Free Biological Molecules MCQs with Answers

41. Water reaches its maximum density at 4 degrees Celsius. The biological importance of this is that

• A. lakes freeze from the surface down, insulating the water below
• B. ice sinks to the bottom of lakes in winter
• C. water boils at a higher temperature than expected
• D. evaporation from the skin removes very little heat

Explanation: Water colder than 4 degrees becomes less dense and stays at the surface, so ice forms on top and acts as an insulating lid while liquid water below shelters fish and other life. If ice were denser than liquid water, lakes would freeze solid from the bottom up.

42. Metabolic water, which keeps some desert animals such as the kangaroo rat alive without drinking, is produced inside cells mainly during

• A. the hydrolysis of glycogen
• B. the conversion of pyruvate to lactate
• C. alcoholic fermentation
• D. the aerobic oxidation of glucose

Explanation: At the end of the electron transport chain, oxygen accepts electrons and hydrogen ions and is reduced to water, so aerobic respiration generates metabolic water. Fermentation pathways do not produce water, and hydrolysis consumes water rather than releasing it.

43. Which statement about water is correct?

• A. Its molecules are held together by ionic bonds
• B. It has an unusually high boiling point for such a small molecule because of hydrogen bonding
• C. Its solid form is denser than its liquid form
• D. It ionises completely into hydrogen and hydroxide ions

Explanation: Hydrogen bonding between polar water molecules means far more energy is needed to separate them than its tiny molecular mass would suggest, giving a boiling point of 100 degrees Celsius. Ice is less dense than liquid water, and water ionises only very slightly.

44. Complete hydrolysis of maltose yields

• A. glucose and fructose
• B. two glucose units
• C. glucose and galactose
• D. fructose and galactose

Explanation: Maltose is two glucose units joined by a glycosidic bond, typically produced when starch is broken down. Sucrose yields glucose plus fructose and lactose yields glucose plus galactose, so the products of hydrolysis identify the disaccharide.

45. Which polysaccharide forms the exoskeleton of insects and the cell walls of fungi?

• A. Cellulose
• B. Glycogen
• C. Pectin
• D. Chitin

Explanation: Chitin is a tough nitrogen-containing polysaccharide that builds arthropod exoskeletons and fungal walls. Cellulose is the structural polymer of plant walls, and glycogen is an animal storage polysaccharide, so each structural role maps to its own polysaccharide.

46. When a reducing sugar is heated with Benedict's solution, the colour changes from blue to

• A. green, then yellow, then brick red
• B. purple within a few seconds
• C. blue black
• D. completely colourless

Explanation: The free aldehyde or ketone group of a reducing sugar reduces copper ions in Benedict's solution, precipitating red copper oxide, with green and yellow as intermediate shades. Iodine gives the blue black colour with starch, which is a common source of confusion between the two tests.

47. Amylose differs from amylopectin in that amylose is

• A. highly branched through 1,6 glycosidic bonds
• B. a structural polymer of beta glucose
• C. largely unbranched and coils into a helix
• D. resistant to digestion by human amylase

Explanation: Amylose is an unbranched chain of alpha glucose that coils into a compact helix, while amylopectin carries frequent branches that give starch its many free ends for rapid enzyme attack. Cellulose, not amylose, is the beta glucose structural polymer that we cannot digest.

48. The exact sequence of amino acids in a polypeptide chain is called the protein's

• A. secondary structure
• B. tertiary structure
• C. quaternary structure
• D. primary structure

Explanation: Primary structure is simply the amino acid order, dictated by the gene. It determines how the chain folds into secondary and tertiary shapes, so a single substitution can wreck the protein's function. Quaternary structure only exists when several chains assemble together.

49. Which of the following is a fibrous protein?

• A. Haemoglobin
• B. Insulin
• C. Collagen
• D. Amylase

Explanation: Collagen forms long rope-like triple helices that give strength to skin, tendons and bone, the hallmark of fibrous proteins. Haemoglobin, insulin and amylase are compact, soluble globular proteins built for transport, signalling and catalysis rather than for strength.

50. Each of the twenty standard amino acids differs from all the others in

• A. the structure of its amino group
• B. the nature of its R group
• C. the structure of its carboxyl group
• D. the number of peptide bonds it can form

Explanation: Every amino acid shares the same amino group, carboxyl group and central carbon, so only the variable R group distinguishes one from another and gives it its chemical character. This is why the R group, not the backbone, determines where an amino acid sits in a folded protein.