What is Photorespiration, and How Do Plants Deal with It?

 by Christine Zhou

Photorespiration is a metabolic process that occurs in plants where photosynthesis takes place. This process is considered to be a waste of energy and a side effect of the enzyme rubisco, which plays a significant role in both photosynthesis and photorespiration. 

Rubisco is an enzyme found in chloroplasts of plant cells and is responsible for the initial step of carbon fixation in the Calvin cycle. In this step, carbon dioxide is combined to RuBp and forms two molecules of glycerate-3-phosphate, which are converted into triose phosphate then organic molecules, leading to the synthesis of glucose or other carbohydrates in the end. However, in photorespiration, rubisco reacts with oxygen rather than with carbon dioxide. This is due to the limitation of rubisco: carbon dioxide and oxygen have oxygen atoms on both sides of the molecules, so the active sites of rubisco can bind to both of the gas molecules. Under normal conditions, rubisco prefers carbon dioxide and catalyses the carboxylation reaction, but under some circumstances such as high temperatures or low carbon dioxide concentrations, the enzyme could bind to oxygen. 

In photorespiration, oxygen is added to RuBP, producing one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate. Unlike the Calvin cycle of photosynthesis, the photorespiratory pathway does not produce sugars. It consumes energy and reduces the overall efficiency of photosynthesis. This process can also lead to a net loss of fixed carbon, reducing the plant’s biomass production.

Since photorespiration is not favourable in plants, different plants use different ways to deal with it. Most plants are called C3 plants, which are adapted to temperate climates and can perform well under moderate temperatures and light conditions. However, they are most susceptible to photorespiration, particularly under high temperatures and in environments with limited water availability. C4 plants are good at enhancing carbon fixation efficiency, especially in high temperature and high light environments. The initial carbon dioxide fixation happens in the mesophyll cells of the leaves, where an enzyme called PEP carboxylase (which has a higher affinity to carbon dioxide compared to oxygen) catalyses the reaction between PEP and carbon dioxide to form oxaloacetate, a four-carbon molecule. It is then converted into another four carbon compounds such as malate or aspartate, which is then transported deeper inside the leaf to the bundle-sheath cells. Inside these cells, carbon dioxide is released from the four-carbon compounds, and then it can be used in the calvin cycle of photosynthesis. This ensures that only carbon dioxide can be around rubisco and increases the concentration of carbon dioxide around this enzyme. For CAM (Crassulacean Acid Metabolism) plants, they have a unique carbon fixation method. They open their stomata at night to take in carbon dioxide in a cooler and more humid environment. Then the carbon dioxide goes through the same pathway as which in C4 plants to become four-carbon compounds. During the day, when sunlight is available, CAM plants close their stomata to reduce water loss through transpiration, and the four-carbon compounds release carbon dioxide at the same time, which effectively concentrates carbon dioxide around rubisco and minimises the possibility of photorespiration. CAM plants are often found in arid and semi-arid regions where water availability is limited and daytime temperatures are high.