Assertion : Photosynthetically $C_4$ plants are less efficient than $\mathrm{C}_3$ plants. Reason : The operation of $\mathrm{C}_4$ pathway requires the involvement of only bundle-sheath cells.

$\mathrm{C}_4$ plants are those plants that produce $\mathrm{C}_4$ compound like malic acid or aspartic acid as the first stable compound of $\mathrm{CO}_2$ fixation cycle, eg. maize, sugar cane etc. These plants can grow in dense tropical, forests where there is poor supply of $\mathrm{CO}_2$ because there is internal supply of $\mathrm{CO}_2$. Also photorespiration is negligible or absent in these plants. So, they are photosynthetically more efficient than $\mathrm{C}_3$ plants. These plants show a special type of leaf anatomy called Kranz anatomy. $\mathrm{C}_4$ plants uses mesophyll cells and bundle sheath cells in the outer part of their leaves. The $\mathrm{C}_4$ pathway shows 2 carboxylation reactions one in mesophyll chloroplast and the other in bundle sheath. Chloroplasts $-\mathrm{CO}_2$ acceptor is PEP and not RuBP and PEP carboxylase is the key enzyme.