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Question : Describe C_(4) pathway.

Answer»

Solution :Plants that are adapted to dry tropical regions have the `C_(4)` pathway.
These plants have the `C_(4)` oxaloacetic acid as the first `CO_(2)` fixation product. They use the `C_(3)` pathway or the Calvin cycle as the main biosynthetic pathway.
Internal specially of `C_(4)` plants leaves :
(1) `C_(4)` plants are special. They have a SPECIA type of leaf anatomy, they tolerate higher temperatures.
They show a response to high light intensities.
They lack a process called photorespirations.
They have greater productivity of biomass.
(5) Invertical sectio of leaves, one of a `C_(4)` plant cells of mesophyll are seen.
(6) The particularly larage cells around the vascular bundles of the `C_(4)` pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have .Kranz. anatomy.
(7) Here "Kranz" means "WREATH" and is a REFLECTION of the arrangement of cells.
(8) The bundle sheath may have SEVERA layers around the vascular bundles.
(9) They are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
(10) There are no inter - cellular spaces around cells. If we cut a section of the leaves of `C_(4)` plants - maize or sorghum - to observe the Kranz anatomy and the distribution of mesophyll cells.

The pathway shown in given figure is called Hatch and slack pathway. This is also a cyclic process.
Stages or steps of `C_(4)` pathway : Steps of `C_(4)` pathway are as follows :
Step 1 : Atomspheric `CO_(2)` enters into the cells of mesophyll where the primary `CO_(2)` acceptor is a 3-carbon molecule phosphenonlpyruvate (PEP)
The enzyme responsible for this `CO_(2)` fixation is PEP carboxylase or PEPCASE.
The mesophyll cells lack RuBisCO enzyme.
The `C_(4)` acid OAA in the mesophyll cells
`CO_(2)+PEP overset("PEPcase")rarrOA A`
Step 2 : It then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll calls itself. which are transported to the bundle sheath cells. In the bundle sheath cells these `C_(4)` acids are broken down to release `CO_(2)` and a 3-carbon molecule.

Step 3 : The 3 carbon molecul is tranported back to the mesophyll where it is converted to PEP again, thus completing the cycle
Pyuruic acid `rarr` Inmesophylll (in presence of ATP)
Pyruvic acid + ATP `rarr` (PEP)
Step 4 : The `CO_(2)` relased in the bundle sheath cells enteres the `C_(3)` or the Calvin pathway, a pathway common to all plants.
The bundle sheath cells arerich in an enzyme Ribulose, biophosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase.
Thus, the basic pathway that result in the formation of the sugars, the calvin pathway is common to the `C_(3) and C_(4)` plants.


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