Biology · Ch 12 — Photosynthesis
Photorespiration
Photorespiration
Photorespiration is a wasteful process, tightly linked to the C3 (Calvin) cycle, that switches on under conditions of high temperature, bright light, high oxygen concentration and low CO2 concentration — instead of CO2 being fixed, it is released. It runs as a series of cyclic reactions spanning three organelles — the chloroplast, peroxisomes and mitochondria — sometimes called the PCO (photosynthetic carbon oxidation) cycle. Under these conditions, the enzyme Rubisco starts acting as an oxygenase rather than a carboxylase: RuBP reacts with O2 instead of CO2, producing one 3-carbon compound (PGA, which can still enter the Calvin cycle normally) and one 2-carbon compound, phosphoglycolate. This phosphoglycolate is converted to glycolate and shuttled out of the chloroplast into a peroxisome. …
What this figure shows. A three-organelle cycle (the PCO cycle) shown spanning the chloroplast, the peroxisome and the mitochondrion. In the chloroplast, RuBP reacts with O2 (oxygenation, catalysed by Rubisco) to give one 3-carbon PGA (which re-enters the Calvin cycle) and one 2-carbon phosphoglycolate, which becomes glycolate and is shuttled out to the peroxisome. In the peroxisome, glycolate is oxidised (via glyoxylate, releasing H2O2) into the amino acid glycine. In the mitochondrion, two glycine molecules are combined into one serine, releasing CO2 in the process; the serine is shuttled back to the peroxisome, converted …