In many tropical grasses, including economically important plants such as maize, sorghum, and sugar cane, the isolation of rubisco is accomplished by a short carboxylation/decarboxylation pathway referred to as the Hatch-Slack cycle, after Marshall D Hatch and C Roger Slack, two plant physiologists who played key roles in the elucidation of the pathway. Plants containing this pathway are referred to as C4 plants because the immediate product of carbon dioxide fixation by the Hatch-Slack cycle is the four-carbon organic acid oxaloacetate. This term distinguishes such plants from C3 plants, in which the first detectable product of carbon dioxide fixation is the three-carbon compound three-phosphoglycerate.

To appreciate the advantage of the Hatch-Slack cycle, one must first consider the arrangement of the Hatch-Slack and Calvin cycles within the leaf of a C4 plant. C4 plants, unlike C3 plants, have in their leaves two distinct types of photosynthetic cells mesophyll cells and bundle sheath cells that differ in their enzyme composition and hence their metabolic activities. 

The first steps of carbon dioxide fixation within a C4 plant are accomplished by the Hatch-Slack cycle in mesophyll cells, which are exposed to the carbon dioxide and oxygen that enter a leaf through its stomata. The carbon dioxide that is fixed in mesophyll cells is subsequently released in bundle sheath cells, which are relatively isolated from the atmosphere. 

The entire Calvin cycle, including rubisco, is confined to chloroplasts in the bundle sheath cells. Because of the activity of the Hatch-Slack cycle, the carbon dioxide concentration in bundle sheath cells may be as much as 10 times the level in the atmosphere, strongly favouring rubisco's carboxylase activity and minimising its oxygenase activity.
The Hatch-Slack cycle begins with the carboxylation of phosphoenolpyruvate to form oxaloacetate. Not only does this carboxylase lack rubisco’s oxygenase activity, it is an excellent scavenger for carbon dioxide. In other words, it has a high affinity (a low Km) for its substrate, bicarbonate (HCO^), and operates very efficiently even when the concentration of bicarbonate is quite low. (Bicarbonate forms when carbon dioxide dissolves in water; its concentration therefore reflects the availability of carbon dioxide gas.)

In one version of the Hatch-Slack pathway, the oxalo-acetate generated by PEP carboxylase is rapidly converted to malate by an NADPH-dependent malate dehydrogenase. Malate is a stable four-carbon acid that carries carbon from mesophyll cells to chloroplasts of bundle sheath cells, where decarboxylation by NADP+ malic enzyme releases C02. The liberated carbon dioxide is then refixed and reduced by the Calvin cycle. 

Because decarboxylation of malate is accompanied by the generation of NADPH, the Hatch-Slack cycle also conveys reducing power from mesophyll to bundle sheath cells. This might limit the demand for non-cyclic electron flow from water to NADP+ in the bundle sheath cells, thereby minimising the formation of oxygen by PSII complexes and further favouring rubisco’s carboxylase activity.

Although less than one per cent of the plant species investigated depends on the Hatch-Slack cycle, the pathway is of particular interest because several economically important species are in this group. Moreover, C4 plants such as maize and sugar cane are characterised by net photo synthetic rates that are often two or three times those of C3 plants such as cereal grains. 

The writer is Associate Professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also fellow, Botanical Society of Bengal, and can be contacted at tapanmaitra59@yahoo.co.in.