Free Macromolecules MCQs with Answers

1. The linkage that joins amino acid units in a protein is

• A. a glycosidic linkage
• B. a peptide linkage, formed by condensation between a carboxyl and an amino group
• C. an ester linkage
• D. a hydrogen bond

Explanation: The carboxyl group of one amino acid condenses with the amino group of the next, releasing water and leaving a CO-NH amide link known as the peptide bond. Glycosidic links join sugars and ester links join glycerol to fatty acids, so each class of macromolecule has its own characteristic linkage. Hydrogen bonds are important in proteins but hold the chain in shape rather than joining the units.

2. An amino acid is described as amphoteric because it

• A. dissolves only in organic solvents
• B. contains a benzene ring
• C. contains both an acidic carboxyl group and a basic amino group, so it reacts with both acids and bases
• D. has no charge under any conditions

Explanation: The carboxyl group can donate a proton and the amino group can accept one, so an amino acid neutralises both acids and alkalis. Within a single molecule the proton transfers from one group to the other, giving the doubly charged zwitterion that exists at intermediate pH. This ionic character is why amino acids are crystalline solids with high melting points and dissolve readily in water.

3. The pH at which an amino acid exists mainly as a zwitterion and does not migrate in an electric field is called its

• A. neutral point
• B. isoelectric point
• C. equivalence point
• D. melting point

Explanation: At the isoelectric point the positive and negative charges within the molecule exactly balance, so the net charge is zero and the amino acid is at its least soluble and does not move towards either electrode. The value differs for each amino acid according to its side chain. Electrophoresis separates amino acids and proteins by exploiting these differences.

4. Proteins are classified as fibrous or globular. A fibrous protein such as keratin is

• A. insoluble in water and structural in function
• B. soluble in water and catalytic in function
• C. always an enzyme
• D. made of a single amino acid repeated

Explanation: Fibrous proteins consist of long parallel chains held in bundles, which makes them tough, insoluble and suited to structural roles in hair, nails, tendons and skin. Globular proteins fold into compact soluble balls with hydrophilic groups outside, which is what enzymes, antibodies and haemoglobin require. Both types are built from the same twenty amino acids in different sequences.

5. The secondary structure of a protein, such as the alpha helix, is maintained by

• A. peptide bonds between amino acids
• B. ionic bonds between side chains only
• C. hydrogen bonds between the carbonyl and amide groups of the backbone
• D. disulphide bridges only

Explanation: Regular coiling or pleating arises because each carbonyl oxygen hydrogen bonds to an amide hydrogen further along the same chain, giving the alpha helix and the beta pleated sheet. Peptide bonds create the primary sequence, and side chain interactions such as disulphide bridges belong to the tertiary level. Heat breaks the weak hydrogen bonds and the structure unwinds, which is denaturation.

6. Denaturation of a protein involves

• A. hydrolysis of all its peptide bonds
• B. loss of its three dimensional shape while the amino acid sequence remains intact
• C. a change in its amino acid sequence
• D. conversion of the protein into a carbohydrate

Explanation: Heat, extreme pH, heavy metal ions and alcohol break the weak interactions holding the fold, so the chain unravels and biological activity is lost, but the covalent peptide backbone survives. This is why a boiled egg sets irreversibly without breaking down into amino acids. Hydrolysis of the peptide bonds is digestion, a different process requiring enzymes or prolonged heating with acid.

7. The complete hydrolysis of a protein produces

• A. glucose molecules
• B. fatty acids and glycerol
• C. a mixture of amino acids
• D. urea and ammonia

Explanation: Every peptide bond is broken by boiling with concentrated acid or by protease enzymes, releasing the constituent amino acids, which can then be separated by chromatography or electrophoresis. Glucose comes from the hydrolysis of carbohydrates and fatty acids with glycerol from fats. Urea is a later metabolic product formed when surplus amino acids are deaminated in the liver.

8. Which of the following is NOT a function of proteins in the body?

• A. Acting as enzymes
• B. Transporting oxygen
• C. Providing antibodies for defence
• D. Serving as the main long term energy store

Explanation: The body stores energy as fat and, in the short term, as glycogen, and proteins are broken down for energy only when those stores are exhausted, since doing so sacrifices functional tissue. Enzymes, haemoglobin and antibodies are all proteins, as are hormones such as insulin and structural materials such as collagen. This breadth of function is why protein is an essential part of the diet.

9. The biuret test for proteins gives

• A. a violet colour with copper two ions in alkaline solution
• B. a brick red precipitate
• C. a silver mirror
• D. a blue black colour

Explanation: Copper two ions form a violet complex with the peptide linkages, so the test detects the presence of peptide bonds rather than any particular amino acid and requires at least two of them. A blue black colour with iodine is the test for starch and a brick red precipitate with Fehling's solution indicates a reducing sugar. The ninhydrin test is the alternative for free amino acids.

10. Enzymes are described as biological catalysts because they

• A. are used up in the reactions they catalyse
• B. raise the activation energy of a reaction
• C. supply energy to drive unfavourable reactions
• D. speed up reactions by lowering the activation energy and are recovered unchanged

Explanation: An enzyme provides an alternative pathway with a smaller energy barrier, so a far larger proportion of collisions are effective at body temperature, and it emerges from each cycle ready to work again. It cannot make an energetically unfavourable reaction happen, only accelerate one that is already possible. A single enzyme molecule may convert thousands of substrate molecules per second.