bio

Photorespiration

The price of Rubisco grabbing oxygen instead of CO₂. Rather than sugar, the reaction yields a useless and mildly toxic two-carbon fragment, forcing the cell to run an entire salvage line: burning ATP, burning reducing power, and handing back one CO₂ for every two mistakes. In other words the leaf spends energy undoing its own error and loses the carbon it had just worked to capture. On Earth two things rescue it: Rubisco does prefer CO₂ by a factor of about ninety, and CO₂ dissolves in water roughly twenty-six times better than oxygen — so in practice a leaf gets about five good turns per mistake. The whole evolutionary arms race that produced C4 and CAM exists to fight this single flaw.