Biomolecules - MCQs with Explanations

1. An organic compound with the formula $C_{6}H_{12}O_{6}$ forms a yellow crystalline solid with phenyl-hydrazine and gives a mixture of sorbitol and mannitol when reduced with sodium. Which among the following could be the compound?

• (a) Fructose
• (b) Glucose
• (c) Mannose
• (d) Sucrose

Correct Answer: (a)

Explanation: The formation of a yellow crystalline osazone with phenylhydrazine indicates the compound is an aldohexose or ketohexose. Since reduction with sodium yields a mixture of two alcohols, sorbitol and mannitol, the compound must be fructose. Glucose only yields sorbitol upon reduction.

2. Which one is diaminodicarboxylic amino acid?

• (a) Cystine
• (b) Lysine
• (c) Cysteine
• (d) Aspartic acid

Correct Answer: (a)

Explanation: Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues. These residues are covalently linked by a disulfide bond, making it a diaminodicarboxylic acid.

3. Which of the following is not a function of proteins?

• (a) Nail formation
• (b) Skin formation
• (c) Muscle formation
• (d) Providing energy for metabolism

Correct Answer: (d)

Explanation: Proteins are essential for the formation of biological structures like nails, skin, and muscles. However, they do not serve as a primary source for providing energy for metabolism.

4. The 'repeating unit' of glycogen is

• (a) fructose
• (b) mannose
• (c) glucose
• (d) galactose.

Correct Answer: (c)

Explanation: Glycogen consists of long polymer chains of glucose units. These monomer units are specifically $\alpha$-D-glucose.

5. Which statement is incorrect about peptide bond?

• (a) C-N bond length in proteins is almost same as usual C-N bond length.
• (b) Spectroscopic analysis shows planar structure of -C-NH- bond.
• (c) C-N bond length in proteins is smaller than usual C-N bond length.
• (d) None of these.

Correct Answer: (a)

Explanation: Due to resonance, the C-N bond in a peptide linkage acquires some double bond character. This makes the bond length shorter than a typical single C-N bond.

6. An organic compound consumes 4 moles of periodic acid to form following compounds per mole of the starting compound: HCHO, 3HCOOH and CHOCOOH. The organic compound is

• (a) glucose
• (b) fructose
• (c) gluconic acid
• (d) sorbitol.

Correct Answer: (c)

Explanation: Gluconic acid, represented by $CH_2OH(CHOH)_4COOH$, undergoes cleavage with 4 moles of periodic acid to produce the listed products.

7. If the sequence of bases in one strand of DNA is ATGACTGTC then, the sequence of bases in its complementary strand is

• (a) TACTGACAG
• (b) TUCTUGUCCUG
• (c) GUAGTUAUG
• (d) none of these.

Correct Answer: (a)

Explanation: In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Therefore, the complementary strand for ATGACTGTC is TACTGACAG.

8. Which one is the most abundant protein in the animal world?

• (a) Collagen
• (b) Insulin
• (c) Trypsin
• (d) Haemoglobin

Correct Answer: (a)

Explanation: Collagen is recognized as the most abundant protein in the animal kingdom, providing necessary structural integrity.

9. Which one of the following bases is not present in DNA?

• (a) Thymine
• (b) Quinoline
• (c) Adenine
• (d) Cytosine

Correct Answer: (b)

Explanation: The standard nitrogenous bases in DNA are adenine, thymine, cytosine, and guanine. Quinoline is not a part of DNA.

10. Observe the given disaccharide and choose the correct statement. [Structure shown in source]

• (a) Ring (B) is furanose with $\beta$-glycosidic link.
• (b) Ring (A) is furanose with $\alpha$-glycosidic link.
• (c) Ring (A) is pyranose with $\alpha$-glycosidic link.
• (d) Both (a) and (c).

Correct Answer: (d)

Explanation: In the structure provided, ring (A) is a pyranose ring with an $\alpha$-glycosidic linkage. Ring (B) is a furanose ring featuring a $\beta$-glycosidic linkage.

11. Which one of the following sets of monosaccharides forms sucrose?

• (a) $\alpha$-D-galactopyranose and $\alpha$-D-glucopyranose
• (b) $\alpha$-D-glucopyranose and $\beta$-D-fructofuranose
• (c) $\beta$-D-glucopyranose and $\alpha$-D-fructofuranose
• (d) $\alpha$-D-glucopyranose and $\beta$-D-fructapyranose.

Correct Answer: (b)

Explanation: Sucrose is produced by the condensation of one $\alpha$-D-glucopyranose unit and one $\beta$-D-fructofuranose unit.

12. Match the list I with list II and select the correct answer. P. Neutral amino acid; Q. Acidic amino acid; R. Basic amino acid. List II: 1. Arginine; 2. Aspartic acid; 3. Valine.

• (a) 1 2 3
• (b) 2 3 1
• (c) 1 3 2
• (d) 3 2 1

Correct Answer: (d)

Explanation: Valine (3) is a neutral amino acid. Aspartic acid (2) is an acidic amino acid. Arginine (1) is a basic amino acid.

13. The bending and folding of a protein molecule would produce a

• (a) tertiary structure
• (b) primary structure
• (c) secondary structure
• (d) linear sequence of amino acids.

Correct Answer: (a)

Explanation: While the primary structure is the sequence of amino acids, the further bending and folding of these chains leads to the complex 3D tertiary structure.

14. A dextrorotatory sugar present in fruits is

• (a) glucose
• (b) fructose
• (c) cellulose
• (d) starch.

Correct Answer: (a)

Explanation: Glucose is a dextrorotatory sugar, often referred to as dextrose. Fructose is levorotatory.

15. Which of the following statements about RNA is not correct?

• (a) It has a single strand.
• (b) It does not undergo replication.
• (c) It does not contain any pyrimidine base.
• (d) It controls the synthesis of proteins.

Correct Answer: (c)

Explanation: RNA contains pyrimidine bases, specifically uracil and cytosine. Therefore, statement (c) is false.

16. The specific rotation of $\alpha$- and $\beta$-forms of a monosaccharide are $+29^{\circ}$ and $-17^{\circ}$ respectively. If the equilibrium mixture is $+14^{\circ}$, the percentage of $\alpha$ and $\beta$ forms are:

• (a) $\alpha\text{-form}=67.4\%,\beta\text{-form}=32.6\%$
• (b) $\alpha\text{-form}=60\%,\beta\text{-form}=40\%$
• (c) $\alpha\text{-form}=70\%,\beta\text{-form}=30\%$
• (d) $\alpha\text{-form}=50\%,\beta\text{-form}=50\%$

Correct Answer: (a)

Explanation: Let the fraction of the $\beta$-form be $x$ and the $\alpha$-form be $1-x$. Solving $x(-17) + (1-x)29 = 14$ yields $x = 0.326$. This corresponds to 32.6% $\beta$-form and 67.4% $\alpha$-form.

17. Which of the following will reduce Tollen's reagent? [Structures provided in source]

• (a) Structure (a)
• (b) Structure (b)
• (c) Both (a) and (b)
• (d) None of these.

Correct Answer: (a)

Explanation: Structure (a) acts as a reducing sugar because its monosaccharide units are not linked through both reducing centers, leaving one free. In structure (b), the units are linked through their reducing centers, making it non-reducing.

18. A carbohydrate X (MW 180) has 1 primary and 4 secondary alcohol groups. It forms a pentaacetate. The molecular weight of the pentaacetate is:

• (a) 180
• (b) 210
• (c) 390
• (d) 42

Correct Answer: (c)

Explanation: Replacing a hydrogen atom (MW 1) with an acetyl group (MW 43) increases the mass by 42 units per group. For five groups, the increase is $5 \times 42 = 210$. $180 + 210 = 390$ g/mol.

19. Mark the incorrect statement regarding fructose ring structures.

• (a) Five-membered ring structures are named as furanose structures.
• (b) The cyclic structures represent two anomers of fructose.
• (c) Five-membered ring structures are named as pyranose structures.
• (d) These are also called Haworth structures.

Correct Answer: (c)

Explanation: Five-membered rings are correctly called furanose rings. Pyranose refers to six-membered rings.

20. Vitamin $B_{6}$ is known as

• (a) pyridoxine
• (b) thiamine
• (c) tocopherol
• (d) riboflavin.

Correct Answer: (a)

Explanation: Pyridoxine is the chemical name for Vitamin $B_{6}$.

21. How many isomeric tripeptides may exist for a tripeptide composed equally of L-valine, L-tyrosine and L-alanine?

• (a) 3
• (b) 4
• (c) 6
• (d) 8

Correct Answer: (c)

Explanation: There are six possible sequences for these three different amino acids: Val-Tyr-Ala, Val-Ala-Tyr, Tyr-Val-Ala, Tyr-Ala-Val, Ala-Tyr-Val, and Ala-Val-Tyr.

22. Which one of the following statements about protein structure is correct?

• (a) Proteins consisting of one polypeptide can have quaternary structure.
• (b) Disulfide bond formation requires two participating cystine residues to be adjacent in primary sequence.
• (c) The stability of quaternary structure is mainly due to covalent bonds among subunits.
• (d) Information for correct folding is contained in the specific sequence of amino acids.

Correct Answer: (d)

Explanation: The specific sequence of amino acids in the polypeptide chain contains the necessary information to guide the correct three-dimensional folding of the protein. Participating cysteine residues for disulfide bonds do not need to be adjacent in the primary sequence as long as folding brings them close.

23. Match List I: I. Anti beri-beri factor; II. Pancreas; III. Palm oil; IV. L(+)-Ascorbic acid. List II: A. Vitamin C; B. Glycerides; C. Vitamin $B_{1}$; D. Insulin.

• (a) I-C; II-D; III-B; IV-A
• (b) I-C; II-D; III-A; IV-B
• (c) I-A; II-B; III-D; IV-C
• (d) I-C; II-B; III-C; IV-D.

Correct Answer: (a)

Explanation: Vitamin $B_{1}$ is the anti beri-beri factor (I-C). Insulin is produced in the pancreas (II-D). Palm oil contains glycerides (III-B). Vitamin C is L(+)-ascorbic acid (IV-A).

24. Which one of the following statements about the structure of double-helical DNA is incorrect?

• (a) There are 10 base pairs per turn of the helix.
• (b) Hydrogen bonds between complementary bases stabilize the helix.
• (c) Separation of strands requires untwisting.
• (d) The molar amount of A+T equals the molar amount of G+C.

Correct Answer: (d)

Explanation: While A equals T and G equals C, the total amount of A+G (purines) equals T+C (pyrimidines). The amount of A+T does not necessarily equal G+C.

25. A compound gives both ninhydrin test and Molisch's test. It is

• (a) a glycoprotein
• (b) a carboxylic acid
• (c) a carbohydrate
• (d) haemoglobin.

Correct Answer: (a)

Explanation: Glycoproteins contain both protein and carbohydrate components, thus they respond positively to both the ninhydrin test (for proteins) and Molisch's test (for carbohydrates).

26. A tripeptide (X) on partial hydrolysis gave Cys-Gly and Glu-Cys. Identify the tripeptide.

• (a) Glu-Cys-Gly
• (b) Gly-Glu-Cys
• (c) Cys-Gly-Glu
• (d) Cys-Glu-Gly

Correct Answer: (a)

Explanation: Based on the overlapping sequences of the dipeptides, cystine must be positioned between glutamic acid and glycine. The correct sequence is Glu-Cys-Gly.

27. The two functional groups present in a typical carbohydrate are

• (a) -OH and-COOH
• (b) -CHO and -COOH
• (c) $>C=O$ and -OH
• (d) -OH and-CHO

Correct Answer: (c)

Explanation: Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones. Thus, they contain a carbonyl group ($>C=O$, as an aldehyde or ketone) and multiple hydroxyl (-OH) groups.

28. Biuret test is not given by

• (a) proteins
• (b) carbohydrates
• (c) polypeptide
• (d) urea.

Correct Answer: (b)

Explanation: The Biuret test is used to detect the presence of the peptide linkage (-CONH-). Carbohydrates do not possess this linkage and will not give a positive result.

29. Which are true about enzymes? (i) Lack nucleophilic groups; (ii) Highly specific; (iii) Catalyse by lowering activation energy; (iv) Pepsin is proteolytic.

• (a) (i)
• (b) (i) and (iv)
• (c) (i) and (iii)
• (d) (ii), (iii) and (iv).

Correct Answer: (d)

Explanation: Enzymes are highly specific catalysts that lower the activation energy of biological reactions. Pepsin is indeed a proteolytic enzyme.

30. In an amino acid, carboxyl group $pK_{a_{1}}=2.34$ and ammonium ion $pK_{a_{2}}=9.60$. The isoelectric point is at pH:

• (a) 5.97
• (b) 2.34
• (c) 9.60
• (d) 6.97

Correct Answer: (a)

Explanation: The isoelectric point (pI) is calculated as the average of the $pK_a$ values. $(2.34 + 9.60) / 2 = 5.97$.

31. What form of glutamic acid would predominate in a strongly basic solution?

• (a) Zwitterionic form
• (b) Di-anionic form
• (c) Mono-anionic form
• (d) All forms.

Correct Answer: (b)

Explanation: In a strongly basic environment, both carboxyl groups of glutamic acid lose their protons, making the anionic form dominant.

32. Which of the following is the major species in a solution of lysine at $pH=3.5$?

• (a) Anionic form
• (b) Zwitterionic form
• (c) Neutral form
• (d) Cationic form.

Correct Answer: (d)

Explanation: At low pH values like 3.5, the basic groups of lysine are protonated, making the cationic form the major species.

33. Match Column I: A. Sucrose; B. Cellulose; C. Proteins; D. Oil. Column II: P. Lipase; Q. Invertase; R. Emulsion; S. Pepsin.

• (a) A-S, B-R, C-Q, D-P
• (b) A-R, B-S, C-P, D-Q
• (c) A-P, B-Q, C-R, D-S
• (d) A-Q, B-R, C-S, D-P

Correct Answer: (d)

Explanation: Invertase hydrolyzes sucrose (A-Q). Proteins are hydrolyzed by pepsin (C-S). Lipase acts on oils (D-P). Cellulose is associated with the intermediate product (B-R).

34. The change in optical rotation with time of freshly prepared solution of glucose is called

• (a) inversion
• (b) glycolysis
• (c) mutarotation
• (d) saponification.

Correct Answer: (c)

Explanation: Mutarotation is the gradual change in optical rotation observed in a fresh sugar solution as $\alpha$ and $\beta$ forms reach equilibrium.

35. Which one of the following is a peptide hormone?

• (a) Testosterone
• (b) Thyroxin
• (c) Adrenaline
• (d) Insulin

Correct Answer: (d)

Explanation: Insulin is a well-known peptide hormone.

36. Starch can be used as an indicator for the detection of the traces of

• (a) glucose in aqueous solution
• (b) proteins in blood
• (c) iodine in aqueous solution
• (d) urea in blood.

Correct Answer: (c)

Explanation: Starch reacts with iodine to produce an intense blue color, making it an excellent indicator.

37. The number of atoms in the ring structure of pyranoses are

• (a) 5 Carbon + 1 Oxygen
• (b) 4 Carbon
• (c) 4 Carbon + 1 Oxygen
• (d) 3 Carbon

Correct Answer: (a)

Explanation: A pyranose ring consists of 5 carbon atoms and 1 oxygen atom.

38. [Uses of glucose]

• (a) manufacture of vitamin C
• (b) as preservative
• (c) in the manufacture of alcohol
• (d) all of the above.

Correct Answer: (d)

Explanation: Glucose is used in the synthesis of Vitamin C, as a preservative, and in alcohol production.

39. Keratin, a structural protein is present in

• (a) hair
• (b) wool
• (c) silk
• (d) all of these.

Correct Answer: (d)

Explanation: Keratin is a fibrous structural protein found in hair, wool, silk, and muscles.

40. Which of the following amino acids contain sulphur and is an essential amino acid?

• (a) Isoleucine
• (b) Methionine
• (c) Proline
• (d) Glutamine

Correct Answer: (b)

Explanation: Methionine is an essential amino acid that contains sulfur in its side chain.

41. Match List I: P. Peptide linkage; Q. Nucleic acid; R. Hydrolysis of cane sugar; S. Starch. List II: 1. Inversion; 2. Polysaccharide; 3. Proteins; 4. Nucleotides.

• (a) P-2, Q-1, R-3, S-4
• (b) P-4, Q-1, R-2, S-3
• (c) P-3, Q-4, R-1, S-2
• (d) P-1, Q-3, R-4, S-2

Correct Answer: (c)

Explanation: Peptide linkages are found in proteins (P-3). Nucleic acids are polymers of nucleotides (Q-4). Hydrolysis of cane sugar results in inversion (R-1). Starch is a polysaccharide (S-2).

42. Which statement is incorrect regarding digestion of proteins?

• (a) Proteins are hydrolysed to peptones in mouth by amylase.
• (b) Proteins are hydrolysed to peptones in stomach by pepsin.
• (c) Peptones are broken to peptides in intestine by trypsin.
• (d) Peptides are broken to amino acids in intestine by peptidase.

Correct Answer: (a)

Explanation: Amylase in the mouth hydrolyzes polysaccharides (carbohydrates), not proteins. Protein digestion begins in the stomach with pepsin.

43. D-(+)-Glucose $\xrightarrow{Ac_{2}O}$ D-(+)-Glucose pentaacetate. Which statement is true?

• (a) Reacts with phenylhydrazine but not Tollens' reagent.
• (b) Reacts with Tollens' reagent but not phenylhydrazine.
• (c) Reacts with both.
• (d) Reacts with neither.

Correct Answer: (d)

Explanation: In the pentaacetate form, the hemiacetal hydroxyl group is acetylated, preventing the formation of the open-chain aldehyde. Thus, it fails to respond to both Tollens' reagent and phenylhydrazine.

44. Which one of the following does not exhibit the phenomenon of mutarotation?

• (a) (+) Sucrose
• (b) (+) Lactose
• (c) (+) Maltose
• (d) (-) Fructose

Correct Answer: (a)

Explanation: Sucrose lacks a free aldehyde or ketone group because its monosaccharide units are linked through their reducing centers. Consequently, it cannot show mutarotation.

45. Fructose reduces Tollen's reagent due to

• (a) asymmetric carbons
• (b) primary alcoholic group
• (c) secondary alcoholic group
• (d) enolisation followed by conversion to aldehyde by base.

Correct Answer: (d)

Explanation: In the basic medium of Tollens' reagent, fructose undergoes enolization and rearrangement to form glucose and mannose, both of which contain the oxidizable aldehyde group.