3.4 Photorespiration and C4/CAM photosynthesis
3 min read•august 7, 2024
is a wasteful process that occurs when fixes oxygen instead of carbon dioxide. It reduces photosynthetic efficiency, especially in . High temperatures and dry conditions make photorespiration worse, leading to significant carbon loss.
C4 and 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
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- 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
- have a unique 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 , 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 (), hence the name
- The 4-carbon compound is then converted to 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
Key Terms to Review (21)
C3 plants: C3 plants are a group of plants that utilize the C3 pathway for photosynthesis, where carbon dioxide is fixed into a three-carbon compound, 3-phosphoglycerate (3-PGA), during the Calvin cycle. This process is efficient under normal conditions but can lead to photorespiration under high temperatures and low carbon dioxide concentrations, making these plants less adaptable to certain environmental stresses compared to C4 and CAM plants.
C3 vs C4 Photosynthesis: C3 and C4 photosynthesis are two distinct pathways that plants use to convert sunlight into energy. C3 photosynthesis, which occurs in most plants, uses the Calvin cycle directly to fix carbon dioxide into a three-carbon compound, while C4 photosynthesis incorporates a preliminary step to concentrate carbon dioxide, allowing for more efficient sugar production in high-temperature environments. Understanding the differences between these pathways is crucial for grasping how plants adapt to varying environmental conditions and the implications of photorespiration.
C4 Photosynthesis: C4 photosynthesis is a metabolic pathway that enables certain plants to efficiently fix carbon dioxide, minimizing photorespiration and maximizing photosynthetic efficiency in hot and dry environments. This process involves an initial fixation of carbon dioxide into a four-carbon compound, which is then transported to specialized cells for further processing. C4 plants adapt to high temperatures and low water availability by utilizing a unique mechanism that allows them to capture sunlight and convert it into energy more effectively than C3 plants under similar conditions.
C4 plants: C4 plants are a group of plants that utilize a unique photosynthetic pathway to efficiently fix carbon dioxide into a four-carbon compound, instead of the typical three-carbon compound seen in C3 plants. This adaptation allows them to minimize photorespiration and thrive in high light, high temperature, and low carbon dioxide environments. The efficiency of this pathway makes C4 plants particularly resilient to climate change impacts, as they can better cope with increased temperatures and variable water availability.
CAM Adaptation: CAM adaptation, or Crassulacean Acid Metabolism adaptation, is a photosynthetic process used by certain plants to minimize water loss while still capturing carbon dioxide. This adaptation allows plants in arid environments to open their stomata at night to take in CO2, which is then stored as organic acids and used during the day for photosynthesis when the stomata are closed. This process helps reduce photorespiration and optimizes water use efficiency, making it an essential mechanism for survival in dry conditions.
CAM Photosynthesis: CAM photosynthesis, or Crassulacean Acid Metabolism, is a photosynthetic adaptation found in certain plants that allows them to conserve water by opening their stomata at night instead of during the day. This process minimizes water loss in arid conditions, making it particularly advantageous for plants in dry environments. By storing carbon dioxide as malate overnight and using it during the day for photosynthesis, these plants can thrive in extreme conditions while still producing energy efficiently.
CAM plants: CAM (Crassulacean Acid Metabolism) plants are a specialized group of plants that utilize a unique photosynthetic pathway to adapt to arid environments by minimizing water loss. They take in carbon dioxide at night and store it as malate, which is then used during the day for photosynthesis while the stomata remain closed to conserve water. This adaptation allows CAM plants to thrive in conditions where water is scarce, connecting their survival strategies to the broader concepts of photorespiration and efficient carbon fixation.
Carbon Fixation: Carbon fixation is the process of converting inorganic carbon dioxide (CO₂) into organic compounds, primarily through photosynthesis in plants. This is crucial for life on Earth, as it forms the foundation of the food chain by producing energy-rich carbohydrates that can be utilized by plants and other organisms. The efficiency and mechanisms of carbon fixation vary among different plants, significantly influencing their photosynthetic pathways and adaptation to environmental conditions.
CO2 Assimilation: CO2 assimilation is the process by which plants convert carbon dioxide from the atmosphere into organic compounds during photosynthesis. This process is crucial for plant growth, as it forms the foundation for building carbohydrates, which serve as an energy source and structural material for the plant. The efficiency of CO2 assimilation can be influenced by various factors including light intensity, temperature, and the type of photosynthetic pathway used by the plant, such as C3, C4, or CAM pathways.
High temperature: High temperature refers to elevated thermal conditions that can significantly impact plant physiological processes, particularly photosynthesis and respiration. In the context of plants, high temperatures can lead to increased rates of photorespiration and alter the efficiency of carbon fixation, particularly in C3 plants. Understanding how high temperatures affect these processes is crucial for comprehending plant responses to environmental stressors.
Leaf Anatomy: Leaf anatomy refers to the structural organization and arrangement of tissues within a leaf, which is essential for its functions such as photosynthesis, gas exchange, and water regulation. Understanding leaf anatomy is crucial because different plants have evolved various adaptations in their leaf structure to optimize these processes, especially in relation to mechanisms like photorespiration and C4/CAM photosynthesis, which are strategies that help plants manage environmental stressors like drought and high temperatures.
Low co2 concentration: Low CO2 concentration refers to a condition where the levels of carbon dioxide in the atmosphere are reduced, which can significantly impact plant processes such as photosynthesis. Under low CO2 conditions, plants may struggle to efficiently fix carbon, leading to changes in metabolic pathways like photorespiration. This situation is particularly relevant for plants using different photosynthetic strategies, influencing their adaptation and efficiency in capturing carbon.
Malate: Malate is a four-carbon organic acid that plays a crucial role in various metabolic pathways, particularly in the context of photosynthesis and respiration. It acts as an important intermediate in the citric acid cycle and serves as a key component in the C4 and CAM pathways of photosynthesis, facilitating the fixation and transport of carbon dioxide in plants. Malate helps to regulate pH levels and transport energy within plant cells, making it a vital compound for plant metabolism.
Oxaloacetate: Oxaloacetate is a four-carbon dicarboxylic acid that plays a crucial role in the citric acid cycle and serves as a key intermediate in various metabolic pathways. It is vital for the process of photosynthesis in C4 and CAM plants, as it helps in carbon fixation and the efficient utilization of CO2. Its formation and regeneration are critical in optimizing photosynthetic efficiency and mitigating photorespiration.
Pep carboxylase: PEP carboxylase is an enzyme that catalyzes the conversion of phosphoenolpyruvate (PEP) and carbon dioxide into oxaloacetate, playing a crucial role in C4 and CAM photosynthesis pathways. This enzyme helps plants efficiently fix carbon dioxide, particularly under conditions where oxygen competes with carbon dioxide for binding sites on Rubisco, reducing the negative effects of photorespiration. By using PEP instead of ribulose bisphosphate (RuBP), plants can enhance their carbon fixation efficiency in hot and dry environments.
Photorespiration: Photorespiration is a metabolic process in plants that occurs when the enzyme RuBisCO oxygenates ribulose-1,5-bisphosphate instead of carboxylating it, leading to the consumption of energy and release of CO2. This process typically takes place under high light intensity and low carbon dioxide concentrations, which can negatively impact plant productivity by reducing the efficiency of photosynthesis.
Resource Allocation: Resource allocation refers to the process by which plants distribute their available resources, such as water, nutrients, and energy, to different growth and metabolic processes. This strategic distribution is crucial for optimizing growth and reproduction while responding to environmental conditions, including light availability and temperature. In the context of photosynthesis, resource allocation plays a significant role in balancing carbon fixation and photorespiration, especially in plants using different photosynthetic pathways like C4 and CAM.
Ribulose bisphosphate: Ribulose bisphosphate (RuBP) is a five-carbon sugar phosphate that plays a crucial role in the process of carbon fixation during photosynthesis. It acts as the substrate for the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the first major step of the Calvin cycle, converting RuBP and carbon dioxide into 3-phosphoglycerate. The efficiency of RuBP in binding to carbon dioxide is essential for the overall success of photosynthesis and impacts plant productivity.
Rubisco: Rubisco, or ribulose-1,5-bisphosphate carboxylase/oxygenase, is an enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle. This enzyme is crucial for converting inorganic carbon dioxide into organic compounds that can be utilized by plants. Rubisco's activity directly influences the efficiency of photosynthesis and the overall rate of carbon assimilation in plants, linking it closely to processes such as photorespiration and alternative photosynthetic pathways like C4 and CAM photosynthesis.
Stomatal Conductance: Stomatal conductance refers to the rate at which carbon dioxide (CO2) enters and water vapor exits a leaf through the stomata, which are tiny openings on the leaf surface. This process plays a crucial role in photosynthesis and transpiration, affecting plant water use efficiency and gas exchange. Stomatal conductance can be influenced by environmental factors such as light, humidity, and soil moisture, as well as plant physiological responses, making it essential in understanding how plants adapt to changing conditions.
Water Use Efficiency: Water use efficiency (WUE) refers to the ratio of carbon dioxide assimilated through photosynthesis to the amount of water lost through transpiration. It is a crucial metric in understanding how effectively plants use water during the process of photosynthesis, directly impacting their growth and survival in various environments. High WUE indicates that a plant is able to produce more biomass while conserving water, which is especially vital under conditions of limited water availability.