The C4 plants are twice as efficient as C3 plants in terms of fixing CO2 but lose only half as much water as C3 plants for the same amount of CO2 fixed. In C3 plants, carbon dioxide is only fixed in the mesophyll leaves but in C4 plants, carbon dioxide is fixed in both mesophyll and bundle sheath cells. C4 plant regulate their constant supply of CO2 by keeping the stomata closed for a short span of time. CO2 fixation requires large amount of energy thus more water is lost in C3 than C4 plants.