Biology THE `C_4` PATHWAY


`star` Difference between `C_3` and `C_4` Plants
`star` Kranz Anatomy
`star` Hatch and Slack Pathway


● `color{violet}"Plants that are adapted"` to `color{violet}"dry tropical regions"` have the `C_4` pathway mentioned earlier.

● Though these plants have the `C_4` `color{violet}"oxaloacetic acid"` as the first `CO_2` `color{violet}"fixation product"` they use the `C_3` pathway or the `color{violet}"Calvin cycle"` as the main `color{violet}"biosynthetic pathway"`.

● They are still `color{violet}"different"` from `C_3` `color{violet}"plants"`.

● `C_4` plants are `color{violet}"special:"`

`star` They have a special type of `color{violet}"leaf anatomy"`,

`star` They `color{violet}"tolerate higher temperatures"`

`star` They show a `color{violet}"response to highlight intensities"`,

`star` They lack a process called `color{violet}"photorespiration"` and have `color{violet}"greater productivity"` of biomass.


● The particularly `color{violet}"large cells"` around the `color{violet}"vascular bundles"` of the `C_4` pathway plants are called `color{Brown}"bundle sheath cells"`, and the leaves which have such anatomy are said to have `color{Brown}"Kranz’ anatomy."`‘

● Kranz’ means `color{violet}"wreath"` and is a reflection of the `color{violet}"arrangement of cells"`.

● The bundle sheath cells may `color{violet}"form several layers"` around the `color{violet}"vascular bundles"`; they are characterised by having a `color{violet}"large number of chloroplasts"`, thick walls `color{violet}"impervious to gaseous exchange"` and no `color{violet}"intercellular spaces"`.


● The pathway shown in the figure has been named the `color{violet}"Hatch and Slack Pathway"`, and is again a cyclic process.

● The primary `CO_2` acceptor is a `color{violet}"3-carbon molecule"` `color{Brown}"phosphoenol pyruvate (PEP)"` and is present in the `color{violet}"mesophyll cells"`.

● The `color{violet}"enzyme responsible"` for this fixation is `color{Brown}"PEP carboxylase or PEPcase"`.

● It is important to register that the `color{violet}"mesophyll cells"` lack `color{violet}"RuBisCO enzyme"`.

● The `C_4` acid `color{violet}"OAA"` is formed in the `color{violet}"mesophyll cells"`.

● It then forms other `color{violet}"4-carbon compounds"` like `color{violet}"malic acid"` or `color{violet}"aspartic acid"` in the mesophyll cells itself, which are transported to the `color{violet}"bundle sheath cells"`.

● In the `color{violet}"bundle sheath cells"` these `C_4` acids are broken down to `color{violet}"release"` `CO_2` and a `color{violet}"3-carbon molecule"`.

● The `color{violet}"3-carbon molecule"` is transported back to the `color{violet}"mesophyll"` where it is converted to `color{violet}"PEP"` again, thus, completing the cycle.

● The `CO_2` released in the `color{violet}"bundle sheath cells"` enters the `C_3` or the `color{violet}"Calvin pathway,"` a pathway common to all plants.

● The bundle sheath cells are rich in an `color{violet}"enzyme Ribulose bisphosphate carboxylase"`-`color{violet}"oxygenase (RuBisCO)"`, but `color{violet}"lack PEPcase"`.

● Thus, the `color{violet}"basic pathway"` that results in the `color{violet}"formation of the sugars"`, the Calvin pathway, is common to the `C_3` and `C_4` plants.

● The `color{violet}"Calvin pathway"` occurs in all the `color{violet}"mesophyll cells"` of the `C_3` plants.

● In the `C_4` plants it does not take place in the `color{violet}"mesophyll cells"` but does so only in the `color{violet}"bundle sheath cells."`