What is photorespiration and how does it affect photosynthesis?

Photorespiration is a process that reduces the efficiency of photosynthesis by consuming oxygen and releasing carbon dioxide.

Photorespiration, also known as C2 photosynthesis, is a metabolic pathway in plants that competes with the Calvin cycle, the main pathway of photosynthesis. It occurs in the stroma of chloroplasts, where it consumes oxygen and releases carbon dioxide, the opposite of what happens in photosynthesis. This process is triggered when the concentration of carbon dioxide inside the leaf is low, and the concentration of oxygen is high, which typically happens in hot and dry conditions.

The process begins when the enzyme RuBisCO, which normally catalyses the reaction of carbon dioxide with Ribulose-1,5-bisphosphate (RuBP) in the Calvin cycle, instead reacts with oxygen. This results in the production of a two-carbon compound called phosphoglycolate, which is then converted into glycolate. The glycolate is transported to peroxisomes and mitochondria, where it is converted back into carbon dioxide in a series of reactions that also consume ATP and NADPH, energy-rich compounds produced in the light-dependent reactions of photosynthesis.

Photorespiration is considered a wasteful process because it reduces the efficiency of photosynthesis. Instead of fixing carbon dioxide into organic compounds, which is the primary function of photosynthesis, photorespiration releases carbon dioxide. Moreover, it consumes energy in the form of ATP and NADPH, which could otherwise be used for the synthesis of glucose and other organic compounds.

However, it's important to note that photorespiration also has some beneficial effects. It helps to protect plants from photooxidative damage, which can occur when there is an excess of light energy. It also plays a role in the metabolism of nitrogen in plants. Despite these benefits, many plants have evolved mechanisms to minimise photorespiration, such as the C4 and CAM pathways, which increase the concentration of carbon dioxide in the vicinity of RuBisCO, thereby favouring photosynthesis over photorespiration.