Pathway:

Under general atmospheric conditions, rubisco adds CO₂ to ribulose 1, 5-bisphosphate. Moreover, when the CO₂ concentration is low, it can add O₂ as an alternative. This generates phosphoglycolate and 3-phosphoglycerate. A phosphoglycolate can be salvaged and used for biosynthetic reactions but the pathway for achieving this releases CO₂ and NH₄ and wastes metabolic energy. Since the net result of this procedure is to consume O₂ and release CO₂, it is known as photorespiration. This is a main problem for plants in hot climates. The plants close the gas replace pores  in their leaves  (stomata)  to conserve  water  but this leads  to a drop in the CO₂ concentration   within  the  leaf,  favoring  photorespiration. Additionally,  as  temperature   increase,  the  oxygenase  activity  of  rubisco  (using  O₂) raise more rapidly than the carboxylase  activity (using CO₂), again favoring photorespiration. To prevent these problems, some plants adapted to live in hot climates, such as corn and sugar cane which have evolved a mechanism to maximize the carboxylase activity of rubisco.  In this plant carbon fixation using the Calvin cycle takes place only in bundle-sheath cells which are protected from the air through mesophyll cells. Since the bundle-sheath cells are not showing to air and the O₂ concentration is low. The CO₂ is transported from the air through the mesophyll cells to the bundle-sheath cells through combining with three-carbon molecules (C3) to produce four-carbon molecules (C4).  These enter the bundle-sheath   cells where they are broken down to C3 compounds and releasing CO₂.  The C3 molecules revisit to the mesophyll cell to accept more CO₂. These cycles ensures a high CO₂ concentration for the carboxylase activity of rubisco action in the bundle- sheath cells.  Because  it  relies  on  CO₂ transport  through  four-carbon  molecules,  it  is known as the C4 pathway and plants which use this mechanism  are known as C4 plants. All other plants are known as C3 plants since they trap CO₂ straightly as the three- carbon compound 3-phosphoglycerate.

Details of the C4 pathway are shown in Figure. The steps included are as follows:

-  phosphoenolpyruvate (C3)   accepts   CO₂ to form oxaloacetate (C4); a reaction catalyzed through phosphoenolpyruvate carboxylase in the   mesophyll   cell

-  Oxaloacetate is transformed to malate (C4) by NADP⁺ linked malate dehydrogenase

-  malate enters the bundle-sheath  cell and releases CO₂, creating pyruvate (C3); catalyzed through NADP⁺    -linked malate enzyme

-  Pyruvate proceeds to the mesophyll cell and is used to regenerate phosphoenolpyruvate.  This reaction, catalyzed through pyruvate-P_i dikinase, is unusual in which it needs ATP and P_i and breaks a high-energy bond to produce AMP and pyrophosphate.

Figure:  The C4 pathway.

The pyrophosphate  from the pyruvate-P_i  dikinase is rapidly degraded so that, whole,  the  total  price  the  plant  pays  for  operation  of  this  CO₂    pump  is  the hydrolysis of two high-energy phosphate bonds for every molecule of CO ₂ transported:

CO₂ (in air) +ATP → CO₂ (bundle-sheath cell) +AMP +2 P_i