Photorespiration occurs when the CO2 levels inside a leaf become low. This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss. If the plant continues to attempt to fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations.
When the CO2 levels inside the leaf drop to around 50 ppm, Rubisco starts to combine O2 with RuBP instead of CO2.
The net result of this is that instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolateis produced.
The plant must get rid of the phosphoglycolate
First it immediately gets rid of the phosphate group, converting the molecule to glycolic acid. The glycolic acid is then transported to the peroxisome and there converted to glycine.
The glycine is then transported into a mitochondria where it is converted into serine.
The serine is then used to make other organic molecules. All these conversions cost the plant energy and results in the net lost of CO2 from the plant.
To prevent this process, two specialized biochemical additions have been evolved in the plant world: C4 and CAM metabolism which we will review in the following pages....