Certain plants—including the important crops sugarcane and corn (maize), as well as other diverse species believed to have evolved in the drier tropical areas—have developed a special mechanism of carbon fixation that largely prevents photorespiration. The C4 pathway acts as a shuttle for carrying carbon dioxide into the chloroplasts of the bundle sheath cells, where it is used in carbohydrate synthesis. The resulting higher level of internal carbon dioxide in these chloroplasts serves to increase the ratio of carboxylation to oxygenation, thus minimizing photorespiration. Although the plant must expend extra energy to drive this shuttle, the energy loss is more than compensated by the near elimination of photorespiration under conditions where it would otherwise occur. Sugarcane and certain other plants that employ this pathway have the highest annual yields of biomass of all species.
Source: Taken from Encyclopaedia Britannica 2002