This destroys the CO2 acceptor molecule and generates unwanted phosphoglycollate, which has to be metabolised to regenerate the lost acceptor. The process is called photorespiration it needs oxygen, releases CO2, and uses about half the plant's photosynthetic energy. Photorespiration can be artificially inhibited by increasing the CO2 concentration so that it can compete better with oxygen. This is done commercially by enriching the air in greenhouses with CO2, when crop yield may be doubled. C4 plants do the same thing with a built-in enrichment system.
Photosynthesis occurs in two layers of cells surrounding the vascular bundles.
The system acts as a CO2 pump, increasing the CO2 concentration in the bundle sheath to a level where photorespiration is inhibited. To do this, I looked for a likely mechanism for the stepwise evolution of the C4 syndrome to see if it could be mimicked by artificial selection. It soon became apparent that the C4 pathway was complex and its mode of evolution totally unknown. The clue to how it could have arisen came from the kranz anatomy of C4 plants and the fact that they evolved in hot dry regions. They have evolved separately many times in different families.