光呼吸（英语：photorespiration）是所有使用卡尔文循环进行碳固定的细胞在光照和高氧低二氧化碳情况下发生的一个生化过程。它是卡尔文循环中一个损耗能量的副反应。过程中氧气被消耗，并且会生成二氧化碳。如果光呼吸发生在进行光合作用的生物中，那幺光呼吸会抵消约30%的光合作用。因此降低光呼吸被认为是提高光合作用性能的途径之一。但是人们后来发现，光呼吸有着很重要的细胞保护作用。

在光呼吸过程中，参与卡尔文循环的反应物1,5-二磷酸核酮糖（英文缩写为RuBP，本文中将简称为二磷酸核酮糖）和催化剂1,5-二磷酸核酮糖羧化酶/加氧酶（英文缩写为RuBisCO，本文中将简称羧化/加氧酶）发生了与其在光合作用中不同的反应。光合作用中，二磷酸核酮糖在羧化/加氧酶的催化下与二氧化碳结合增加一个碳原子，再经过一系列反应，最终生成3-磷酸甘油酸。后者再经过部分卡尔文循环中的步骤，可再次重新生成为二磷酸核酮糖（插图1和插图2）。但光呼吸过程中，二磷酸核酮糖在羧化/加氧酶的催化下生成2-磷酸乙醇酸。

Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, however approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C₃ plants. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.