enzymatic Differences: Rubisco vs. PEP Carboxylase

C3 plants: Only Rubisco involved in CO₂ fixation
C4 plants: Initial fixation by PEP carboxylase followed by Rubisco in a high-CO₂ environment.

Comparative Efficiency In Various Environmental Conditions

C3 plants: More productive in cooler, wetter climates
C4 plants: superior in hot and dry conditions due to minimised photorespiration and efficient use of water.

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Frequently Asked Questions (FAQs)

1. What is photorespiration, and why does it take place?

Photorespiration is a process in which, on account of oxygenation by the enzyme RuBP, energy and carbon dioxide get wasted. It mainly takes place in plants of the C3 category under high oxygen and low carbon-di-oxide conditions.

2. How do plants of the C4 category minimise photorespiration?

C4 plants reduce the levels of photorespiration by taking up a two-step photosynthetic process to ensure high CO2 concentration around the Rubisco enzyme, excluding the interaction of oxygen with it.

3. What are the main differences between C3 and C4 plants?

C3 plants fix CO2 directly through the Calvin cycle, while C4 plants use an additional step to concentrate CO2, making them more efficient in hot, dry environments.

4. Why is photorespiration considered inefficient for plants?

Photorespiration is regarded as inefficient because it results in the loss of reduced carbon and energy, hence reducing the overall photosynthetic efficiency of the plant.

5. How does climate change impact photorespiration?

Climate change can elevate photorespiration due to increasing temperatures and variable CO2. This can lower crop yields.

6. What are the key differences between C3 and C4 photosynthesis?

The main differences are: 1) C3 plants use only the Calvin cycle, while C4 plants use both the C4 cycle and Calvin cycle. 2) C3 plants have higher rates of photorespiration, while C4 plants minimize it. 3) C3 plants have a single type of chloroplast, while C4 plants have two types (in mesophyll and bundle sheath cells). 4) C3 plants are less efficient in hot, dry environments, while C4 plants are better adapted to these conditions.

7. What is the primary enzyme involved in photorespiration?

The primary enzyme involved in photorespiration is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO can fix both CO2 (carboxylation) and O2 (oxygenation). When it fixes O2, it initiates the photorespiratory pathway. The dual function of RuBisCO is a result of its evolutionary history and the changing composition of Earth's atmosphere over time.

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

C3 plants have a uniform leaf anatomy with mesophyll cells containing chloroplasts. C4 plants have a specialized "Kranz" anatomy with two distinct types of photosynthetic cells: mesophyll cells in an outer ring and bundle sheath cells in an inner ring around the vascular bundles. This arrangement allows C4 plants to spatially separate the initial carbon fixation from the Calvin cycle, reducing photorespiration.

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

The CO2 compensation point is the CO2 concentration at which photosynthesis exactly balances respiration, resulting in no net CO2 fixation. C3 plants have a higher CO2 compensation point (around 50-100 ppm) due to photorespiration. C4 plants have a much lower CO2 compensation point (0-10 ppm) because they can continue fixing CO2 efficiently even at very low concentrations, thanks to their CO2 concentrating mechanism.

10. What are some examples of C3 and C4 plants?

Examples of C3 plants include wheat, rice, barley, oats, soybeans, and most trees and flowering plants. C4 plants include corn (maize), sugarcane, sorghum, millet, and many tropical grasses. It's important to note that only about 3% of flowering plant species are C4, but they contribute significantly to global primary productivity, especially in tropical and subtropical regions.

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

PEP carboxylase (phosphoenolpyruvate carboxylase) is a key enzyme in C4 photosynthesis. It catalyzes the initial fixation of CO2 in mesophyll cells, combining it with phosphoenolpyruvate to form oxaloacetate. This step is crucial because PEP carboxylase has a higher affinity for CO2 than RuBisCO and doesn't react with O2, allowing efficient carbon fixation even when CO2 concentrations are low.

12. How does the energy cost of C4 photosynthesis compare to C3?

C4 photosynthesis requires more energy than C3 photosynthesis. The C4 pathway uses 2 ATP per CO2 fixed in the initial steps, in addition to the 3 ATP and 2 NADPH used in the Calvin cycle. This totals to 5 ATP and 2 NADPH per CO2 fixed in C4 plants, compared to 3 ATP and 2 NADPH in C3 plants. However, this extra energy cost is often offset by increased efficiency and reduced photorespiration, especially in hot or dry conditions.

13. How does the efficiency of photosynthesis compare between C3 and C4 plants?

C4 plants generally have higher photosynthetic efficiency than C3 plants, especially in hot, dry, or high-light environments. This is because C4 plants minimize photorespiration and can continue photosynthesizing even when stomata are partially closed to conserve water. However, the C4 pathway requires more energy (ATP) per CO2 fixed. In cooler or more moderate environments, C3 plants can be equally or more efficient.

14. How do environmental factors affect the relative advantages of C3 vs C4 photosynthesis?

Environmental factors greatly influence the relative advantages of C3 vs C4 photosynthesis. C4 plants have an advantage in hot, dry, high-light environments because they can concentrate CO2 and reduce water loss. C3 plants perform better in cooler, more humid, or shaded environments where photorespiration is naturally lower. Climate change and increasing CO2 levels may alter these dynamics, potentially reducing the advantage of C4 plants in some areas.

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

Carbonic anhydrase plays a crucial role in C4 photosynthesis by catalyzing the rapid interconversion of CO2 and bicarbonate (HCO3-) in mesophyll cells. This enzyme ensures a steady supply of CO2 for PEP carboxylase, which uses bicarbonate as its substrate. By maintaining the equilibrium between CO2 and bicarbonate, carbonic anhydrase facilitates efficient carbon fixation in the initial step of the C4 pathway.

16. What is the role of the enzyme RuBisCO activase in photosynthesis?

RuBisCO activase plays a crucial role in maintaining the activity of RuBisCO in both C3 and C4 plants. It removes inhibitory sugar phosphates from RuBisCO's active sites and facilitates the carbamylation of RuBisCO, which is necessary for its catalytic activity. Without RuBisCO activase, RuBisCO would gradually become inactive, severely limiting photosynthesis. The activity of RuBisCO activase is sensitive to high temperatures, which can contribute to decreased photosynthetic efficiency under heat stress.

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

The C4 pathway generally increases the light saturation point of photosynthesis compared to C3 plants. The light saturation point is the light intensity above which photosynthetic rate does not increase. C4 plants can continue to increase their photosynthetic rate at higher light intensities because their CO2

18. What is the quantum yield of photosynthesis, and how does it differ between C3 and C4 plants?

The quantum yield of photosynthesis is the amount of CO2 fixed or O2 evolved per quantum of light absorbed. C3 plants typically have a higher quantum yield than C4 plants under low light conditions. This is because the C4 pathway requires more energy (ATP) per CO2 fixed. However, under high light and temperature conditions, C4 plants often have a higher effective quantum yield due to reduced photorespiration. This difference in quantum yield contributes to the ecological distribution of C3 and C4 plants and their performance under various environmental conditions.

19. What is photorespiration and why is it considered wasteful?

Photorespiration is a process that occurs in C3 plants when the enzyme RuBisCO fixes oxygen instead of carbon dioxide. It's considered wasteful because it consumes energy and reduces the efficiency of photosynthesis by up to 25%. During photorespiration, no glucose is produced, and previously fixed carbon is released as CO2.

20. What are the main products of the photorespiratory pathway?

The main products of the photorespiratory pathway are CO2, NH3 (ammonia), and 3-phosphoglycerate. The pathway begins when RuBisCO fixes O2 instead of CO2, producing 2-phosphoglycolate. This compound is then converted through a series of reactions in chloroplasts, peroxisomes, and mitochondria. The released CO2 represents a loss of previously fixed carbon, while the NH3 must be re-assimilated, both processes consuming energy.

21. Why does photorespiration occur more in hot and dry conditions?

Photorespiration increases in hot and dry conditions because: 1) Higher temperatures decrease the solubility of CO2 relative to O2 in leaf tissues. 2) Plants close their stomata to conserve water, reducing CO2 intake. 3) The oxygenase activity of RuBisCO increases with temperature more than its carboxylase activity. These factors increase the likelihood of RuBisCO fixing O2 instead of CO2, leading to more photorespiration.

22. How does photorespiration affect crop yields in agriculture?

Photorespiration can significantly reduce crop yields, especially in C3 crops like wheat and rice. It can decrease photosynthetic efficiency by up to 25% in normal conditions and even more in hot, dry environments. This has led to research into engineering C3 crops with C4-like traits or enhancing photorespiratory bypasses to improve yields. Understanding photorespiration is crucial for developing strategies to increase food production in the face of climate change.

23. How does temperature affect the relative efficiency of C3 and C4 photosynthesis?

Temperature significantly affects the relative efficiency of C3 and C4 photosynthesis. As temperature increases, photorespiration in C3 plants increases more rapidly than photosynthesis, reducing efficiency. C4 plants, with their CO2 concentrating mechanism, maintain efficiency at higher temperatures. Consequently, C4 plants generally outperform C3 plants in hot environments. However, at lower temperatures (below about 25°C), C3 plants can be more efficient due to the lower energy cost of their photosynthetic pathway.

24. How do C4 plants minimize photorespiration?

C4 plants minimize photorespiration through a specialized leaf anatomy and biochemical pathway. They concentrate CO2 around RuBisCO using PEP carboxylase in mesophyll cells, then transport the fixed carbon to bundle sheath cells where RuBisCO operates in a CO2-rich environment. This spatial separation reduces oxygen's competition with CO2, effectively suppressing photorespiration.

25. What is the evolutionary significance of photorespiration?

Photorespiration is often seen as an evolutionary relic. When RuBisCO evolved, Earth's atmosphere had much higher CO2 and lower O2 levels, so its oxygenase activity wasn't problematic. As O2 levels rose due to photosynthesis, photorespiration became more common. Rather than evolving a new carboxylase, plants developed ways to deal with photorespiration, including the C4 pathway. Understanding this helps explain why such an apparently inefficient process persists in modern plants.

26. How do C4 plants concentrate CO2 around RuBisCO?

C4 plants concentrate CO2 around RuBisCO through a spatial and biochemical mechanism. In mesophyll cells, PEP carboxylase fixes CO2 into four-carbon compounds. These compounds are transported to bundle sheath cells, where they are decarboxylated, releasing CO2. This creates a high CO2 concentration around RuBisCO in the bundle sheath cells, promoting carboxylation and suppressing oxygenation. This CO2 pump can increase CO2 levels up to 10 times higher than in C3 plants.

27. 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 the Calvin cycle and CO2 concentration. They have thick walls that prevent CO2 leakage and contain numerous chloroplasts. The four-carbon compounds produced in mesophyll cells are transported to bundle sheath cells, where they release CO2. This creates a CO2-rich environment around RuBisCO, promoting carboxylation and suppressing oxygenation, thus minimizing photorespiration.

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

C3 and C4 plants respond differently to increasing atmospheric CO2 levels. C3 plants generally show a more pronounced positive response, with increased photosynthetic rates and reduced photorespiration as CO2 levels rise. C4 plants, already having a CO2 concentrating mechanism, show a more limited response to elevated CO2. This differential response could potentially alter plant community compositions and crop competitiveness in the future as atmospheric CO2 continues to increase.

29. What is the significance of the C4 cycle in terms of water use efficiency?

The C4 cycle significantly improves water use efficiency in plants. By concentrating CO2 around RuBisCO, C4 plants can achieve high photosynthetic rates even when stomata are partially closed to conserve water. This allows them to maintain productivity in hot, dry environments where C3 plants would struggle. As a result, C4 plants typically have higher water use efficiency, requiring less water per unit of biomass produced compared to C3 plants.

30. What is the role of malate in C4 photosynthesis?

Malate plays a crucial role in C4 photosynthesis as a carrier of fixed carbon between mesophyll and bundle sheath cells. In many C4 plants, after CO2 is initially fixed by PEP carboxylase, the resulting four-carbon compound is converted to malate. Malate then diffuses into bundle sheath cells, where it is decarboxylated, releasing CO2 for the Calvin cycle. This malate shuttle is key to the CO2 concentrating mechanism in these C4 plants.

31. How do C3 and C4 plants differ in their nitrogen use efficiency?

C4 plants generally have higher nitrogen use efficiency than C3 plants. This is because C4 plants can achieve higher photosynthetic rates with less RuBisCO, which is a major nitrogen investment for plants. The CO2 concentrating mechanism in C4 plants allows RuBisCO to operate more efficiently, so less of the enzyme is needed. Consequently, C4 plants can allocate more nitrogen to other processes or maintain high photosynthetic rates with lower leaf nitrogen content compared to C3 plants.

32. What is the significance of the enzyme phosphoenolpyruvate carboxykinase (PEPCK) in some C4 plants?

Phosphoenolpyruvate carboxykinase (PEPCK) is an important enzyme in some C4 plants, particularly in what's known as the PEPCK-type C4 pathway. In these plants, PEPCK catalyzes the decarboxylation of oxaloacetate in bundle sheath cells, releasing CO2 for the Calvin cycle. This is an alternative to the more common NAD-malic enzyme or NADP-malic enzyme decarboxylation steps. The PEPCK pathway has slightly different energetics and may offer advantages in certain conditions, contributing to the diversity of C4 photosynthetic subtypes.

33. How does the distribution of C3 and C4 plants vary globally, and why?

The global distribution of C3 and C4 plants is largely influenced by climate. C4 plants are more prevalent in tropical and subtropical regions, particularly in hot, dry areas with high light intensity. They dominate in grasslands and savannas in these regions. C3 plants are more common in temperate and cooler climates, and in shaded environments. This distribution reflects the adaptive advantages of each photosynthetic pathway: C4 plants' ability to minimize photorespiration gives them an edge in hot, high-light environments, while C3 plants perform well in cooler or more moderate conditions.

34. How do C3 and C4 plants differ in their stomatal conductance and water use?

C4 plants generally have lower stomatal conductance than C3 plants under similar conditions. This means they can partially close their stomata to conserve water while still maintaining high photosynthetic rates due to their CO2 concentrating mechanism. As a result, C4 plants typically have higher water use efficiency, losing less water per unit of carbon gained. This adaptation allows C4 plants to thrive in hot, dry environments where C3 plants would struggle due to water loss and increased photorespiration.

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

In some C4 plants, particularly those using the NAD-malic enzyme subtype, aspartate plays a crucial role as a carbon carrier. After the initial fixation of CO2 by PEP carboxylase in mesophyll cells, the resulting oxaloacetate is converted to aspartate. Aspartate then moves to the bundle sheath cells, where it is converted back to oxaloacetate and then decarboxylated. This aspartate shuttle is an alternative to the malate shuttle and contributes to the diversity of C4 photosynthetic mechanisms.