Plants perform photosynthesis to convert light energy into chemical energy, primarily sugars. However, plants also undergo photorespiration, a natural metabolic pathway that consumes oxygen and releases carbon dioxide. This process seemingly undoes some carbon fixation achieved by photosynthesis.
The Role of RuBisCO
The central player in both photosynthesis and photorespiration is the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enzyme is arguably the most abundant protein on Earth. RuBisCO has a dual nature, binding to either carbon dioxide (CO2) or oxygen (O2).
In photosynthesis, RuBisCO’s primary role is carboxylation, catalyzing the reaction of CO2 with ribulose-1,5-bisphosphate (RuBP) to initiate the Calvin cycle and produce sugars. However, RuBisCO can also bind to O2 in a process called oxygenation, which starts the photorespiration pathway. The competition between CO2 and O2 for RuBisCO’s active site determines which reaction proceeds, influencing carbon fixation efficiency.
The Photorespiratory Pathway
When RuBisCO binds with oxygen, it initiates the photorespiratory pathway, a multi-step process involving three plant organelles: chloroplasts, peroxisomes, and mitochondria. This pathway begins in the chloroplast where RuBP reacts with O2, producing 3-phosphoglycerate (3-PGA) and phosphoglycolate. While 3-PGA can proceed in the Calvin cycle, phosphoglycolate cannot and is considered a toxic byproduct.
Phosphoglycolate undergoes transformations across these organelles. It converts to glycolate in the chloroplast, then moves to the peroxisome. In the peroxisome, glycolate becomes glyoxylate and then glycine. Two glycine molecules then travel to the mitochondrion, converting into serine and releasing CO2 and ammonia.
Serine then returns to the peroxisome, becoming glycerate, which re-enters the chloroplast to convert back into 3-PGA, rejoining the Calvin cycle. This pathway consumes ATP and reducing power (NADPH) that would otherwise be used for sugar synthesis. It also results in the net loss of previously fixed carbon as CO2, making it an energetically costly process for the plant.
Environmental Triggers
Photorespiration is heavily influenced by environmental conditions, particularly temperature and the concentrations of carbon dioxide and oxygen within the plant cell. High temperatures increase the solubility of oxygen more than carbon dioxide, making it more likely for RuBisCO to bind with O2. This shift favors the oxygenation reaction.
Conditions leading to low CO2 and high O2 concentrations inside the leaf promote photorespiration. On hot, dry days, plants often close their stomata to conserve water and prevent dehydration. This closure restricts external CO2 intake while photosynthesis produces O2, leading to O2 buildup and CO2 depletion within the leaf. This imbalance pushes RuBisCO towards its oxygenase activity, increasing photorespiration.
Impact on Plant Efficiency
Photorespiration is often viewed as a wasteful process because it consumes energy and releases carbon dioxide without producing sugars. Unlike photosynthesis, which fixes carbon to create glucose, photorespiration releases a CO2 molecule that was previously fixed. This means the energy (ATP and NADPH) generated during photosynthesis’s light-dependent reactions is expended in photorespiration rather than contributing to carbohydrate synthesis.
The direct consequence of photorespiration is a reduction in net photosynthetic yield and overall plant productivity, especially in C3 plants. Under certain environmental conditions, such as high temperatures and low CO2 availability, photorespiration can decrease photosynthetic output by up to 25% in C3 plants. This loss of fixed carbon and wasted energy can significantly limit plant growth and biomass production, particularly in agriculture.
Plant Strategies to Reduce Photorespiration
Over evolutionary time, some plants have developed adaptations to minimize photorespiration, particularly in hot, dry environments. Two prominent strategies are C4 photosynthesis and Crassulacean Acid Metabolism (CAM). Both aim to increase CO2 concentration around the RuBisCO enzyme, favoring its carboxylation activity over oxygenation.
C4 plants, such as corn and sugarcane, spatially separate initial CO2 fixation from the Calvin cycle. They first capture CO2 in outer mesophyll cells using an enzyme with high affinity for CO2 that does not react with O2. This CO2 is converted into a four-carbon compound, transported to specialized bundle sheath cells deep within the leaf. Here, the compound releases CO2, creating a high CO2 concentration around RuBisCO, minimizing photorespiration.
CAM plants, including cacti and pineapples, separate these processes temporally. They open stomata and take in CO2 only at night when temperatures are cooler and humidity is higher, reducing water loss. This CO2 is stored as organic acids in vacuoles. During the day, with stomata closed to conserve water, the stored CO2 is released within plant cells, providing a concentrated supply for RuBisCO to perform photosynthesis with reduced photorespiration.