Britannica Sources used in this answer

How does photorespiration reduce the efficiency of photosynthesis?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Photorespiration reduces the efficiency of photosynthesis by causing the enzyme Rubisco to act as an oxygenase rather than a carboxylase.12

Normally, Rubisco initiates the Calvin-Benson cycle by fixing carbon dioxide ( $C O_{2}$ ) to RuBP. However, in the presence of molecular oxygen, Rubisco can catalyze the oxygenation of RuBP instead. This reaction produces only one molecule of PGA and one molecule of phosphoglycolate, a two-carbon acid that is subsequently converted in part to $C O_{2}$ .12

Because this process releases $C O_{2}$ that was already fixed and consumes energy without producing sugar, it directly competes with the productive carboxylation reaction. This inefficiency is particularly pronounced under conditions of high temperature, high light intensity, and water limitation, where the internal concentration of oxygen relative to $C O_{2}$ increases. By diverting resources away from the synthesis of organic molecules, photorespiration significantly lowers the net fixation of carbon and overall photosynthetic productivity.123