bio

C4 photosynthesis

A carbon pump some plant lineages evolved to head off Rubisco's oxygen mistake. The method: fix CO₂ first with a different enzyme that cannot confuse it with oxygen, out in the leaf's outer cell layer; ship the product into a more sealed inner compartment; then release the CO₂ there, so Rubisco always sits in a chamber where CO₂ is artificially concentrated — above a thousand ppm — and oxygen barely gets a look in. Maize, sugarcane and most tropical grasses run this way. The cost is two extra units of ATP per CO₂ delivered. That is a shrewd investment in carbon-thin air and money thrown away in carbon-rich air — so a CO₂-thick world would have no reason to evolve it at all.