Kranz anatomy or C4 pathway or Hatch and Slack Pathway

Biosynthesis phase or dark reaction is slightly different in some plants from C3 Plants.

In those plants during the biosynthesis phase or fixation of carbon dioxide the first compound that formed is four carbon-containing compounds named oxalo acetic acid hence called C4 plants.

Characterstic of C4 Plants

They have a special type of leaf anatomy. They can withstand higher temperatures.They provide a response against high light intensity. They  do not exhibit  photorespiration. They  produce  greater productivity of biomass.

Features of special anatomy of leaf in C4 plants 

In the leaves of C4 plants,there are large cells around the vascular bundle named  bundle sheath cells along with the mesophyll cells. Such anatomy of  the leaves are called Kranz anatomy. The bundle sheaths cells form  several layers around the vascular bundles and have 

a large number of chloroplasts. The thick wall of kranz anatomy  impervious to gaseous exchange. The presence of the bundle sheath cells are typical features of the C4 Plants.

Hatch and Slack pathway or C4 cycle 

This pathway is a cyclic process for  the fixation of carbon dioxide described by the  Hatch and Slack .This cycle can be understood with the following steps -

FIXATION

Carbon dioxide is first accepted by the three carbons containing compound phosphoenol pyruvic acid.  This three-carbon containing compound  is present in the mesophyll cell of the leaf of C4 plants. The fixation of carbon dioxide by phosphoenol pyruvic acid catalyzed by the enzyme phosphoenol pyruvic carboxylase or PEPcase. As a result four carbon-containing compounds named oxalo acetic acid are formed.

👌👌Remember  The mesophyll cells do not have Ribulose bisphosphate carboxylase-oxygenase or RuBisCO enzyme.

TRANSPORTATION

The  Oxalo acetic acid  is formed in the mesophyll cells as described in fixation. Now oxalo acetic acid is transformed into    malic acid or aspartic acid in the mesophyll cells and transported into bundle sheath cells.

DECARBOXYLATION

In the bundle sheath cells these malic or aspartic acids are broken down. As a result, carbon dioxide is released under the decarboxylation. Along with carbon dioxide, three carbon containing  molecule  pyruvic acids are also formed. The carbon dioxide that is released in the bundle sheath cells now  enters into the Calvin pathway.

The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase but lack 

phosphoenol pyruvic carboxylase

REGENERATION

Pyruvic acid is  transported back from bundle sheath cells  to the mesophyll where it is involved in regeneration of  phosphoenol pyruvic acid for continuation of cycle.