C3 plants are simply the plants we have been looking at already in photosynthesis. In C3 plants, CO2 is fixed from the atmosphere into the 3 carbon compound GP using the RuBisCO enzyme. Remember the problem here; above 25OC the RuBisCO starts to prefer oxygen over CO2 fixing (the CO2 and O2 are competing), reducing the rate of photosynthesis. C4 plants avoid this problem by having a more efficient way to fix the CO2.
Essentially, the CO2 is used in another biochemical pathway using pyruvate (3C) which is converted eventually to oxaloacetate (4C), and eventually to the compound malate (4C). This allows a ‘store’ of CO2 to be built up that can be released directly into the Calvin cycle. In these higher concentrations the CO2 is more readily absorbed by the RuBisCO in preference to O2.
The problem is the CO2 has to be fixed twice, which requires more energy than a C3 pathway. C3 plants require 18 molecules of ATP to synthesise 1 molecule of glucose; C4 plants require around 30 ATP per molecule. It turns out to be a balancing act – C4 plants are found in places where there is a higher temperature that would favour RuBisCO taking O2, for example the tropics. C3 plants are found in more temperate regions. C4 plants are also more water efficient, doing better in dry conditions.
Despite being only around 3% of all terrestrial plants, C4 plants are responsible for around 30% of terrestrial carbon fixation.