Biomolecules — Practice

Question 1

Why is sucrose a non-reducing sugar but maltose a reducing sugar?

Accepted Answer

In sucrose the glycosidic bond links C1 of glucose to C2 of fructose, involving both anomeric carbons, so there is no free aldehyde/ketone group. In maltose the bond is between C1 of one glucose and C4 of another, leaving one anomeric carbon free, which makes maltose reducing.

Question 2

What is a zwitterion? Give an example.

Accepted Answer

A zwitterion is a dipolar ion bearing both positive and negative charges but with net zero charge. Amino acids exist in aqueous solution as H3N+-CH(R)-COO-.

Question 3

Distinguish between DNA and RNA.

Accepted Answer

DNA has deoxyribose sugar, bases A/T/G/C, and is double-stranded. RNA has ribose sugar, bases A/U/G/C, and is generally single-stranded.

Question 4

Name two water-soluble and two fat-soluble vitamins.

Accepted Answer

Water-soluble: vitamin C and the B-complex vitamins. Fat-soluble: vitamins A, D, E, and K (any two).

Question 5

Differentiate between the alpha-helix and beta-sheet of protein secondary structure.

Accepted Answer

The alpha-helix is a right-handed coiled structure stabilised by hydrogen bonds between the N-H of one residue and the C=O of a residue four positions ahead. The beta-sheet is a pleated arrangement stabilised by hydrogen bonds between adjacent extended polypeptide segments.

Practice — Biomolecules

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