3.4 Photorespiration and C4/CAM photosynthesis

Photorespiration is a wasteful process that occurs when Rubisco fixes oxygen instead of carbon dioxide. It reduces photosynthetic efficiency, especially in C3 plants. High temperatures and dry conditions make photorespiration worse, leading to significant carbon loss.

C4 and CAM plants have evolved special mechanisms to concentrate CO2 around Rubisco. This reduces photorespiration and allows them to thrive in hot, dry environments where C3 plants struggle. These adaptations make photosynthesis more efficient in challenging conditions.

Photorespiration in C3 Plants

The Photorespiration Process

Top images from around the web for The Photorespiration Process

• 

  Frontiers | The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in ... View original

  Is this image relevant?

• 

  Photosynthesis | Biology for Majors I View original

  Is this image relevant?

• 

  Using Light Energy to Make Organic Molecules | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Frontiers | The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in ... View original

  Is this image relevant?

• 

  Photosynthesis | Biology for Majors I View original

  Is this image relevant?

1 of 3

Top images from around the web for The Photorespiration Process

• 

  Frontiers | The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in ... View original

  Is this image relevant?

• 

  Photosynthesis | Biology for Majors I View original

  Is this image relevant?

• 

  Using Light Energy to Make Organic Molecules | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Frontiers | The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in ... View original

  Is this image relevant?

• 

  Photosynthesis | Biology for Majors I View original

  Is this image relevant?

1 of 3

• Photorespiration occurs when Rubisco fixes O2 instead of CO2 due to high O2 concentration or high temperatures
• Leads to the production of 2-phosphoglycolate (2-PG) which is toxic and must be recycled
• 2-PG is converted to glycolate in the chloroplast then transported to peroxisomes and mitochondria for recycling
• Recycling process converts glycolate back into 3-PGA which can re-enter the Calvin cycle
• Photorespiration is an energy-consuming process that reduces the efficiency of photosynthesis (up to 50% of fixed carbon can be lost)

Factors Affecting Photorespiration

• Photorespiration is more prevalent in C3 plants which include most major crops (wheat, rice, soybeans)
• High temperatures increase the rate of photorespiration because Rubisco's affinity for CO2 decreases while its affinity for O2 increases
• Dry conditions can cause stomata to close, reducing CO2 concentration in the leaf and increasing photorespiration
• Photorespiration is less significant in C4 and CAM plants due to their CO2 concentration mechanisms

C4 Photosynthesis

C4 Plant Anatomy and Physiology

• C4 plants have a unique leaf anatomy known as Kranz anatomy which consists of two distinct photosynthetic cell types: mesophyll cells and bundle sheath cells
• Mesophyll cells form a ring around the bundle sheath cells and contain PEP carboxylase, an enzyme with high affinity for CO2
• Bundle sheath cells contain Rubisco and carry out the Calvin cycle
• C4 plants have a CO2 concentration mechanism that allows them to concentrate CO2 in the bundle sheath cells, reducing photorespiration

The C4 Photosynthetic Pathway

• In mesophyll cells, CO2 is initially fixed by PEP carboxylase to form a 4-carbon compound (oxaloacetate), hence the name C4 photosynthesis
• The 4-carbon compound is then converted to malate or aspartate and transported to the bundle sheath cells
• In the bundle sheath cells, the 4-carbon compound is decarboxylated, releasing CO2 which is then fixed by Rubisco in the Calvin cycle
• The remaining 3-carbon compound is transported back to the mesophyll cells to regenerate PEP
• This CO2 concentration mechanism allows C4 plants to maintain high rates of photosynthesis even in hot, dry conditions (examples: corn, sugarcane, sorghum)

Crassulacean Acid Metabolism (CAM)

CAM Plant Adaptations

• CAM plants are adapted to hot, dry environments and include many succulents (cacti, aloe, jade plant)
• CAM plants have a unique CO2 concentration mechanism that allows them to open their stomata at night and close them during the day to conserve water
• At night, CAM plants fix CO2 using PEP carboxylase to form malic acid which is stored in the vacuole
• During the day, the stomata close and the stored malic acid is decarboxylated, releasing CO2 which is then fixed by Rubisco in the Calvin cycle

The CAM Photosynthetic Pathway

• The CAM pathway is divided into four phases that occur over a 24-hour period
• Phase I (night): Stomata open, CO2 is fixed by PEP carboxylase to form malic acid which is stored in the vacuole
• Phase II (early morning): Stomata close, malic acid continues to be stored
• Phase III (day): Malic acid is decarboxylated, releasing CO2 which is fixed by Rubisco in the Calvin cycle
• Phase IV (late afternoon/evening): Malic acid reserves are depleted, stomata may reopen to fix more CO2 if conditions allow
• The temporal separation of CO2 fixation and the Calvin cycle in CAM plants allows them to conserve water while still maintaining photosynthesis