The balance between $CO_2$ and $O_2$ in the atmosphere is maintained by:

Give differences between:
$(1)$ Grana and Stroma
$(2)$ $PS-I$ and $PS-II$

Sugar produced in photosynthesis is converted into starch in

$I.$ In $C_{3}$-plants,the Calvin pathway takes place in mesophyll cells.
$II.$ In $C_{4}$-plants,the Calvin pathway takes place in the mesophyll cells.
Which of the following statements is true?

Name the two important enzymes of $C_3$ and $C_4$ pathways, respectively. What important role do they play in fixing $CO_2$?

$A -$ Calvin cycle starts with the carboxylation of $RuBP$.
$R -$ Photorespiration starts with the oxygenation of $RuBP$.