JEE Chemistry Biomolecules MCQs Set 05

Answer: (b) cofactor

Explanation:
1. Many enzymes need a non-protein part for activity.
2. That part is the cofactor, which may be a metal ion or an organic coenzyme.
3. The protein part is the apoenzyme.

Teacher's Note:
1. Holoenzyme is apoenzyme plus cofactor.
2. Vitamins often act as coenzymes.

Answer: (a) coenzyme

Explanation:
1. A coenzyme binds loosely and only during the reaction.
2. A prosthetic group is tightly bound permanently.

Teacher's Note:
1. Many coenzymes are derived from vitamins.
2. NAD and FAD are common examples.

Answer: (b) Uracil and cytosine

Explanation:
1. RNA contains the bases adenine, guanine, cytosine and uracil.
2. Of these, cytosine and uracil are pyrimidines.
3. Thymine is found in DNA, and adenine is a purine.

Teacher's Note:
1. Purines have two rings (A, G) and pyrimidines have one ring (C, T, U).
2. Thymine is 5-methyluracil.

Answer: (a) replication

Explanation:
1. DNA makes an identical copy of itself by replication.
2. Transcription makes RNA from DNA, and translation makes protein from RNA.

Teacher's Note:
1. Replication is semi-conservative.
2. Each new DNA has one old and one new strand.

Answer: (d) Both glycogen and amylopectin

Explanation:
1. Glycogen and amylopectin both have alpha-1,6 branch points.
2. Cellulose is an unbranched chain of beta-D-glucose.

Teacher's Note:
1. Amylose is the unbranched part of starch.
2. Amylopectin makes up 80 to 85 percent of starch.

Answer: (b) antiscurvy vitamin

Explanation:
1. Vitamin C (ascorbic acid) prevents scurvy.
2. The antisterility vitamin is vitamin E.

Teacher's Note:
1. Citrus fruits and amla are rich sources.
2. Vitamin C is water soluble and not stored.

Answer: (a) They reduce Fehling's solution

Explanation:
1. Reducing sugars have a free hemiacetal or hemiketal group, which opens to give a free aldehyde or ketone.
2. They reduce both Fehling's solution and Tollens' reagent.
3. So only statement (a) is true.

Teacher's Note:
1. Glucose, fructose, maltose and lactose are reducing sugars.
2. Sucrose is non-reducing.

Answer: (a) avitaminosis

Explanation:
1. Disease due to the lack of vitamins is called avitaminosis.
2. Xerophthalmia is caused by vitamin A deficiency specifically.

Teacher's Note:
1. Deficiency of a single vitamin gives a specific disease.
2. Hypervitaminosis means excess.

Answer: (a) Vitamin A

Explanation:
1. Carotene is provitamin A.
2. It is converted to retinol (vitamin A) in the liver.

Teacher's Note:
1. Carrots and papaya are rich in carotene.
2. Vitamin A deficiency causes night blindness.

Answer: (a) Glucagon

Explanation:
1. Glucagon is a polypeptide of 29 amino acids made by the pancreas.
2. Testosterone is a steroid, and thyroxine and adrenaline are amino acid derivatives.

Teacher's Note:
1. Peptide hormones include insulin, glucagon and oxytocin.
2. Steroid hormones are made from cholesterol.

Answer: (c) pepsin and trypsin

Explanation:
1. Pepsin in the stomach breaks proteins into polypeptides.
2. Trypsin in the intestine continues the hydrolysis to smaller peptides and amino acids.

Teacher's Note:
1. Amylase and maltase act on carbohydrates, and lipase acts on fats.
2. Match each enzyme to its substrate.

Answer: (a) 5-methyluracil

Explanation:
1. Thymine is uracil with a methyl group at position 5 of the pyrimidine ring.

Teacher's Note:
1. This is why DNA contains thymine and RNA contains uracil.
2. The methyl group adds stability to DNA.

Answer: (d) 8 to 9

Explanation:
1. An amino acid is least soluble at its isoelectric point, where it is a zwitterion.
2. Lysine has two amino groups, so its isoelectric point is high (about 9.7).
3. Among the options, 8 to 9 is closest.

Teacher's Note:
1. Basic amino acids have pI above 7.
2. Acidic amino acids have pI below 7.

Answer: (a) D-sugar component

Explanation:
1. The sugars ribose and 2-deoxyribose are D-sugars with several stereocentres.
2. The bases and phosphate groups are achiral.

Teacher's Note:
1. Natural nucleic acids contain only D-sugars.
2. Natural amino acids in proteins are L.

Answer: (d) adenine and thymine; guanine and cytosine

Explanation:
1. Adenine pairs with thymine through two hydrogen bonds.
2. Guanine pairs with cytosine through three hydrogen bonds.

Teacher's Note:
1. Uracil appears only in RNA.
2. Purine always pairs with pyrimidine.

Answer: (d) Thyroxine

Explanation:
1. Thyroxine is made from the amino acid tyrosine and is an amine hormone.
2. Insulin is a peptide and progesterone is a steroid.

Teacher's Note:
1. Adrenaline is another amine hormone.
2. Identify hormones by their chemical class.

Answer: (d) gene

Explanation:
1. A gene is a segment of DNA carrying the code for a polypeptide.

Teacher's Note:
1. A nucleotide is only one unit of the DNA chain.
2. A gene is a sequence of many nucleotides.

Answer: (c) enolisation of fructose followed by conversion to an aldehyde by the base

Explanation:
1. Tollens' reagent is basic.
2. In base, fructose enolises and rearranges to glucose and mannose, which have aldehyde groups, so it reduces the reagent.

Teacher's Note:
1. Fructose is a ketose but is still a reducing sugar.
2. Bromine water can tell glucose from fructose.

Answer: (a) Lysine

Explanation:
1. Lysine has an additional amino group on its side chain.
2. Its basicity is clearly more than the other options.
3. Glutamine and glycine are neutral.

Teacher's Note:
1. Arginine is more basic still, but it is not offered.
2. Lysine, arginine and histidine are basic.

Answer: (c) (+) Lactose is a reducing sugar and does not exhibit mutarotation

Explanation:
1. Lactose has a free hemiacetal group on its glucose unit, so it reduces Tollens' reagent and also shows mutarotation.
2. Statement (c) is therefore false.
3. The other statements are true.

Teacher's Note:
1. Count of OH groups: 11 oxygen atoms minus two ring oxygens and one glycosidic oxygen leaves 8.
2. Reducing disaccharides always show mutarotation.

Answer: (b) Vitamin B complex

Explanation:
1. Vitamins A, D, E and K are fat soluble.
2. Vitamin B complex is water soluble.

Teacher's Note:
1. Fat-soluble vitamins are stored in the body.
2. So they are less likely to be needed daily.