CBSE Class 12 Chemistry Biomolecules MCQs Set 15

Answer: (b) amylopectin

Explanation:
1. Glycogen is a branched polymer of α-D-glucose with C₁–C₄ chains and C₁–C₆ branches.
2. It is more highly branched than amylopectin, and so is called animal starch.

Teacher's Note:
1. Amylose is unbranched; amylopectin is branched.
2. Cellulose is made of β-D-glucose.

Answer: (c) Galactose

Explanation:
1. Glucose, mannose and fructose differ only at C-1 and C-2 and so give the same osazone.
2. Galactose differs at C-4 as well, so its osazone is different.

Teacher's Note:
1. Osazone formation destroys the configuration at C-1 and C-2.
2. Epimers at C-4 therefore give different osazones.

Answer: (c) C-1

Explanation:
1. On ring closure the aldehyde carbon C-1 becomes a new chiral centre (anomeric carbon).
2. The two anomers differ only in the position of –OH at C-1.

Teacher's Note:
1. The anomeric carbon is the former carbonyl carbon.
2. For fructose it is C-2.

Answer: (d) glucose and fructose both

Explanation:
1. Invertase hydrolyses the glycosidic linkage of sucrose.
2. One molecule of glucose and one of fructose are formed.

Teacher's Note:
1. Ethyl alcohol arises only on fermentation with zymase.
2. The product mixture is invert sugar.

Answer: (a) Vitamin B₁₂

Explanation:
1. Water-soluble vitamins are excreted in urine and are not stored, and must be supplied regularly.
2. Vitamin B₁₂ is an exception and is stored in the liver.

Teacher's Note:
1. Fat-soluble vitamins A, D, E and K are stored in fat and liver.
2. B₁₂ is the water-soluble vitamin that is stored.

Answer: (c) Both Fehling's solution and Tollens' reagent

Explanation:
1. Cellulose is a non-reducing polysaccharide with no free aldehyde or ketone group.
2. It therefore does not respond to either reagent.

Teacher's Note:
1. Non-reducing sugars: sucrose, starch, cellulose.
2. Reducing sugars have a free anomeric –OH.

Answer: (b) Sucrose

Explanation:
1. In sucrose, C-1 of glucose and C-2 of fructose are joined, so both reducing groups are used up in the glycosidic bond.
2. Hence sucrose has no free hemiacetal group and is non-reducing.

Teacher's Note:
1. Free hemiacetal means reducing sugar.
2. Sucrose is the standard non-reducing disaccharide.

Answer: (d) basic

Explanation:
1. Amino acids are classified by the number of –NH₂ and –COOH groups.
2. More –NH₂ groups (e.g. lysine) make the amino acid basic.

Teacher's Note:
1. More –COOH: acidic; more –NH₂: basic; equal: neutral.
2. Lysine, arginine and histidine are basic.

Answer: (d) can be synthesised in the body

Explanation:
1. The body can make non-essential amino acids itself.
2. Essential ones must be taken in the diet.

Teacher's Note:
1. Essential or not has nothing to do with the number of functional groups.
2. Glycine, alanine and serine are non-essential.

Answer: (a) Serine

Explanation:
1. Lysine, threonine and tryptophan cannot be made in the body.
2. Serine is synthesised in the body, so it is non-essential.

Teacher's Note:
1. Learn a few essential amino acids: valine, leucine, isoleucine, lysine, threonine.
2. Glycine, alanine and serine are non-essential.

Answer: (a) a positive ion and a negative ion

Explanation:
1. In acid the –COO⁻ group takes up H⁺, leaving –NH₃⁺ and giving a cation.
2. In alkali the –NH₃⁺ group gives up H⁺, leaving –COO⁻ and giving an anion.

Teacher's Note:
1. The zwitter ion exists only near the isoelectric point.
2. Amino acids are amphoteric.

Answer: (c) Both fibrous and globular

Explanation:
1. Tertiary structure is the overall folding of the polypeptide chain.
2. It gives rise to two major molecular shapes: fibrous and globular.

Teacher's Note:
1. Fibrous: keratin, myosin.
2. Globular: insulin, albumins.

Answer: (a) Haemoglobin

Explanation:
1. Globular proteins are spherical in shape and generally soluble in water.
2. Haemoglobin is globular, whereas silk fibroin and collagen are fibrous.

Teacher's Note:
1. Insulin and albumins are other examples.
2. Fibrous proteins are usually insoluble.

Answer: (c) a denatured protein

Explanation:
1. Heat, a change in pH or certain chemicals break the hydrogen bonds.
2. The helix uncoils and globules unfold, so the protein loses its biological activity.

Teacher's Note:
1. Denaturation affects the secondary and tertiary structures only.
2. The primary structure stays intact.

Answer: (b) irreversible denaturation of protein

Explanation:
1. Heat unfolds the globular albumin and it becomes insoluble, a white solid.
2. The change cannot be reversed by cooling the egg.

Teacher's Note:
1. Curdling of milk is another example of denaturation.
2. Peptide bonds are not broken.

Answer: (d) a cofactor

Explanation:
1. Some enzymes need a non-protein part for activity, such as a metal ion or an organic molecule.
2. This non-protein part is the cofactor (the protein part is the apoenzyme).

Teacher's Note:
1. Cofactors can be coenzymes or prosthetic groups.
2. Many vitamins act as coenzymes.

Answer: (b) nucleus

Explanation:
1. The nucleus of the cell carries chromosomes, which are made up of proteins and nucleic acids.
2. DNA is mainly present in the nucleus.

Teacher's Note:
1. Chromosomes carry heredity.
2. RNA is found mainly in the cytoplasm.

Answer: (a) (b) and (c)

Explanation:
1. RNA contains A, G, C and U.
2. Of these only uracil and cytosine are pyrimidines (adenine is a purine and thymine is found in DNA).

Teacher's Note:
1. Thymine is replaced by uracil in RNA.
2. Pyrimidines: C, T, U.

Answer: (a) replication

Explanation:
1. DNA makes exact copies of itself, passing hereditary information from one generation to the next.
2. This process is replication.

Teacher's Note:
1. Replication: DNA to DNA.
2. Transcription: DNA to RNA.

Answer: (c) Glycogen and amylopectin

Explanation:
1. Both glycogen and amylopectin have C₁–C₄ chains with C₁–C₆ branches.
2. Amylose and cellulose are unbranched.

Teacher's Note:
1. Glycogen is more highly branched.
2. Cellulose is a β-glucose polymer.