Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.9 Photorespiration

Photorespiration is a light-activated process involving the release of CO₂ and the uptake of O₂. Because glycolic acid is its primary product, photorespiration is also known as the glycolate pathway. It is enhanced under conditions of low CO₂ and high O₂ concentrations in the ambient air.

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Figure 4.7 - Organic acid METABOLISM during Crassulacean Acid Metabolism (CAM Photosynthesis) (adapted from V. V. Polovyi)

Under these conditions, chloroplast ribulose bisphosphate carboxylase catalyzes not the carboxylation of ribulose-1,5-bisphosphate, but rather its Cleavage into 3-phosphoglyceric and 2-phosphoglycolic acids. The latter is subsequently dephosphorylated to yield glycolic acid.

Glycolic acid is transported from the chloroplast into the peroxisome, where it is oxidized to glyoxylic acid by glycolate oxidase. The hydrogen peroxide generated in this reaction is decomposed by catalase present within the Peroxisomes.

Glyoxylic acid undergoes amination to form Glycine. Glycine is then transported to the Cell/35.html">Mitochondria, where two glycine molecules are condensed to synthesize Serine with the release of CO₂.

Serine can also enter the peroxisome, where, through the action of an aminotransferase, it transfers its amino group to pyruvic acid to form Alanine, while being converted into hydroxypyruvic acid. The latter is reduced to glyceric acid in a NADPH-dependent reaction. Glyceric acid then returns to the Chloroplasts, where it enters The Calvin Cycle (Fig. 4.8).

In C₄ plants, carbon dioxide released during photorespiration reacts in mesophyll Cells with phosphoenolpyruvate to produce oxaloacetic and malic acids. Malic acid is then transported to bundle sheath cells, where it serves as a CO₂ donor.