$(i)$ Name two biologically important compounds that contain both oxygen and nitrogen.
$(ii)$ What is the role of nitrogen-fixing bacteria?

(N/A) $(i)$ Two biologically important compounds containing both oxygen and nitrogen are:
$(a)$ Nucleic acids ($DNA$ and $RNA$): These contain nitrogenous bases,sugars,and phosphate groups.
$(b)$ Proteins: These are polymers of amino acids,which contain both amino groups (nitrogen) and carboxyl groups (oxygen).
$(ii)$ Nitrogen-fixing bacteria (such as $Rhizobium$) live in the root nodules of leguminous plants. They convert atmospheric nitrogen $(N_2)$ into soluble forms like nitrates $(NO_3^-)$ and nitrites $(NO_2^-)$,which plants can easily absorb and utilize for the synthesis of proteins and other nitrogenous compounds.