Question

(A) Define the following term:

(a) Reducing sugar

(b) Differentiate between the following:

(i) Fibrous proteins and globular proteins

(ii) Nucleotide and nucleoside

Hint:

A reducing sugar has a free aldehyde or ketone group. Fibrous proteins are insoluble threads and globular proteins are soluble spheres. A nucleotide is a nucleoside plus a phosphate group.

Answer 1

Step 1: Plan the answer.
Part (a) is a definition. Part (b) has two differences to write, one for proteins and one for the nucleic acid units.

Step 2: Define a reducing sugar.
A reducing sugar is a carbohydrate that reduces Tollens' reagent and Fehling's solution.
It has a free aldehyde group or a free ketone group (a free hemiacetal or hemiketal carbon) in its structure.
Glucose, fructose, maltose and lactose are reducing sugars. Sucrose is not, because its two anomeric carbons are tied up in the glycosidic bond.

Step 3: Fibrous proteins versus globular proteins.
Fibrous proteins have long, thread-like chains held side by side by hydrogen and disulphide bonds. They are insoluble in water. Examples are keratin and myosin.
Globular proteins have chains folded into a spherical or rounded shape. They are soluble in water. Examples are insulin and albumin.

Step 4: Nucleotide versus nucleoside.
A nucleoside has two units: a nitrogenous base and a pentose sugar.
A nucleotide has three units: a base, a pentose sugar and a phosphate group. So a nucleotide is a nucleoside with a phosphate on the sugar.

Final Answer:
A reducing sugar has a free aldehyde or ketone group and reduces Tollens' reagent and Fehling's solution. Fibrous proteins are thread-like and insoluble; globular proteins are spherical and soluble. A nucleoside is base + sugar; a nucleotide is base + sugar + phosphate. \[ \boxed{\text{Nucleotide} = \text{Nucleoside} + \text{Phosphate}} \]