Question

In case of $C_4$ plants, the acceptor of $C{O}_2$ is

• A. Phosphoglyceraldehyde.
• B. Ribulose monophosphate.
• C. Phosphoenolpyruvate.
• D. Ribulose diphosphate.

Answer 1

The correct option is C Phosphoenolpyruvate.

Tropical grasses like sugarcane, sorghum etc are $C_4$ plants. They fix carbon dioxide by Hatch and Slack cycle. These plants do not show photorespiration and are hence better photosynthesizers than $C_3$ plants. These plants have Kranz anatomy. These plants have a special ring of thick walled bundle sheath cells around vascular bundle. The bundle sheath cells lack intercellular spaces. They have large chloroplasts without grana. In the bundle sheath cells carbon dioxide is fixed by the enzyme RuBisCO. After carbon dioxide fixation by RuBisCO, Calvin cycle proceeds as in $C_3$ plants. The mesophyll cells are the usual cell type as found in $C_3$ plants also. Initial carbon dioxide fixation into organic acids occurs in the mesophyll cells. The initial carbon dioxide acceptor is phosphoenolpyruvic acid (PEP) and the initial carboxylating enzyme is PEP carboxylase.