(i) A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside. In nucleosides, the sugar carbons are numbered as 1′,2′,3′, etc. in order to distinguish these from the bases. When nucleoside is linked to phosphoric acid at 5'-position of sugar moiety, it forms a nucleotide.

(ii) These are the cyclic monosaccharides which are differ at C-1 in aldose and C-2 in ketose also known as epimers or stereoisomers.
(iii) An amino acid that cannot be produced by the organism and must thus be given in the food is known as an essential amino acid or indispensable amino acid.