Alternative Carbon Fixation Mechanisms
Alternative Mechanisms of Carbon Fixation
Plants require both light and raw materials such as carbon dioxide and water for photosynthesis, a process that converts light energy into chemical energy. The efficiency of this process is heavily influenced by the availability of these key components.
Terrestrial plants and aquatic plants with floating leaves have direct access to atmospheric carbon dioxide, ensuring a constant supply for their photosynthetic needs.
Submerged aquatic plants, however, must utilize dissolved carbonic acid in the water as their source of carbon dioxide, which can sometimes limit their photosynthetic capacity depending on water conditions.
The concentration of atmospheric carbon dioxide is only 0.04 \%, significantly lower than that of oxygen. This imbalance presents challenges for plants, particularly concerning the efficiency of carbon fixation.
Preventing Water Loss
Water is not only essential for photosynthesis but also for maintaining turgor pressure and transporting nutrients. Land plants have evolved various adaptations to limit water loss, allowing them to thrive in drier environments.
These adaptations often affect the plant's ability to exchange gases with the environment, necessitating a delicate balance between water conservation and carbon dioxide uptake.
A waxy cuticle covers the leaves of terrestrial plants, acting as a barrier to prevent water loss by evaporation. The thickness and composition of this cuticle can vary depending on the plant species and its environment.
Stomata, tiny pores on the leaves, play a crucial role in controlling gas exchange. They open during the day to allow carbon dioxide intake for photosynthesis and close at night to conserve water.
Transpiration, the loss of water vapor through the stomata when they are open, is an inevitable consequence of gas exchange. Environmental factors like humidity, temperature, and wind speed can influence the rate of transpiration.
Stomata close during the day when plants are at risk of water loss, a response often triggered by hormonal signals like abscisic acid (ABA). This closure helps prevent dehydration but can also limit carbon dioxide availability.
Photorespiration: The Problem with Rubisco
Enzymes, including rubisco, catalyze biological reactions. Their efficiency is affected by factors such as temperature, pH, and the presence of inhibitors or activators. Understanding these factors is vital for optimizing plant metabolism
Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase, is the enzyme responsible for the initial step in carbon fixation. Despite being abundant, it is relatively slow, catalyzing only about three molecules of carbon dioxide per second.
Rubisco can bind with oxygen instead of carbon dioxide, leading to a wasteful process called photorespiration. This occurs because oxygen and carbon dioxide compete for the same active site on the rubisco enzyme.
Rubisco's dual capability to catalyze reactions with either carbon dioxide or oxygen gives it the full name ribulose-1,5-bisphosphate carboxylase oxygenase, reflecting its role in both photosynthesis and photorespiration.
The reaction between oxygen and RuBP produces a molecule not useful to the cell, which must be converted back through a complex process that consumes ATP and releases carbon dioxide, negating some of the energy gained through photosynthesis.
Under equal concentrations of oxygen and carbon dioxide, rubisco binds carbon dioxide more frequently, but the actual ratio is influenced by environmental conditions.
In nature, the concentration of oxygen is much higher than that of carbon dioxide, increasing the likelihood of photorespiration.
Under normal atmospheric conditions and moderate temperatures, rubisco binds with carbon dioxide about 75 \%, and with oxygen 25 \%, leading to a drain on cell resources. This inefficiency has driven the evolution of alternative carbon fixation pathways like C4 and CAM.
Photorespiration becomes a significant concern when the carbon dioxide supply is significantly reduced, such as when plants close their stomata to conserve water.
Terrestrial plants in hot, dry climates face the dual challenges of photorespiration and water loss, making them particularly susceptible to reduced photosynthetic efficiency.
Closing stomata to conserve water reduces carbon dioxide intake and increases photorespiration, creating a trade-off between water conservation and carbon assimilation.
The solubility of oxygen and carbon dioxide decreases as temperature increases, with carbon dioxide decreasing more rapidly, further exacerbating photorespiration. This is because rubisco is more likely to bind with oxygen as the temperature rises.
In high temperatures, photorespiration can waste up to 50 \%$ of the plant's energy, severely limiting growth and productivity. This necessitates the evolution of mechanisms to overcome this limitation.
C4 Plants
C4 plants, commonly found in hot, dry climates, minimize photorespiration through a specialized leaf structure and carbon fixation method, enhancing their photosynthetic efficiency.
Bundle-sheath cells, tightly packed cells surrounding leaf veins, are the sites where Calvin cycle reactions occur in C4 plants. This spatial separation helps concentrate carbon dioxide around rubisco.
Mesophyll cells, located around bundle-sheath cells, reduce their exposure to oxygen and operate a C4 cycle, an initial carbon fixation pathway that precedes the Calvin cycle.
In the C4 cycle, carbon dioxide combines with phosphoenolpyruvate (PEP) to produce oxaloacetate (4-carbon), a reaction catalyzed by PEP carboxylase.
Oxaloacetate is reduced to malate, which diffuses into bundle-sheath cells, where it is oxidized to pyruvate, releasing carbon dioxide. This carbon dioxide is then used in the Calvin cycle.
This mechanism effectively increases the carbon dioxide concentration around rubisco while simultaneously reducing oxygen exposure, minimizing photorespiration.
PEP carboxylase catalyzes the initial binding of carbon dioxide to PEP in the C4 cycle and has a greater affinity for carbon dioxide than oxygen, enabling efficient binding regardless of oxygen concentration.
Many tropical plants, including corn and sugarcane, utilize C4 metabolism, which allows them to thrive in hot, high-light environments.
C4 plants also use the Calvin cycle but in bundle-sheath cells, where the carbon dioxide concentration is elevated due to the C4 cycle.
The C4 cycle requires the double hydrolysis of ATP to AMP to regenerate PEP from pyruvate, equivalent to six ATP per G3P produced by the Calvin cycle. This energy cost is significant but justified under conditions favoring photorespiration.
In hot climates, the C4 pathway is worth the energy cost as photorespiration can decrease carbon fixation efficiency by over 50 \%$$. The ATP requirement is easily met due to lots of sunshine, and enhanced efficiency has a number of implications.
C4 plants can open their stomata less than C3 plants, enabling them to survive arid environments by reducing water loss.
C4 plants also require one-third to one-sixth less rubisco and have a much lower nitrogen demand, enabling them to survive in more nutrient-poor soil conditions. This is because rubisco constitutes a large portion of a plant's nitrogen investment.
In temperate climates, lower temperatures mean that photorespiration is less of a problem, and the additional ATP requirement is harder to meet with less sunshine, making C3 plants more competitive.
CAM Plants
C4 plants run Calvin and C4 cycles simultaneously but in different locations, whereas Crassulacean Acid Metabolism (CAM) plants run them in the same cells but at different times of day, providing another adaptation to minimize water loss in arid conditions.
CAM plants were first observed in the Crassulaceae family, known for their ability to store water in their leaves and stems, with nighttime accumulation of malic acid.
CAM plants typically live in hot and dry regions and exhibit several adaptations, including a low surface-to-volume ratio and fewer stomata. Furthermore, their stomata open only at night, when they release the oxygen that accumulates from photosynthesis during the day and allow carbon dioxide to enter.
Carbon dioxide is fixed by a C4 pathway into malate, which is stored as malic acid in cell vacuoles during the night, ensuring a supply of carbon for use during the day.
During the day, stomata close, reducing water loss and cutting off gas exchange, and malic acid diffuses from vacuoles into the cytosol, where malate is oxidized to pyruvate, releasing carbon dioxide.
The high carbon dioxide concentration favors rubisco's carboxylase activity, allowing efficient Calvin cycle operation with little photorespiration loss. This temporal separation of carbon fixation and the Calvin cycle is a hallmark of CAM plants.
Pyruvate produced during the day is converted back to malate during the night, requiring ATP expenditure. This regeneration of malate ensures a continuous cycle of carbon fixation and release.