Question

Explain the Hatch-Slack pathway.

Answer 1

Hint: We know that in most of the plants, Oxaloacetic acid which is a 3-carbon containing compound is the first ${CO}_2$ fixation product. But some plants also make use of Phospho-enol pyruvate (PEP) as the first product of ${CO}_2$ fixation which is a 4-carbon containing compound.

Complete answer:
The plants which make use of phospho-enol pyruvate (PEP) as the first product of ${CO}_2$ fixation is called as $C_4$ plants as PEP is a 4-carbon containing compound. The $C_4$ pathway is also called the Hatch-Slack pathway.
Let us take a look at this pathway step by step:
- First, phospho-enol pyruvate (PEP) which is present in the mesophyll cells accepts ${CO}_2$ under the influence of the enzyme PEP carboxylase. (Mesophyll cells of $C_4$ plants lack RuBisCO enzyme and hence the normal pathway with OAA is not used).
- After accepting ${CO}_2$, Malic acid or aspartic acid form in the mesophyll cell itself which are then transported to bundle sheath cells. In bundle sheath cells these acids are broken down in order to release ${CO}_2$ and a 3-carbon molecule.
- This 3-carbon containing molecule is then transported back to mesophyll cells where it again forms PEP and hence one turn of the cycle is completed.
- The ${CO}_2$ which was released in bundle sheath cells participates in the regular $C_3$ pathway since these cells are rich in RuBisCO enzyme but they lack PEPcarboxylase.
For better understanding, let us take a look at the diagram below –

Note:
Plants that show $C_4$ pathways are called as $C_4$ plants are showing a specific type of anatomy in the leaf called Kranz anatomy. They also respond to high light intensities and lack photorespiration.