Photorespiration

Photorespiration is a process in plants that occurs alongside photosynthesis, particularly in C₃ plants. It involves the uptake of oxygen (O₂) and the release of carbon dioxide (CO₂) by the plant. This process is considered inefficient in terms of carbon fixation because it counteracts photosynthesis by consuming energy and carbon compounds without producing ATP or sugar.

1.0What is Photorespiration? 

• Photorespiration represents a costly side reaction of photosynthesis, primarily due to the lack of specificity of the enzyme Rubisco. In this process, CO₂ serves as the preferred substrate for Rubisco, rather than molecular oxygen (O₂). However, O₂ and CO₂ compete for the active site of Rubisco, and after a few turnovers, Rubisco catalyzes the reaction with O₂, leading to the production of 3-phosphoglycerate and 2-phosphoglycolate, the latter being considered a metabolically wasteful byproduct. This occurs because of the oxygenase activity of Rubisco, where the process fails to contribute to carbon fixation, generates the unproductive byproduct 2-phosphoglycolate, and consumes considerable amounts of cellular energy along with the release of previously fixed CO₂.
• The solubility of O₂ and CO₂ varies with temperature, increasing as temperature rises. At higher temperatures, Rubisco's affinity for CO₂ diminishes, and its oxygenase activity becomes more prominent, steering it towards the less efficient oxygenase reaction. The byproduct, 2-phosphoglycolate, is processed through the glycolate pathway in the chloroplast, a pathway that, although salvages some products, incurs high costs. A phosphatase enzyme converts 2-phosphoglycolate to glycolate, which is then transported to the peroxisome. There, in the presence of molecular oxygen, glycolate is oxidized to glyoxylate by glycolic acid oxidase. 
• Subsequent transamination produces glycine, which is then transported to the mitochondria. In the mitochondria, the glycine decarboxylase complex catalyzes the oxidative carboxylation of glycine, resulting in the formation of serine, along with the release of CO₂ and NH₃, and the reduction of NAD⁺ to NADH. 
• A carbon fragment from glycine is transferred to the cofactor tetrahydrofolate. Serine hydroxymethyltransferase then transfers a carbon unit from tetrahydrofolate to another glycine molecule, producing serine. This serine is then converted to hydroxypyruvate, and subsequently to glycerate by α-hydroxy acid reductase. Glycerate is transported back to the chloroplast stroma, where it is converted into 3-phosphoglycerate, ready to enter the Calvin cycle, thereby salvaging the pathway. Thus, the combined activities of Rubisco and the glycolate pathway contribute to photorespiration, also known as the C₂ cycle or the oxidative photosynthetic carbon cycle.

2.0Adaptation to Countering Photorespiration 

Plants have evolved various adaptations to counteract or minimize the effects of photorespiration, a process that can significantly impact their efficiency in carbon fixation and energy use. These adaptations are particularly crucial in hot, arid environments where plants are more susceptible to the detrimental effects of photorespiration due to higher oxygenase activity of Rubisco. Here are some of the key adaptations:

C4 Photosynthesis

• C4 photosynthesis is an adaptation that has evolved in some plants to minimize photorespiration. In C4 plants, carbon dioxide is first fixed into a four-carbon compound in mesophyll cells by the enzyme PEP carboxylase, which has a higher affinity for CO₂ and no affinity for O₂, unlike Rubisco. This four-carbon compound is then transported to bundle-sheath cells, where CO2 is released and refixed by Rubisco for the Calvin cycle. 
• This mechanism effectively concentrates CO₂ around Rubisco, significantly reducing its oxygenase activity and thus photorespiration. C4 plants include many of the world’s most productive crops and grasses, such as maize, sugarcane, and sorghum.

CAM Photosynthesis

• Crassulacean Acid Metabolism (CAM) is another adaptation that reduces photorespiration, common in plants living in arid conditions. Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.
• CAM plants fix CO₂ at night when the stomata are open, converting it into organic acids stored in vacuoles. During the day, when the stomata are closed to conserve water, these acids are decarboxylated to release CO₂, which is then fixed by Rubisco. This temporal separation of CO₂ fixation allows CAM plants to maintain high internal concentrations of CO₂ during the day, reducing Rubisco’s oxygenase activity and thus photorespiration. Examples of CAM plants include succulents, cacti, and some orchids.