Question

In $C_4$ plants, the bundle sheath cells
A. Have thin walls to facilitate gaseous exchange
B. Have large intercellular spaces
C. Are rich in PEP carboxylase
D. Have a high density of chloroplast.

Answer 1

Hint: The $C_4$ pathway requires the presence of two types of cells namely mesophyll cells and bundle sheath cells. The particularly large cells around the vascular bundles of $C_4$ plants are called bundle sheath cells. These cells may form several layers around the vascular bundles. This special anatomy of $C_4$ leaves is called Kranz anatomy. ‘Kranz’ means wreath and is a reflection of the arrangement of cells.

Complete answer:

- Bundle sheath cells have walls that are impervious for gaseous exchange but the walls are thick. If the walls are thin the gaseous exchange will occur but these cells do not involve gaseous exchange. So the option is not correct.

- These cells have no intercellular spaces between them . These cells are present so closely that bundle sheath cells and mesophyll cells have no space between them. So this option is also incorrect.

- They are rich in RuBisCO the enzyme which has affinity for both carbon dioxide and oxygen but lacks PEP( phosphoenolpyruvate) carboxylase. Hence this option is not correct.

- They are specialised for photosynthesis the plant performs its own food due to this pathway so they have more chlorophyll which means more chloroplast. Hence this option is not correct.

Hence, the correct answer is option (D).

Note: Bundle sheath cells have large numbers of chloroplast. They do not possess grana. They have thick walls that are impervious to gaseous exchange and have no intercellular spaces. So $C_4$ plants can tolerate high temperatures. They show a response to high light intensities due to presence of these cells.