JEE Chemistry Biomolecules MCQs Set 04

Answer: (b) acts as an oxygen carrier in the blood

Explanation:
1. Haemoglobin is a conjugated protein containing iron in its haem group.
2. Iron binds oxygen reversibly, so it carries oxygen from lungs to tissues.

Teacher's Note:
1. Blood sugar is controlled by insulin, antibodies are immunoglobulins and catalysts are enzymes.
2. Match each protein with its function to avoid confusion.

Answer: (c) Insulin

Explanation:
1. Insulin lowers blood glucose by promoting its conversion into glycogen in the liver and muscles.
2. Glucagon does the opposite job.

Teacher's Note:
1. Insulin is a peptide hormone made of two chains joined by disulphide bridges.
2. Its deficiency causes diabetes mellitus.

Answer: (a) hydrogen bonds

Explanation:
1. The alpha-helix is a secondary structure.
2. It is held by hydrogen bonds between the C=O of one amino acid and the N-H of the fourth amino acid ahead of it in the chain.

Teacher's Note:
1. Peptide bonds build the primary structure, not the helix.
2. Hydrogen bonds are the key forces in secondary structure.

Answer: (b) Thiol

Explanation:
1. The amino acid cysteine has a thiol (-SH) group.
2. Two -SH groups are oxidised to form a -S-S- disulphide bridge between chains or parts of a chain.

Teacher's Note:
1. Disulphide links help hold the tertiary structure.
2. Hair keratin is rich in them, which is why perming works by breaking and re-forming them.

Answer: (a) phospholipids

Explanation:
1. The membrane is a phospholipid bilayer with embedded proteins.
2. Phospholipids form the main structural framework.

Teacher's Note:
1. Heads are hydrophilic and tails hydrophobic, so they arrange as a bilayer in water.
2. Proteins and carbohydrates are present in smaller amounts.

Answer: (a) alpha-Keratin

Explanation:
1. Keratin is a fibrous protein, and fibrous proteins are insoluble in water.
2. Haemoglobin and ribonuclease are globular proteins, which are soluble.

Teacher's Note:
1. Fibrous proteins (keratin, collagen, myosin) are structural.
2. Globular proteins (enzymes, haemoglobin, insulin) are soluble.

Answer: (d) 2nd

Explanation:
1. Ribose in RNA has an OH at C-2.
2. 2-Deoxyribose in DNA lacks the OH at C-2, which is the difference.

Teacher's Note:
1. The extra 2-OH makes RNA more reactive and easier to hydrolyse.
2. That is why DNA is the stable store of genetic information.

Answer: (c) Quinoline

Explanation:
1. DNA contains adenine, guanine, cytosine and thymine.
2. Quinoline is a heterocyclic compound and not a nucleic acid base.

Teacher's Note:
1. RNA has uracil in place of thymine.
2. Learn the two purines (A, G) and three pyrimidines (C, T, U).

Answer: (a) alpha-amino acid

Explanation:
1. One NH₂ group sits on the carbon next to COOH, so it is an alpha-amino acid.
2. The second amino group is on the side chain.
3. Lysine is essential, so it is not made in the body.

Teacher's Note:
1. Proteins are made only of alpha-amino acids.
2. Lysine is basic because it has two amino groups and one carboxyl group.

Answer: (b) disulphide linkages (together with hydrogen bonds)

Explanation:
1. Fibrous proteins have parallel chains held together by hydrogen bonds and disulphide bonds.
2. Among the given options, disulphide linkage is correct.

Teacher's Note:
1. Fibrous proteins are insoluble in water.
2. Keratin and collagen are common examples.

Answer: (b) amylopectin

Explanation:
1. Both glycogen and amylopectin are branched polymers of alpha-D-glucose.
2. They have alpha-1,4 links in chains and alpha-1,6 links at the branches.
3. Glycogen is more highly branched.

Teacher's Note:
1. Glycogen is called animal starch.
2. It is stored in liver and muscles.

Answer: (c) Galactose

Explanation:
1. Glucose, mannose and fructose differ only at C-1 and C-2, and these two carbons become part of the osazone.
2. So all three give the same osazone.
3. Galactose differs at C-4, so it gives a different osazone.

Teacher's Note:
1. Osazone formation uses carbons 1 and 2 only.
2. Epimers at C-2 and the corresponding ketose give the same osazone.

Answer: (d) both glucose and fructose

Explanation:
1. Invertase hydrolyses sucrose into equal amounts of D-glucose and D-fructose.
2. The mixture is called invert sugar because the rotation changes from dextro to laevo.

Teacher's Note:
1. Sucrose is dextrorotatory (+66.5 degrees).
2. The product is laevorotatory since fructose (-92.4 degrees) rotates more than glucose (+52.5 degrees).

Answer: (d) Vitamin B₁₂

Explanation:
1. Water-soluble vitamins are generally excreted and need regular supply.
2. Vitamin B₁₂ is an exception and is stored in the liver.

Teacher's Note:
1. Vitamins A, D, E and K are fat soluble and stored.
2. B12 is the water-soluble vitamin that can be stored.

Answer: (d) i, ii, iii and iv

Explanation:
1. Starch, cellulose and glycogen are polymers of glucose.
2. Dextrins are shorter polysaccharide chains obtained by partial hydrolysis of starch.
3. So all four are polysaccharides.

Teacher's Note:
1. Polysaccharides give many monosaccharide units on hydrolysis.
2. Starch, cellulose and glycogen are the main three.

Answer: (c) both of these

Explanation:
1. Cellulose is a non-reducing polysaccharide with glycosidic links between all anomeric carbons except one terminal end, which is negligible.
2. So it gives no test with Fehling's or Tollens' reagent.

Teacher's Note:
1. Reducing tests need a free hemiacetal group in appreciable amount.
2. Large polymers practically do not respond.

Answer: (c) Sucrose

Explanation:
1. In sucrose the anomeric carbons of glucose and fructose are both used in the glycosidic bond.
2. So there is no free hemiacetal or hemiketal group, and it is non-reducing.

Teacher's Note:
1. A free hemiacetal group makes a sugar reducing.
2. Sucrose is the standard non-reducing disaccharide.

Answer: (a) which can be synthesised in the body

Explanation:
1. Amino acids that the body can make are non-essential.
2. Those that must come from the diet are essential.

Teacher's Note:
1. Essential amino acids include valine, leucine, lysine and tryptophan.
2. The classification has nothing to do with the number of functional groups.

Answer: (a) Serine

Explanation:
1. Lysine, threonine and tryptophan have to be supplied by the diet.
2. Serine can be made in the body, so it is non-essential.

Teacher's Note:
1. Remember essential ones for adults with the name PVT TIM HALL.
2. Serine, glycine, alanine and others are non-essential.

Answer: (c) both fibrous and globular

Explanation:
1. The tertiary structure is the overall folding of the chain.
2. It can be fibrous (rod-like) or globular (spherical).

Teacher's Note:
1. Tertiary structure is stabilised by disulphide bridges, hydrogen bonds and ionic forces.
2. Hydrophobic interactions also help.

Answer: (c) haemoglobin

Explanation:
1. Haemoglobin is folded into a compact spherical shape and is water soluble.
2. Silk fibroin and collagen are fibrous.

Teacher's Note:
1. Albumin, insulin and enzymes are also globular.
2. Globular proteins are soluble in water.