Free Macromolecules MCQs with Answers

31. Proteins are described as polymers because they are

• A. large molecules built from repeating amino acid units
• B. made of two identical halves
• C. always branched
• D. insoluble in all solvents

Explanation: A polymer is any large molecule assembled from many small repeating units, and in a protein those units are amino acids joined by peptide bonds. Unlike a synthetic polymer, the sequence is precisely specified rather than random, which is what allows a protein to have a defined function. Polysaccharides and nucleic acids are the other biological polymers.

32. The number of amino acids commonly found in natural proteins is

• A. 4
• B. 20
• C. 64
• D. 100

Explanation: Twenty amino acids account for almost all natural protein, and their enormous number of possible sequences is what allows such variety of structure and function. The figure 64 is the number of codons in the genetic code, which is why several codons specify the same amino acid. Four is the number of bases in DNA.

33. A zwitterion of an amino acid carries

• A. no charged groups
• B. a positive charge on the amino group and a negative charge on the carboxyl group at the same time
• C. two positive charges
• D. a single negative charge only

Explanation: The carboxyl group donates its proton to the amino group within the same molecule, so the ion has both charges and a net charge of zero at the isoelectric point. This internal ionisation explains why amino acids are crystalline solids with high melting points rather than the low melting organic liquids their size would suggest. They are also very soluble in water.

34. Which factor would NOT denature a protein?

• A. Boiling
• B. Strong acid
• C. Heavy metal ions
• D. Dissolving it in pure water at room temperature

Explanation: Many proteins dissolve in water without any loss of structure, since water is the medium they normally function in. Heat, extremes of pH, heavy metals, alcohol and vigorous agitation all disrupt the weak interactions holding the fold. Denaturation destroys function while leaving the peptide backbone intact.

35. Insulin is a protein hormone whose function is to

• A. raise blood glucose concentration
• B. lower blood glucose concentration by promoting uptake and storage
• C. digest proteins in the stomach
• D. carry oxygen

Explanation: Secreted by the beta cells of the pancreas, insulin increases the permeability of cells to glucose and stimulates its conversion into glycogen, so blood glucose falls. Glucagon raises it, and the two form a negative feedback pair. Insulin was the first protein to have its full amino acid sequence determined.

36. Collagen is the most abundant protein in the human body and its main role is

• A. catalysing reactions
• B. providing tensile strength in skin, tendon and bone
• C. transporting oxygen
• D. storing genetic information

Explanation: Its triple helix of three chains wound together gives collagen enormous tensile strength, which is why tendons and ligaments resist stretching. Vitamin C is needed for its synthesis, and a deficiency produces the weakened connective tissue of scurvy. Genetic information is stored by nucleic acids rather than proteins.

37. The rate of an enzyme catalysed reaction levels off at high substrate concentration because

• A. the substrate begins to inhibit the enzyme
• B. all the active sites are occupied, so the enzyme is saturated
• C. the enzyme is denatured by the substrate
• D. the products stop the reaction

Explanation: Once every active site is engaged, adding more substrate cannot increase the rate, and the plateau reached is Vmax. At that point the enzyme concentration rather than the substrate is limiting, so adding more enzyme would raise the rate again. This saturation behaviour is one of the clearest differences between enzymes and inorganic catalysts.

38. A coenzyme differs from a cofactor in that a coenzyme is

• A. an inorganic metal ion
• B. an organic molecule, often derived from a vitamin
• C. part of the polypeptide chain
• D. a type of substrate

Explanation: NAD, FAD and coenzyme A are organic helpers built from niacin, riboflavin and pantothenic acid respectively, whereas cofactors such as zinc and magnesium ions are inorganic. A helper bound tightly and permanently is called a prosthetic group instead. This is why vitamin deficiencies cause such widespread metabolic effects.

39. Which of the following is a conjugated protein?

• A. albumin
• B. haemoglobin
• C. keratin
• D. fibroin

Explanation: Haemoglobin carries a non protein haem prosthetic group that does the actual oxygen binding, which makes it conjugated. Albumin, keratin and fibroin consist of polypeptide alone and yield only amino acids on hydrolysis. Glycoproteins and lipoproteins are conjugated in the same sense, with carbohydrate or lipid attached.

40. An enzyme used to decompose the lipids into fatty acids in our alimentary canal is

• A. Amylase
• B. Protease
• C. Lipase
• D. Urease

Explanation: Pancreatic lipase hydrolyses triglycerides into fatty acids and glycerol in the duodenum, working on the enormous surface created after bile salts have emulsified the fat. Amylase acts on starch and protease on protein, each enzyme being specific to one class of substrate. Urease breaks urea down and is found in bacteria and plants rather than the human gut.