JEE Chemistry Biomolecules MCQs Set 27

Answer: (b) 8

Explanation:
1. Open-chain fructose has three chiral carbon atoms (C-3, C-4, C-5).
2. Number of optical isomers = 2 raised to 3 = 8.

Teacher's Note:
1. Count the chiral centres first.
2. Ketohexoses have three, aldohexoses have four.

Answer: (b) vitamin A

Explanation:
1. Vitamin A (retinol) is needed to form rhodopsin, the visual pigment in the retina.
2. Its deficiency causes night blindness and xerophthalmia.

Teacher's Note:
1. Retinol and vision are linked.
2. B12 deficiency causes pernicious anaemia.

Answer: (a) glucose only

Explanation:
1. Starch is a mixture of amylose and amylopectin.
2. Both are polymers whose only monomer is D-glucose, so complete hydrolysis gives only glucose.

Teacher's Note:
1. Starch hydrolysis goes through dextrins and maltose to glucose.
2. Sucrose gives glucose and fructose.

Answer: (d) L-lactic acid

Explanation:
1. During vigorous exercise oxygen supply is insufficient, so pyruvic acid is converted to lactic acid by anaerobic respiration.
2. The build-up of lactic acid causes muscle fatigue and cramps.

Teacher's Note:
1. Anaerobic respiration gives lactic acid.
2. Glycogen is the store, not the product.

Answer: (d) sucrose

Explanation:
1. Mutarotation needs a free hemiacetal group that can open to the aldehyde form.
2. Sucrose has both anomeric carbons involved in the glycosidic link, so it has no free hemiacetal group and shows no mutarotation.

Teacher's Note:
1. Sucrose is a non-reducing sugar.
2. Maltose and lactose have a free anomeric OH.

Answer: (b) 3.25

Explanation:
1. For an acidic amino acid the zwitterion with a net charge of zero lies between the two carboxylic acid pKa values.
2. pI = (2.2 + 4.3) / 2 = 3.25.

Teacher's Note:
1. Average the two pKa values that surround the neutral form.
2. Acidic amino acids have low pI.

Answer: (d) I-C, II-D, III-B, IV-A

Explanation:
1. Vitamin C deficiency causes scurvy.
2. Retinol (vitamin A) deficiency causes night blindness.
3. Riboflavin (B2) deficiency causes cracked lips, and thiamine (B1) deficiency causes beri-beri.

Teacher's Note:
1. Learn each vitamin with its disease.
2. Water-soluble B vitamins cause beri-beri and cheilosis.

Answer: (b) 3',5'-phosphodiester bonds

Explanation:
1. A phosphate group joins the 3' carbon of one sugar to the 5' carbon of the next sugar.
2. This gives the sugar-phosphate backbone of the polynucleotide chain.

Teacher's Note:
1. Hydrogen bonds join the two strands, not the nucleotides.
2. Glycosidic bonds join base and sugar.

Answer: (a) TACGAACT

Explanation:
1. Adenine pairs with thymine, and guanine pairs with cytosine.
2. Pairing each base gives A-T, T-A, G-C, C-G, T-A, T-A, G-C, A-T, that is TACGAACT.

Teacher's Note:
1. Complementary base pairing: A-T and G-C.
2. Pair each base one at a time.

Answer: (c) sucrose

Explanation:
1. In sucrose the anomeric carbons of glucose and fructose are both involved in the glycosidic bond.
2. So there is no free aldehyde or hemiketal group and it does not reduce Fehling's or Tollens' reagent.

Teacher's Note:
1. Lactose and maltose have a free hemiacetal group.
2. Fructose is a reducing ketose.

Answer: (c) sucrose

Explanation:
1. Glucose, fructose (via enolisation to an aldose) and aldehydes reduce Benedict's solution.
2. Sucrose has no free anomeric OH and does not reduce it.

Teacher's Note:
1. Sucrose is the standard non-reducing disaccharide.

Answer: (d) no characteristic colour or precipitate will be formed

Explanation:
1. Methanolic HCl converts the hemiacetal OH of glucose into a methyl glycoside (an acetal).
2. The acetal cannot open to the aldehyde form, so Tollens' reagent is not reduced.

Teacher's Note:
1. Glycosides are non-reducing.
2. Only the free aldehyde form gives a silver mirror.

Answer: (d) none of these

Explanation:
1. Alanine, valine and proline each have an alpha carbon bonded to four different groups, so all are chiral.
2. The only achiral protein amino acid is glycine, which is not listed.

Teacher's Note:
1. Glycine has two H atoms on the alpha carbon.
2. None of the listed acids is glycine.

Answer: (c) the human body can synthesise all the amino acids it needs

Explanation:
1. Essential amino acids cannot be synthesised by the human body and must come from the diet.
2. The other statements are correct.

Teacher's Note:
1. Valine, leucine and lysine are essential.
2. Zwitterions predominate near neutral pH.

Answer: (a) pyridoxine

Explanation:
1. Vitamin B6 is pyridoxine.
2. Thiamine is B1, riboflavin is B2 and tocopherol is vitamin E.

Teacher's Note:
1. Pyridoxine = B6.
2. Its deficiency causes convulsions.

Answer: (a) three

Explanation:
1. A codon is a triplet of three consecutive nucleotides in mRNA that specifies one amino acid.

Teacher's Note:
1. Genetic code is a triplet code.
2. 64 codons exist for 20 amino acids.

Answer: (d) guanidine

Explanation:
1. Nucleic acids contain the bases adenine, guanine, cytosine, thymine (DNA) and uracil (RNA).
2. Guanidine is not a nucleic acid base, although it resembles guanine in name.

Teacher's Note:
1. Do not confuse guanine with guanidine.
2. Guanidine is a strongly basic compound.

Answer: (b) pH = 6

Explanation:
1. At the isoelectric point the amino acid exists mainly as the neutral zwitterion.
2. Moving away from pH 6 produces the cation or anion.

Teacher's Note:
1. At pI net charge is zero.
2. Solubility is also lowest at pI.

Answer: (d) proteins can only contain the elements C, H, O and N

Explanation:
1. Proteins decompose or denature on heating and have no sharp melting point, they contain peptide (-CONH-) linkages, and they have high molecular masses.
2. Many proteins also contain sulphur (cysteine, methionine) and some contain phosphorus or metals.

Teacher's Note:
1. Cysteine contains sulphur.
2. So protein elements are not limited to C, H, O, N.

Answer: (d) hydrogen bonding

Explanation:
1. The two strands are held together by hydrogen bonds between complementary bases: two for A-T and three for G-C.
2. These specific interactions give the double helix.

Teacher's Note:
1. Base pairing uses hydrogen bonds.
2. G-C pairs are stronger than A-T pairs.

Answer: (c) butyric acid

Explanation:
1. Rancid butter contains butyric acid (butanoic acid) formed by hydrolysis of the glycerides of butter.
2. It is responsible for the unpleasant smell.

Teacher's Note:
1. Butter fat contains butyrate esters.
2. Hydrolysis releases the free acid.

Answer: (d) it will react with neither phenylhydrazine nor Tollens' reagent

Explanation:
1. Acetylation of all five OH groups, including the anomeric one, fixes the ring form.
2. There is no free CHO group in equilibrium, so there is no reaction with phenylhydrazine or Tollens' reagent.

Teacher's Note:
1. Pentaacetate means every OH including the hemiacetal OH is acetylated.
2. Evidence for the cyclic structure.

Answer: (c) 5-methyluracil (5-methylpyrimidine-2,4-dione)

Explanation:
1. Thymine is the pyrimidine base found in DNA, and it is uracil with a methyl group at position 5.
2. RNA contains uracil in its place.

Teacher's Note:
1. Thymine = 5-methyluracil.
2. It pairs with adenine.