Explain and give digramatic representation of Hatch and Slack pathway.

C4 plants like maize, sugarcane;have a special type of mechanism avoid photorespiration,.

In these plants three or 2 types of photosynthetic cells namely mesophyll cells and bundle sheath cell are present. The bundle sheath cells are arranged in a wreath like manner, thus calling them as kranz anatomy .

C4 contain dimorphic chloroplasts, that is chloroplasts in bundle sheath cells are agranal. The presence of two types of cells allow the occurrence of light reactions and carbon reactions separately

In C4 plants, light reactions occur in the mesophyll cells, whereas CO2 assimilation occurs in the bundle sheath cells. This type of separation does not allow O2 released in mesophyll cells to escape in to the bundle sheath cells. This prevents the oxygenation of RuBP, which is present in the bundle sheath cells.

diagram of maize leaf with mesophyll and bundle-sheath cells

 The C4 pathway takes place in the mesophyll cells, and the C3 pathway (Calvin cycle) operates in the bundle-sheath cells. Both types of cells contain chloroplasts.

To further avoid photorespiration, C4 plants have evolved a CO2 concentrating mechanism called the C4 pathway. The main objective of C4 pathway is to build up a high concentration of CO2 in the place of Rubisco, thus favoring carboxylation and thus photorespiration is suppressed.

Many plants like maize and sugar cane, are far more efficient at taking up CO2 than C3 plants. The CO2 acceptor in C4 plants is phosphoenolpyruvate (PEP). PEP reacts with CO2 to form oxaloacetic acid which is reduced by NADPH to form maleic acid. The maleic acid then reacts with RUBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as C3 plants. These reactions are called the Hatch - Slack pathway.

hatch and slack pathway

 

These plants, however first fix carbon dioxide in the four - carbon compound oxaloacetic acid. C4 plants have two rings of cells surrounding the vascular bundles. The outer mesophyll cells fix CO2 using PEP and pass maleic acid to the inner bundle sheath cells. Here CO2 is released for acceptance by RUBP and the eventual formation of triose phosphate. The photosynthetic efficiency of C4 plants is due to their ability to fix CO2 in environmental conditions where CO2 is the limiting factor. The C4 photosynthetic pathway is more efficient than the C3 pathway due to the absence of photorespiration in C4 plants.

 

 

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