Explain the phases of `C_(4)` pathway.

Explanation

`C_(4)` pathway is ocmpleted in two phases, first phase takes place in strom of mesophyll cells, where the `CO_(2)` acceptor molecule is 3 - carbon compound , phospho enol pyruvate (PEP) to form 4 - carbon Oxalo acetic acid (OAA). The first product is a 4 - carbon and so it is named as `C_(4)` cycle. Oxalo acetic acetic acid is a dicarboxylic acid and hence this cycle is also know as dicarboxylic acid pathway. Carbon dioxide fixation takes place in two places one in mesophyll and another in bundle sheath cell (dicarboxylation pathway) . It is the adaptation of tropical and sub tropical plants growing in warm and dry conditions . Fixation of `CO_(2)` with minimal loss is due to absence of photorespiration .C4 plants requires 5ATP and 2NADPH `+ H^(+)` to fix one molecule of `CO_(2)`.

Stage : I Mesophyll Cells .
Phosphoenol Pyruvate + `CO_(2)`
(PEP) (3C)
PEP carboxylase ` darr`
Oxaloacetic acid (OAA) (4C)
Oxaloacetic acid (OAA) is converted into malic acid or aspartic acid and is transported to the bundle sheat cells through plasmodesmata.
Stage : II Bundle Sheath Cells : Malic acid undergoes decarboxylation and produce a 3 carbon compound Pyruvic acid and `CO_(2)` . The released `CO_(2)` combies with RUBP and follows the calvin cycle and finally sugar is released to the pholem. Pyruvic acid is transported to the mesophyll cells .