Q.Given below is the diagrammatic representation of one of the categories of small molecular weight organic compounds in the living tissues. Identify the category shown & one blank component "X" in it.

Step 1. Identify what the ring represents. The figure is a five-membered ring containing one ring oxygen (shown at the top) and four ring carbons - this is a furanose sugar ring, the type of pentose sugar found in nucleic acids. It is not a steroid nucleus and not an amino acid skeleton.

Step 2. Read the substituents around the ring. One ring carbon (C4') carries a -CH2OH group (HOH2C, i.e. the C5' carbon hanging off the ring), and two ring carbons (C2' and C3') each carry an -OH group, identifying the sugar as ribose. The remaining ring carbon (C1', the anomeric carbon) carries the unlabelled group 'X' - this is precisely the position at which a nitrogenous base attaches to the sugar through an N-glycosidic bond in a nucleoside or nucleotide.

Step 3. Check whether a phosphate group is present. A nucleotide is defined as pentose sugar + nitrogenous base + phosphate group, while a nucleoside is defined as pentose sugar + nitrogenous base only, with no phosphate. The diagram shows only a plain -CH2OH at C5', and no phosphate group is drawn anywhere on the molecule. So the structure must be a nucleoside, not a nucleotide. …

Identify the molecular category from the drawn skeleton (ring type, substituents, and presence or absence of a phosphate group) and match it against the definitions of the candidate biomolecule categories, eliminating any option whos …

• Assuming any sugar-ring diagram with an attached base must be a nucleotide, overlooking that a nucleotide specifically requires a phosphate group that is not drawn here.
• Confusing the pentose sugar ring with cholesterol's steroid ring system, which is built from four fused carbocyclic rings rather than a single 5-membered oxygen-containing ring. …