Question

${C_4}$​ cycle is
(A) Adjunct to Calvin cycle
(B) Independent cycle
(C) With high RuBP carboxylase efficiency
(D) With PEP carboxylase in bundle sheath cells

Answer 1

${C_4}$ cycle is also known as the Dicarboxylic acid cycle or $C{O_2}$ concentrating mechanism. ${C_4}$ Plants are known to be more efficient photosynthetic plants at present $C{O_2}$ concentration. The major point which differentiates ${C_4}$ plants from ${C_3}$ plants is the presence of bundle sheath.

Complete answer:
Hatch & slack proposed the dark reactions pathway. They studied the pathway in detail in sugarcane and maize. ${C_4}$ Pathway is observed in the spies belonging to the family of angiosperm. It is mostly found in monocots that belong to Gramineae and Cyperaceae. Following are the main features of ${C_4}$ cycle:
The presence of Bundle sheath cells are found around the vascular bundles. These bundle sheath cells are green in colour.
Rubisco is present in the bundle sheath cells in ${C_4}$ cycle while PEPcase is present in the mesophyll cells.
The bundle sheath cells which is a site of Rubisco does not release ${O_2}$ which leads to the lack of photorespiration. Hence, it is said that ${C_4}$ plants are photosynthetically more efficient.
The Calvin cycle happens in the cells of the Bundle sheath.
There is no photorespiratory loss witnessed in ${C_4}$ plants. These plants are known to have adapted themselves in tropical regions with high temperature, low water availability, and intense light.
The primary $C{O_2}$ acceptor in these plants is Phosphoenol Pyruvate.
The primary $C{O_2}$ fixation product is OAA.

Therefore, the correct answer is (A) adjunct to Calvin cycle.

Note:
The $C{O_2}$ compensation points are known to be low in ${C_4}$ plants. The productivity in these plants does not increase with increased $C{O_2}$ concentration above the atm level. The present level of atmospheric $C{O_2}$ is generally not the limiting factor for these plants.