Which is an essential constituent of diet?

The number of disulphide linkages present in insulin are

At which $pOH$ values is the isoelectric point of an amphoteric amino acid typically found?

The most important energy source in every living cell is $......$.

Assertion $(A)$: An optically active amino acid can exist in three forms depending on the $pH$ of the solution.
Reason $(R)$: Amino acids contain both acidic and basic groups; they exist as a zwitterion in aqueous medium,anionic form in basic medium,and cationic form in acidic medium.

Identify the correct statement related to amino acids.