What does the secondary structure of proteins represent?

$1.78 \ g$ of an optically active $L$-amino acid $(A)$ is treated with $NaNO_{2} / HCl$ at $0^{\circ}C$. $448 \ cm^{3}$ of nitrogen at $STP$ is evolved. $A$ sample of protein has $0.25 \%$ of this amino acid by mass. The molar mass of the protein is

Which one of the following structures represents the peptide chain?

Which functional group participates in disulphide bond formation in proteins?

The most important energy carrier in all the living cells is

Which of the following is used in our body as a fuel for muscles and nerves and to build and repair body tissues?