Question

The first stable product of $C_4$ /HSK photosynthesis is
A) Phosphoglyceric acid
B) Ribulose 1,5 bi-phosphate
C) Malic acid
D) Phosphoglyceraldehyde

Answer 1

Hint: The Hatch-Slack Pathway is utilised by the plants which thrive on hot and dry climate. The first stable product is the 4-carbon compound. The $C_4$ plants produce more energy than $C_3$ plants. The HSK pathway takes place in the mesophyll cells.

Complete answer
To answer this question we must know about the Hatch-Slack pathway. The Plants living in the desert follow the HSK pathway rather than Calvin cycle. In these plants, they have special adaptations. In C-4 plants the carbon dioxide is fixed in the mesophyll cells. The 4-Carbon compound called the oxaloacetic acid is the first product.

In the HSK pathway the cycle begins with the formation of phosphoenolpyruvate, a 3-carbon compound. This PEP gets converted into a 4-C compound called Oxaloacetic acid. This is converted into malic acid which is stable. This is then transferred from mesophyll to bundle sheath cells.

Phosphoglyceric acid- It is a 3-Carbon compound which is intermediate in between Calvin cycle and glycolysis.
Ribulose 1,5 bisphosphate- It is a 5-C compound involved in Calvin cycle.
Phosphoglyceraldehyde- It is an intermediate in photosynthesis and glycolysis. It is a 3-C compound.

Thus, the correct answer is option ‘C’.

Note: The HSK is the alternative pathway of the Calvin cycle. It takes place during the dark phase of photosynthesis. Corn, shrubs, sugarcane are the plants which follow the HSK pathway. The Calvin pathway is common in both C-3and C-4plants. It takes place in the mesophyll cells of the $C_4$ Plants.