Hatch-Slack photosynthetic cycle

On the other hand, the Hatch-Slack photosynthetic cycle is important for tropical grasses, com and sugarcane. Phosphenol-pyr-uvate carboxylase is responsible for the primary carboxylation reaction. The first stable carbon compound is a C-4 acid, which is subsequently decarboxylated (101,102). The fundamentals and sys-tematics of the non-statistical distributions of isotopes in natural compounds have been reviewed (103). 

The most significant is the photos5mthetic mechanism. There are two classes of the photosynthetic cycle, the Calvin-Benson photosynthetic cycle and the Hatch-Slack photos5mthetic cycle. 

C4 plants green plants in which the primary product of CO2 fixation is not 3-phosphoglycerate (cf. C3 plants) but a C4 acid such as oxaloacetate, malate or aspartate. These plants possess two types of photi>-synthesizing cells. In mesophyll cells near the leaf surface, CO2 is fixed into C4-compounds. This prefixation of CO2 is due to the action of the cytosolic enzyme, phosphoeno/pyruvate carboxylase (EC 4.1.1.31), which carboxylates phosphoenolpyruvate to oxaloacetic acid (see Hatch-Slack-Kortschak cycle). The Calvin cycle (see) operates in the the vascular bundle cells of C4 plants, and CO2 for the Calvin cycle is derived from the decarboxylation C4 compounds rather than directly from the atmosphere. This Kranz anatomy , i.e. photosynthetically active bundle sheath cells with a photosynthetically active layer... 

Recall that the oxygenase activity of ruhisco increases more rapidly with temperature than does its carboxylase activity. How then do plants, such as sugar cane, that grow in hot climates prevent very high rates of wastefiil photorespiration Their solution to this problem is to achieve a high local concentration of CO2 at the site of the Calvin cycle in their photosynthetic cells. The essence of this process, which was elucidated by M. D. Hatch and C. R. Slack, is that four-carbon (Cf compounds such as oxaloacetate and malate carry CO2 from mesophyll cells, which are in contact with air, to bundle-sheath cells, which are the major sites ofphotosynthesis (Figure 20.17). Decarboxylation of the four-carbon compound in a bundle-sheath cell maintains a high concentration of CO2 at the site of the Calvin cycle. The three-carbon compound pyruvate returns to the mesophyll cell for another round of carboxylation. 

Some plants, such as corn and sugar cane, have evolved an auxiliary C4-dicarboxylic acid cycle< > that cooperates with the reductive pentose cycle in the photosynthetic assimilation of CO2. In plants with this cycle (sometimes referred to as the Hatch and Slack cycle), chloroplasts in the mesophyll cells near the surface on the leaf contain three C4-pathway specific enzymes pyruvate, phosphate-dikinase that directly converts pyruvate into phosphoenolpyruvate (PEP) with ATP, PEP carboxylase that catalyzes the carboxyla-tion of PEP to oxaloacetate, and malate dehydrogenase that finally reduces oxaloacetate to malate with NADPH. The purpose of these steps is apparently to incorporate CO2 and NADPH into malate in order to translocate them to the vascular bundle sheath cells, where they are again released by the action of a NADP-dependent malic enzyme. The malic enzyme is located in the bundle sheath chloroplasts together with the en mes of the Calvin cycle. CO2 is then reduced to carbohydrates while pyruvate is presumably transported back to the mesophyll cells. Besides the malate-type C4-plants, there is a second and larger group of species (aspartate type) that contains little malic enzyme and utilizes aspartate as the COj carrier. 

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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