There are about 8000 species of angiosperms have developed adaptations which minimize the losses to photorespiration. They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence, these plants are called C4 plants.
- After entering through stomata CO2 diffuses into a mesophyll cell. Being close to the leaf surface, these cells are exposed to high levels of O2, but have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle).
- Instead, the CO2 is inserted into a 3-carbon compound called phosphoenolpyruvic acid (PEP) forming the 4-carbon compound oxaloacetic acid.
- Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which isTransported (plasmodesmata) into a bundle sheath cell. Bundle sheath cells are deep in the leaf so atmospheric oxygen cannot diffuse easily to them; often havethylakoids with reduced photosystem II complexes (the one that produces O2). Both of these features keep oxygen levels low.
- Here the 4-carbon compound is broken down into
- carbon dioxide, which enters the Calvin cycle to form sugars and starch.
- pyruvic acid (3 carbons), which is transported back to a mesophyll cell where it is converted back into PEP.