Photorespiration

RuBisCO can bind both CO₂ and O₂ at its active site, and the binding is competitive: whichever gas is relatively more concentrated tends to bind. RuBisCO has a much greater affinity for CO₂ when CO₂ and O₂ are present in similar amounts.

In C₃ plants some O₂ does bind RuBisCO. RuBP then combines with O₂ to give one molecule of phosphoglycerate and one of phosphoglycolate; this oxygenase route is photorespiration.

Photorespiration makes no sugar and no ATP or NADPH. Instead it uses ATP and releases CO₂, making it a wasteful process.

In C₄ plants photorespiration does not occur, because the mechanism concentrates CO₂ at the enzyme site in bundle sheath cells, so RuBisCO acts as a carboxylase and its oxygenase role is minimised.

Lacking photorespiration helps explain why C₄ plants give better productivity and yields. They also tolerate higher temperatures. The biological function of photorespiration is still not known.