In leaves of C_4 -plants malic acid synthesis during CO_2 -fixation...

$\mathrm{C}_4$-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. In $\mathrm{C}_4$-plants the Hatch and Slack pathway involves two carboxylation reaction one taking place in chloroplast of mesophyll cells and other in chloroplast of bundle sheath cells. The initial $\mathrm{CO}_2$ fixation occurs in mesophyll cells where primary $\mathrm{CO}_2$ acceptor is phosphoenol pyruvate and formed oxaloacetate. The oxaloacetate is reduced to malic acid. From chloroplasts of mesophyll cells the malic acid is transferred to chloroplast of bundle sheath cells and decarboxylated to form $\mathrm{CO}_2$ and pyruvic acid. The $\mathrm{CO}_2$ is again fixed by $\mathrm{C}_3$ cycle. In plants, stoma is surrounded by two small, specialized, green epidermal cells called guard cells. The guard cells are small in size and therefore, rapidly influenced by turgor changes.