4.3.4 C4 Photosynthesis: An Adaptation To the Problem of Photorespiration

Introduction

Some plants that grow in tropical climates have evolved an adaptation to reduce photorespiration. This adaptation is called the C4 pathway or Hatch-Slack cycle. Plants that use this pathway are called C4 plants. They have a special mechanism to concentrate CO₂ in the bundle sheath cells, which reduces photorespiration and increases photosynthetic efficiency.

Definition

C4 Photosynthesis: C4 photosynthesis is an adaptation to the problem of photorespiration in which plants use the Hatch-Slack cycle in addition to the Calvin cycle. The first stable product of CO₂ fixation is a 4-carbon compound (oxaloacetate), hence the name C4.

In simple words: C4 photosynthesis is a special pathway that some plants use to concentrate CO₂ and reduce photorespiration.

Core Concept

• Some plants which grow in tropical climate have an adaptation to the problem of photorespiration.
• They have an additional metabolic pathway in the light-independent phase of photosynthesis beside the Calvin cycle.
• This metabolic pathway is called the Hatch-Slack cycle or C4 pathway.
• In this pathway, phosphoenol pyruvate (PEP) carboxylase is used instead of RuBisCO to fix CO₂ to phosphoenol pyruvate (a C3 molecule).
• The result is oxaloacetate, a C4 molecule.
• It takes place in the cytoplasm of mesophyll cells.
• As the first product of CO₂ fixation is a 4-carbon compound oxaloacetate, the plants are called C4 plants.
• Examples: Maize, sugarcane, sorghum, etc.

Detailed Explanation

The C4 Pathway (Hatch-Slack Cycle)

Step 1: Fixation of CO₂ in Mesophyll Cells

• CO₂ from the atmosphere enters the mesophyll cells.
• In the cytoplasm of mesophyll cells, CO₂ is fixed to phosphoenol pyruvate (PEP) (a C3 molecule).
• The enzyme is PEP carboxylase.
• The product is oxaloacetate (a C4 molecule).

Equation: PEP + CO₂ PEP carboxylase→ Oxaloacetate

Key Point: PEP carboxylase has a high affinity for CO₂ and does not bind O₂. This is the key to reducing photorespiration.

Step 2: Conversion of Oxaloacetate to Malate

• Oxaloacetate is then transported to the chloroplasts of mesophyll cells.
• It is converted to another 4-C compound, malate.
• This conversion uses NADH produced in the photochemical phase.

Equation: Oxaloacetate + NADH → Malate + NAD⁺

Step 3: Transport of Malate to Bundle Sheath Cells

• Malate is then transported to the chloroplasts of bundle sheath cells.
• Bundle sheath cells surround the vascular bundles.

Step 4: Release of CO₂ in Bundle Sheath Cells

• In the bundle sheath cells, malate is converted to pyruvate (C3) with the release of CO₂.
• This increases the concentration of CO₂ in the bundle sheath cells.

Equation: Malate → Pyruvate + CO₂

Key Point: The release of CO₂ in bundle sheath cells creates a high concentration of CO₂, which favors the Calvin cycle and suppresses photorespiration.

Step 5: Calvin Cycle in Bundle Sheath Cells

• The bundle sheath cells contain the enzymes of the Calvin cycle.
• Because of the high concentration of CO₂, RuBisCO participates in the Calvin cycle and not in photorespiration.
• Sugar formed in the Calvin cycle is transported into the phloem.

Step 6: Regeneration of PEP in Mesophyll Cells

• Pyruvate generated in the bundle sheath cells re-enters the mesophyll cells.
• Pyruvate is converted back to phosphoenol pyruvate (PEP) by consuming one ATP.

Equation: Pyruvate + ATP → PEP + ADP + Pi

Summary of the C4 Pathway

Comparison of C3 and C4 Photosynthesis

Advantages of C4 Photosynthesis

• PEP carboxylase has a high affinity for CO₂ and does not bind O₂.
• CO₂ is concentrated in bundle sheath cells.
• RuBisCO is saturated with CO₂.
• Photorespiration is minimized.

• More CO₂ is fixed per unit of light.
• Higher photosynthetic rate.
• More sugar is produced.

• C4 plants have an optimum temperature of 30–40°C.
• They are more efficient at high temperatures.
• They are more tolerant to drought.

• C4 plants can use high light intensities efficiently.
• They have higher maximum photosynthetic rates.

• C4 plants can maintain higher CO₂ concentrations with partially closed stomata.
• They lose less water per CO₂ fixed.

• C4 plants need less RuBisCO because CO₂ is concentrated.
• They use nitrogen more efficiently.

Disadvantages of C4 Photosynthesis

• C4 pathway requires 2 extra ATP per CO₂ fixed.
• This makes C4 photosynthesis less efficient in cool, shaded conditions.

• C4 plants have specialized Kranz anatomy (bundle sheath cells).
• This requires more complex leaf structure.

• C4 plants are less efficient in cool, moist conditions.
• C3 plants dominate in temperate regions.

Examples of C4 Plants

Economic Importance of C4 Plants

• Maize, sorghum, and millet are important food crops.
• They are staple foods in many tropical countries.

2. Sugar Production: - Sugarcane is a major source of sugar.

3. Biofuel Production: - Maize and sugarcane are used for ethanol production.

4. Fodder: - Sorghum and millet are used as animal feed.

5. Industrial Uses: - Sugarcane is used to produce ethanol, molasses, and bagasse.

Intermediate Level

C4 Pathway in Detail

Comparison of C3 and C4 Plants

Advantages of C4 Photosynthesis

Economic Importance of C4 Plants

Important Points

• 1. Some plants which grow in tropical climate have an adaptation to the problem of photorespiration.
• 2. They have an additional metabolic pathway in light independent phase of photosynthesis beside Calvin cycle.
• 3. This metabolic pathway is called Hatch-Slack cycle of C4 pathway in which phosphoenol pyruvate (PEP) carboxylase is used instead of RuBisCO to fix CO₂ to phosphoenol pyruvate (a C3 molecule), and the result is oxaloacetate, a C4 molecule.
• 4. It takes place in cytoplasm of mesophyll cells.
• 5. As the first product of CO₂ fixation is a 4-carbon compound oxaloacetate, so the plants are called C4 plants e.g., maize, sugarcane, sorghum, etc.
• 6. Oxaloacetate is then transported to the chloroplasts of mesophyll cells.
• 7. It is then converted to another 4-C compound, the malate, with the help of NADH, produced in the photochemical phase.
• 8. The malate is then transported to the chloroplasts of bundle sheath cells.
• 9. Here malate is converted to pyruvate (C3) with the release of CO₂.
• 10. Thus concentration of CO₂ increases in the bundle sheath cells.
• 11. These cells contain enzymes of Calvin cycle.
• 12. Because of high concentration of CO₂, RubisCO participates in Calvin cycle and not in photorespiration.
• 13. Sugar formed in Calvin cycle is transported into the phloem.
• 14. Pyruvate generated in the bundle sheath cells re-enters mesophyll cells and regenerates phosphoenol pyruvate (PEP) by consuming one ATP.

Remember

• - C4 photosynthesis = adaptation to photorespiration
• - First product = oxaloacetate (4C)
• - **Enzyme** = PEP carboxylase
• - Location = mesophyll cells and bundle sheath cells
• - **Photorespiration** = low
• - Examples = maize, sugarcane, sorghum
• - Kranz anatomy = specialized leaf anatomy
• - Advantages = reduced photorespiration, increased efficiency, better at high temperatures
• - **Extra ATP cost** = 2 ATP per CO₂

Common Mistakes

• 1. Confusing C3 and C4 plants – C3 = first product 3-PGA; C4 = first product oxaloacetate.
• 2. Thinking C4 plants do not use the Calvin cycle – C4 plants use the Calvin cycle in bundle sheath cells.
• 3. Thinking C4 plants do not have photorespiration – C4 plants have reduced photorespiration, not zero.
• 4. Confusing PEP carboxylase and RuBisCO – PEP carboxylase fixes CO₂ in mesophyll cells; RuBisCO fixes CO₂ in bundle sheath cells.
• 5. Thinking C4 pathway occurs in all plants – C4 pathway occurs only in C4 plants.
• 6. Forgetting the role of bundle sheath cells – Bundle sheath cells concentrate CO₂ and run the Calvin cycle.
• 7. Thinking C4 plants are always better – C4 plants are better at high temperatures; C3 plants are better at low temperatures.
• 8. Confusing Kranz anatomy – Kranz anatomy is the specialized leaf anatomy of C4 plants.
• 9. Thinking C4 pathway does not require ATP – C4 pathway requires 2 extra ATP per CO₂.
• 10. Forgetting the examples – Maize, sugarcane, sorghum.
• 11. Thinking C4 plants are not important – C4 plants are important food and biofuel crops.
• 12. Confusing C4 and CAM plants – C4 plants separate CO₂ fixation spatially; CAM plants separate it temporally.

Quick Revision

• Some plants which grow in tropical climate have an adaptation to the problem of photorespiration.
• They have an additional metabolic pathway in light independent phase of photosynthesis beside Calvin cycle.
• This metabolic pathway is called Hatch-Slack cycle of C4 pathway in which phosphoenol pyruvate (PEP) carboxylase is used instead of RuBisCO to fix CO₂ to phosphoenol pyruvate (a C3 molecule), and the result is oxaloacetate, a C4 molecule.
• It takes place in cytoplasm of mesophyll cells.
• As the first product of CO₂ fixation is a 4-carbon compound oxaloacetate, so the plants are called C4 plants e.g., maize, sugarcane, sorghum, etc.
• Oxaloacetate is then transported to the chloroplasts of mesophyll cells.
• It is then converted to another 4-C compound, the malate, with the help of NADH, produced in the photochemical phase.
• The malate is then transported to the chloroplasts of bundle sheath cells.
• Here malate is converted to pyruvate (C3) with the release of CO₂.
• Thus concentration of CO₂ increases in the bundle sheath cells.
• These cells contain enzymes of Calvin cycle.
• Because of high concentration of CO₂, RubisCO participates in Calvin cycle and not in photorespiration.
• Sugar formed in Calvin cycle is transported into the phloem.
• Pyruvate generated in the bundle sheath cells re-enters mesophyll cells and regenerates phosphoenol pyruvate (PEP) by consuming one ATP.

60-Second Revision

• C4 photosynthesis = adaptation to photorespiration
• First product = oxaloacetate (4C)
• **Enzyme** = PEP carboxylase
• Location = mesophyll cells and bundle sheath cells
• **Photorespiration** = low
• Examples = maize, sugarcane, sorghum
• Kranz anatomy = specialized leaf anatomy
• Advantages = reduced photorespiration, increased efficiency, better at high temperatures
• **Extra ATP cost** = 2 ATP per CO₂
• Warning: C4 plants still use the Calvin cycle. They have reduced photorespiration, not zero. PEP carboxylase fixes CO₂ in mesophyll cells; RuBisCO fixes CO₂ in bundle sheath cells. C4 plants are better at high temperatures. C4 pathway requires 2 extra ATP per CO₂. C4 plants have Kranz anatomy. Maize, sugarcane, and sorghum are C4 plants.