$(a)$ Study the given figure of the nitrogen cycle and mention what $A, B, C, D,$ and $E$ represent.
$(b)$ What will happen if step $A$ does not occur?
$(c)$ Write the role of nitrogen-fixing bacteria in the biosphere.
$(d)$ Name two biologically important compounds that contain both $O_2$ and $N_2$.

(A) Based on the nitrogen cycle diagram:
$A$ - Ammonification
$B$ - Ammonia $(NH_3)$
$C$ - Nitrites $(NO_2^-)$
$D$ - Nitrates $(NO_3^-)$
$E$ - Denitrification
$(b)$ If step $A$ (Ammonification) does not occur,the organic nitrogen present in dead plants and animals will not be converted into ammonia. Consequently,nitrogen will not be recycled back into the soil,leading to a deficiency of nitrogenous compounds for plants.
$(c)$ Nitrogen-fixing bacteria (e.g.,Rhizobium) play a crucial role in the biosphere by converting atmospheric nitrogen $(N_2)$ into nitrogenous compounds like ammonia,which can be easily absorbed and utilized by plants for protein synthesis.
$(d)$ Two biologically important compounds that contain both oxygen and nitrogen are proteins (amino acids) and nucleic acids ($DNA$ and $RNA$).