Photorespiration or C2 Cycle or Photosynthetic Carbon Oxidation (PCO) Cycle

Photorespiration is an extra, light-dependent form of respiration that occurs specifically in photosynthetic cells; Decker (1959) first noted that respiration rate is measurably higher in light than in darkness, and this excess light-driven respiration is what photorespiration refers to. It is triggered whenever CO2 becomes scarce and oxygen relatively abundant at RuBisCO's active site, which shifts RuBisCO's behaviour from its normal carboxylase role toward an oxygenase role instead. When this happens, RuBisCO fixes O2 rather than CO2 onto RuBP, splitting the 5-carbon RuBP into one molecule of the 3-carbon PGA (which re-enters the ordinary Calvin cycle normally) and one molecule of the 2-carbon compound phosphoglycolate - and because this first distinctive product is a 2-carbon compound, the whole pathway is also called the C2 cycle, or the Photosynthetic Carbon Oxidation (PCO) cycle. The pathway is structurally unusual in spanning three separate organelles in sequence: phosphoglycolate, formed in the chloroplast, loses a phosphate group to become glycolate, which then moves into the peroxisome, where it is oxidised (consuming O2 and generating hydrogen peroxide as a byproduct) into glyoxylate; glyoxylate is transaminated into the amino acid glycine, which then moves into the mitochondrion, where two glycine molecules condense together to form one molecule of serine, releasing CO2 and ammonia (NH3) in the process; serine then moves back to the peroxisome, where it is converted, via the intermediate hydroxypyruvate and using NADH+H+, into glyceric acid; and glyceric acid finally re-enters the chloroplast, where it is phosphorylated at the cost of one ATP back into PGA, rejoining the mainstream Calvin cycle and closing the pathway. Unlike ordinary respiration, photorespiration yields no usable ATP at all - it is a pure net energy cost to the plant rather than an energy-generating process - and under conditions that favour it (high light, high oxygen, low CO2, warm temperatures), it can waste as much as 50% of a plant's potential photosynthetic productivity. Table 13.5 contrasts photorespiration point-by-point with ordinary dark (mitochondrial) respiration: photorespiration is confined to green, photosynthetic cells and occurs only in the presence of light, spans three organelles (chloroplast, peroxisome and mitochondrion) rather than the mitochondrion alone, does not involve glycolysis, the Krebs cycle or the …