C4 pathway: a photosynthetic pathway in which the carbon dioxide taken into the plant is first used to form a molecule that contains four carbon atoms. In the C3 pathway, the reaction in which carbon dioxide combines with ribulose bisphosphate to produce the three-carbon molecule glycerate 3-phosphate is very slow, particu larly if the level of carbon dioxide falls much below its normal level. In hot, dry con ditions the stomata of plants often close during the daylight hours and the carbon dioxide level in the leaf falls as it is used for photosynthesis. The C4 pathway involves a carbon dioxide-concentrating system that helps avoid this problem. Carbon diox ide is taken into the mesophyll cells of the plant and converted to a molecule that contains four carbon atoms. This reaction can take place in very low concentrations of carbon dioxide. It can then be released into the relatively few cells in which the C3 pathway takes place producing high concentrations of carbon dioxide in these cells. The disadvantage of this pathway is that it requires rather more energy in the form of ATP. It is therefore only effective in conditions of high light intensity. Plants that have the C4 pathway include a number of tropical species such as sugar cane and corn.