The plant special that are adapted to dry tropical region have the C4 pathway mentioned earlier. Though these plants have the C4 oxaloacetic acid as the first CO2 fixation product they use the C3 pathway or the calvin cycle as the main biosynthetic pathway.
C4 plants are : They have a special types of leaf anatomy, they tolerate higher tempertures, they show a response to high intensities, they lack a process called photorespiation and have greater productivity of biomass, Let us understand these one by one.
The particularly large cells around the vascular bundles of the C4 plants are called bundle sheath cells, and leaves which have such anatomy are said to have kranz anatomy . Kranz means wreath and is a reflection of the arrangement of cells . The bundle sheath cells may form several layers around the vascular bundles, they are characterised by haveing a large number of chloroplast, thick walls impervious to gaseous exchange and no intercellular spaces. You may like to cut a section of the leaves of C4 plants - maize or sorghum - to observe the kranz anatomy and the distribution of messophyll cells.
https://biologylibrarywith2023.blogspot.com/2023/03/introduction-of-biology-and-important_16.html
some special point
The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate(PEP) and is present in the mesophyll cells. The enzyme resposible foe this fixtion is PEP carboxylase or PEPcase. The C4 acid OAA is formed in the mesophyll cells.
It then form form other 4- carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, which are transported to the bundle sheath cells. In the bundle sheath cells these C4 caid are broken down to release CO2 and a 3- carbon molecule.
The 3- carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle.
The CO2 released in the bundle sheath cells enters the C3 or the calvin pathway , a pathway common to all plants . The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO),but lack PEPcase. Thus, the basic pathway that results in the formation of the sugars , the calvin pathway, is common to the C3 and C4 plants.
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