Carbon dioxide Hatch-Slack pathway

The lithoautotrophs have to form cellular materials from carbon dioxide. The process to change carbon dioxide into organic compounds is called fixation of carbon dioxide. On the basis of the knowledge to date, all algae and cyanobacteria, and many of the plants, fix carbon dioxide through the Calvin-Benson cycle (or reductive pentose phosphate cycle) (Bassham et al., 1954), while the plants of 20 families and 1200 species have been known to fix carbon dioxide through the Hatch-Slack pathway (or C4 dicarboxylate pathway) (Hatch et al., 1967). 

As a result, oxaloacetate (OAA, C4-compound) is formed unlike the case of the Calvin-Benson cycle in which 3-phosphoglycerate (C3-compound) is formed. The pathway in the fixation of carbon dioxide by the catalysis of PEP-carboxylase is observed in sugar cane, corn, etc., and is called the Hatch-Slack pathway (Hatch et al., 1967). The plants having the Hatch-Slack pathway have chloroplasts both in mesophyll cells and in vascular bundle sheath cells, and the Hatch-Slack pathway occurs in the mesophyll cells. Oxaloacetate formed by the fixation of carbon dioxide in the mesophyll cells is reduced to malate. Malate thus formed moves to the vascular bundle sheath cells and releases carbon dioxide there. Carbon dioxide released is fixed by the catalysis of Rubisco, and the organic compounds are formed through the Calvin-Benson cycle. (Fig. 6.3). 

The plants producing organic compounds from carbon dioxide through the Calvin-Benson cycle are called C3-plants, while the plants producing organic compounds from carbon dioxide through the Hatch-Slack pathway are called C4-plants. 

Hart A, Murrel JC, Poole RK, Norris PR (1991) An acid-stable cytochrome in iron-oxidizing Leptospirillum ferrooxidans. FEMS Microbiol Lett 81 89-94 Hatch MD, Slack CR, Johnson HS (1967) Further studies on a new pathway of photosynthetic carbon dioxide fixation in sugar-cane and its occurrence in other plant species. Biochem J 102 417-422... 

Slack CR and Hatch MD (1967) Comparative studies on the activity of carboxylases and other enzymes in relation to the new pathway of photosynthetic carbon dioxide fixation in tropical grasses. Biochem J 103 660-665... 

It has been suggested that the Hatch and Slack pathway is a modification of the Calvin cycle of advantage to plants growing in dense stands of tropical vegetation where the carbon dioxide concentration may be reduced to a very low level. The reduction of atmospheric CO2 concentration which has occurred since the evolution of photosynthetic reactions may also have contributed to the selection of this reaction sequence. The discovery of the Hatch and Slack pathway, although it is not yet fully authenticated, has demonstrated the possible existence of photosynthetic reactions other than the conventional Calvin cycle and suggests that other and as yet undiscovered variations on the photosynthetic theme may exist. 

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