The C_4 -plants are photosynthetically more efficient than C_3...

$\mathrm{C}_4$-plants show Kranz anatomy. In the leaves of $\mathrm{C}_4$-plants, the vascular bundles are surrounded by bundle sheath of parenchymatous cells, which in turn are surrounded by mesophyll cells. Chloroplasts in bundle sheath cells are larger and always contain grana whereas chloroplasts in mesophyll cells are smaller. The $\mathrm{C}_4$-plants are photosynthetically more efficient than $\mathrm{C}_3$-plants because they have more chloroplasts. Photorespiration takes place only in $\mathrm{C}_3$-plants. It occurs in chlorophyllous tíssue. At high temperature and high oxygen concentration the affinity of RuBP carboxylase for $\mathrm{CO}_2$ decrease and for $\mathrm{O}_2$ increases. Photosynthesis takes $\mathrm{CO}_2$ and release $\mathrm{O}_2$, while respiration utilizes $\mathrm{O}_2$ and release $\mathrm{CO}_2$. If the light factor is saturating, there will be certain concentration of $\mathrm{CO}_2$ in the atmosphere at which the rate of photosynthesis will be equal to rate of respiration. This is called carbon dioxide compensation point. $\mathrm{C}_4$-plants have very low $\mathrm{CO}_2$ compensation point