Question

Explain the Hatch-Slack pathway.

Answer 1

HSK pathway (Hatch-Slack pathway) is also called as C${}_4$ pathway. In C${}_4$ plants , the CO${}_2$ is not directly absorbed by RuBP in presence of enzyme helps to absorb CO${}_2$ having concentration of 550 ppm or more. However, C${}_4$ plants, due to high temperature and high light intensity, keep their stomata partially closed due to which the CO${}_2$ concentration goes down to 2 ppm and calvin cycle cannot be started. Therefore C${}_4$ plants follow C${}_4$ pathway in which CO${}_2$ is absorbed by PEP in presence of enzyme PEP-carboxylase.

The main steps of C${}_4$ pathway are given below:

1. CO${}_2$ of low concentrations enters the mesophyll cells through stomata and is absorbed by phosphoenol pyruvic acid (3C) with the help of PEP-carboxylase to form oxaloacetic acid (4C).
2. OAA (4C), being unstable, gets converted into stable 4C acids like malic acid or aspartic acid.
3. These acids diffuse into bundle sheath where they release CO${}_2$ to form pyruvic acid (3C).
4. When enough of CO${}_2$ is accumulated, itis absorbed by RuBP-carboxylase of calvin cycle.
5. Pyruvic acid diffuses back to mesophyll cells.
6. Pyruvic acid (3C) is phosphorylated by ATP to form phosphoenol pyruvic acid to continue HSK pathway.