Give scientific reasons:
$(1)$ The productivity of $C_4$ plants is more than $C_3$ plants.
$(2)$ It is necessary that the Calvin cycle runs for six times.

(N/A) $(1)$ In the $C_4$ pathway,the concentration of $CO_2$ in the chloroplasts of bundle sheath cells remains high. Since the biosynthetic process of carbon fixation occurs in these cells,more $CO_2$ is available,leading to higher carbohydrate production.
Furthermore,the Kranz anatomy prevents $O_2$ produced in the light reaction of mesophyll cells from reaching the bundle sheath cells. Consequently,there is no possibility of photorespiration. Thus,the productivity of the $C_4$ pathway is higher than that of the $C_3$ pathway.
$(2)$ $ATP$ and $NADPH$ produced during the light reaction are utilized in the biosynthetic phase. $CO_2$ combines with the molecule $RuBP$ to form $PGA$. If this cycle runs six times,$6$ molecules of $RuBP$ combine with $6$ molecules of $CO_2$ to form $12$ molecules of $PGAL$. During reduction,two molecules of $PGAL$ are used to synthesize one molecule of glucose,while the remaining $10$ molecules of $PGAL$ regenerate $6$ molecules of $RuBP$. Thus,the cycle continues,allowing for the fixation of atmospheric $CO_2$ and the synthesis of glucose.