Explanation

Malic acid or aspartic acid is translocated to bundle sheath cells through plasmodesmata. Inside the bundle sheath cell they are decarboxylated (malic acid) or deaminated. In case of aspartic acid to form $CO_{2}$ and pyruvate. After the fixing of $CO_{2}$ to Oxaloacetic Acid (OAA) in $C_{4}$ cycle, the oxaloacetic acid changes into the malic aspartic acid $OAA + NADPH\overset{\ Malic\ acid\ dehydrogenase\ }{\rightarrow}$ Malic acid + $NADP^{+}$ $OAA + NH_{3} + NADPH\overset{\ Transaminase\ }{\rightarrow}$ Aspartic acid + $NADP^{+} + H_{2}O$ Both of these reactions occur in mesophyll cell