Question

Describe the mechanism of Hatch and Slack pathway in $C_4$ plants?

Answer 1

Hint: HSK pathway is an important pathway for photosynthesis in \[C4\] plants. This pathway takes place in mesophyll cells which are present near the bundle sheath cells. Plants which are habituated in hot and dry climates adapt this type of pathway to save water loss. In this \[C4\] acid is broken to produce carbon dioxide and three carbon by-products.

Complete answer:
HSK pathway is adapted by the plants which belong to the tropical climate like sugarcane, sorghum. It occurs in the chloroplasts.

1)In this pathway, the primary acceptor of carbon dioxide is a three- carbon molecule called the phosphoenol pyruvate(PEP). This reaction converts ATP and Pi to AMP and PPi.
2)The fixation of carbon dioxide in \[C4\]plants is carried out by PEP carboxylase enzyme.
3)The PEP is converted to a 4-Carbon compound called the OxaloAcetic Acid in the mesophyll cells. This is not a stable product. It is converted into malic acid which is a 4-Carbon compound and is stable. It is transported to bundle sheath cells where it breaks into a 3-carbon compound and carbon dioxide.
5)The initial steps are carried out in the mesophyll cells whereas the cycle is completed in bundle sheath cells.
6)HSK pathway requires 30 ATP to synthesize one glucose molecule.

Note: HSK pathway takes place in the dark cycle of photosynthesis so as to prevent loss of water. It is an alternative to Calvin cycle. In the Calvin cycle the fixation of carbon dioxide takes place at one place whereas in HSK it happens at different places.