9.3 Photorespiration and C4/CAM pathways
3 min read•july 22, 2024
can significantly impact photosynthesis efficiency, especially in . This process occurs when fixes oxygen instead of carbon dioxide, reducing overall carbon gain. It's particularly problematic in hot, dry environments.
Plants have evolved different photosynthetic pathways to combat photorespiration. C3 is the most common, while C4 and CAM are adaptations for harsh conditions. These pathways help plants thrive in various environments, from grasslands to deserts.
Photorespiration and Its Impact on Photosynthesis
Photorespiration and efficiency impact
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- Photorespiration occurs when RuBisCO enzyme fixes oxygen instead of carbon dioxide competes with in photosynthesis, reducing overall photosynthetic efficiency
- Favored by high temperatures, low CO2 concentrations, and high light intensities (deserts, hot climates)
- Consumes energy and releases previously fixed carbon dioxide, reducing net carbon gain from photosynthesis
- Can decrease photosynthetic efficiency by up to 50% in C3 plants under unfavorable conditions (drought, heat stress)
Comparison of C3, C4, and CAM Photosynthetic Pathways
C3, C4, and CAM pathways
- C3 photosynthesis: most common pathway used by majority of plant species (wheat, rice, soybeans)
- Carbon fixation in mesophyll cells using RuBisCO enzyme
- Directly fixes CO2 into 3-carbon compound 3-phosphoglycerate (3-PGA)
- Vulnerable to photorespiration under high temperatures and low CO2 conditions
- C4 photosynthesis: adaptation to minimize photorespiration and enhance efficiency in hot, dry environments (grasslands, savannas)
- Carbon fixation in two stages and cell types: mesophyll and bundle sheath cells
- Mesophyll cells initially fix CO2 into 4-carbon compound using phosphoenolpyruvate carboxylase (PEPC) enzyme
- 4-carbon compound transported to bundle sheath cells, decarboxylated to release CO2 for RuBisCO
- High CO2 concentration in bundle sheath cells suppresses photorespiration
- CAM photosynthesis: adaptation to conserve water and minimize photorespiration in arid environments (deserts, rock outcrops)
- Carbon fixation in same cell but at different times (temporal separation)
- At night when stomata open, CO2 fixed into 4-carbon compound by PEPC, stored as malic acid in vacuoles
- During day when stomata closed, stored malic acid decarboxylated, releasing CO2 for RuBisCO in chloroplasts
- Temporal separation of CO2 fixation and Calvin cycle minimizes water loss and photorespiration
Adaptations for harsh environments
- C4 adaptations:
- Spatial separation of initial CO2 fixation (mesophyll) and Calvin cycle (bundle sheath)
- High CO2 concentration in bundle sheath suppresses photorespiration by favoring RuBisCO's carboxylase over oxygenase activity
- Efficient CO2 fixation even under low atmospheric CO2 and high temperatures
- CAM adaptations:
- Temporal separation of CO2 fixation (night) and Calvin cycle (day)
- Nocturnal CO2 fixation when stomata open reduces water loss
- Fixed CO2 stored as malic acid in vacuoles provides CO2 source for Calvin cycle during day when stomata closed
- Minimizes photorespiration by maintaining high CO2 around RuBisCO during day
Ecological roles of C4 and CAM plants
- C4 plants dominate grasslands, savannas, subtropical regions with high temperatures and moderate to low rainfall
- Important crops like maize, sugarcane, sorghum use
- Contribute significantly to global primary productivity and carbon fixation
- CAM plants adapted to arid and semi-arid environments like deserts, rock outcrops
- Examples: cacti, agaves, many succulents
- Crucial for ecosystem function and biodiversity in water-limited environments
- Provide food and shelter for various desert animals
- Both C4 and CAM plants contribute to resilience and productivity of ecosystems under changing climatic conditions, especially in regions facing increasing temperatures and water scarcity
Key Terms to Review (12)
C3 plants: C3 plants are a type of plant that utilizes the C3 carbon fixation pathway during photosynthesis, where carbon dioxide is directly fixed into a three-carbon compound, 3-phosphoglycerate. This process occurs in the mesophyll cells and is the most common form of photosynthesis, especially in temperate climates. However, C3 plants are less efficient in hot and dry conditions, leading to the potential for photorespiration.
C4 pathway: The C4 pathway is a photosynthetic process that allows certain plants to efficiently fix carbon dioxide in conditions of high temperature and low carbon dioxide concentration. This pathway is characterized by the initial fixation of CO2 into a four-carbon compound, which is then converted to malate or aspartate, allowing plants to minimize photorespiration and optimize photosynthesis, especially in environments where water loss is a concern.
Carbon fixation: Carbon fixation is the process by which inorganic carbon dioxide ($$CO_2$$) is converted into organic compounds, primarily through the actions of plants, algae, and some bacteria during photosynthesis. This essential step in the carbon cycle allows for the incorporation of carbon into sugars and other organic molecules that serve as energy sources for living organisms, linking it directly to metabolic processes such as photorespiration and specialized pathways like C4 and CAM.
Drought tolerance: Drought tolerance refers to the ability of a plant to survive and thrive in conditions of limited water availability. This adaptation allows plants to maintain their physiological functions despite experiencing drought stress, which can be crucial for survival in arid environments. Drought tolerance is often linked to various physiological, biochemical, and morphological adaptations that reduce water loss and enhance water-use efficiency.
High temperature: High temperature refers to elevated environmental conditions that can impact biological processes, particularly in plants during photosynthesis. In the context of plants, high temperatures can lead to increased rates of photorespiration, affecting the efficiency of carbon fixation and overall energy production. This stress condition often prompts plants to adopt alternative pathways such as C4 and CAM to mitigate the adverse effects of heat.
Low co2 concentration: Low CO2 concentration refers to a situation in which the levels of carbon dioxide in the atmosphere or within plant cells are reduced compared to typical levels. This condition is significant because it directly impacts photosynthesis, particularly in plants that rely on specific pathways such as photorespiration or C4/CAM processes for efficient carbon fixation.
Nitrogen Use Efficiency: Nitrogen use efficiency (NUE) is a measure of how effectively plants utilize nitrogen for growth and development, particularly in relation to the amount of nitrogen applied through fertilizers. High NUE indicates that a plant is able to maximize its growth while minimizing nitrogen wastage, which is crucial for agricultural productivity and environmental sustainability. This efficiency is particularly significant in understanding plant responses to different photosynthetic pathways, as varying pathways can influence nitrogen requirements and utilization.
Pep carboxylase: PEP carboxylase is an enzyme that catalyzes the conversion of phosphoenolpyruvate (PEP) and carbon dioxide into oxaloacetate, playing a crucial role in the C4 and CAM pathways of photosynthesis. This enzyme is essential for plants that have adapted to minimize photorespiration by effectively fixing carbon dioxide, which enhances their efficiency in carbon fixation under high light intensity and temperature conditions.
Photorespiration: Photorespiration is a metabolic pathway that occurs in plants when the enzyme RuBisCO oxygenates ribulose bisphosphate instead of carboxylating it, leading to a loss of fixed carbon and energy. This process typically occurs under high oxygen and low carbon dioxide concentrations, causing inefficiencies in photosynthesis, particularly when the Calvin cycle is active. It connects with other pathways like C4 and CAM, which have evolved to minimize photorespiration in certain plant species.
Photorespiratory carbon loss: Photorespiratory carbon loss refers to the reduction in carbon dioxide fixation that occurs when the enzyme RuBisCO catalyzes the reaction of oxygen instead of carbon dioxide during the process of photosynthesis. This inefficient reaction can lead to a significant loss of fixed carbon, ultimately impacting plant productivity and efficiency in carbon assimilation. The phenomenon is especially prevalent in C3 plants, which rely on RuBisCO's activity for the initial step of carbon fixation, making them particularly vulnerable to photorespiration under certain environmental conditions.
Rubisco: Rubisco, or ribulose-1,5-bisphosphate carboxylase/oxygenase, is an essential enzyme that catalyzes the first step of carbon fixation in the Calvin cycle, where it helps incorporate carbon dioxide into organic molecules. This enzyme plays a critical role in photosynthesis, enabling plants to convert inorganic carbon into biomass. However, Rubisco is also known for its tendency to catalyze a reaction with oxygen instead of carbon dioxide, leading to photorespiration, which can be inefficient for plants. This connection makes Rubisco a key player in both the Calvin cycle and alternate carbon fixation pathways.
Stomatal Conductance: Stomatal conductance refers to the measure of how easily carbon dioxide (COâ‚‚) and water vapor can move through the stomata, small openings on the surface of leaves. This process is crucial for photosynthesis and transpiration, as it directly influences the exchange of gases between the leaf and the atmosphere, affecting plant growth and efficiency in using water and COâ‚‚.