CBSE Class 12 Chemistry Biomolecules MCQs Set I

Question: Which base is present in RNA but not in DNA?
a) Thymine
b) Uracil
c) Guanine
d) Cytosine
Answer: b

Question: Antibodies are –
a) Lipids
b) Enzymes
c) Proteins
d) Carbohydrates
Answer: c

Question: ADP and ATP differ in the number of
a) Ribose units
b) Nitrogen atoms
c) Phosphate units
d) Adenine base
Answer: c

Question: Which of the following properties of glucose can be explained only by its cyclic structure
a) Glucose is oxidized by nitric acid to gluconic acid.
b) Glucose reacts with hydroxylamine to form an oxime.
c) Glucose forms pentaacetate
d) Pentaacetate of glucose does not react with hydroxylamine.
Answer: d

Question: In both DNA and RNA, heterocyclic base and phosphodiester linkages are at –
a) C5′ and C2′ respectively of the sugar molecule
b) C1′ and C5′ respectively of the sugar molecule
c) C5′ and C1′ respectively of the sugar molecule
d) C2′ and C5′ respectively of the sugar molecule
Answer: b

Question: Which statement is incorrect about peptide bond?
a) C–N bond length in proteins is smaller than usual C–N bond length
b) Spectroscopic analysis shows planar structure of –CO–NH– group
c) N bond length in proteins is longer than usual C–N bond length
d) None of the above
Answer: c

Question: Protein is a polymer made of
a) Nucleic acids
b) Amino acids
c) Carbohydrates
d) Carboxylic acids
Answer: b

Question: Which of the following statements is not true about glucose?
a) It does not give 2,4-DNP test
b) It is present in furanose form
c) It is an aldohexose
d) On heating with HI it forms n-hexane
Answer: b

Question: Nucleic acids are the polymers of
a) Bases
b) Sugars
c) Nucleosides
d) Nucleotides
Answer: d

Assertion-Reason Question

DIRECTION: Mark the option which is most suitable:
(a) Assertion and Reason both are correct statements and Reason is correct explanation for Assertion.
(b) Assertion and Reason both are correct statements but Reason is not correct explanation for Assertion.
(c) Assertion is correct statement but reason is false statement.
(d) Assertion is false statement but reason is correct statement.

Question: Assertion: Glycine exists as Zwitter ion but oand p-aminobenzoic acids do not.
Reason: Due to the presence of –NH₂ and –COOH group within the same molecule, they neutralise each other and hence, a-amino acids exist as dipolar ions or zwitter ions.
Answer: b

Question: Assertion: Fructose reduces Fehling’s solution and Tollens’ reagent.
Reason: Fructose does not contain any aldehyde group.
Answer: b

Question: Assertion: a-Amino acids exits as dipolar ions or zwitter ions.
Reason: a-Amino acids are the building blocks of proteins. 
Answer: b

Question: Assertion: Glucose when treated with CH3OH in presence of dry HCl gas gives a-and b-methyl glucosides.
Reason: Glucose reacts with phenyl hydrazine to form crystalline osazone.
Answer: b

Question: Assertion: Uracil occurs in DNA.
Reason: DNA undergoes replication. 
Answer: d

Question: Assertion: DNA undergoes replication.
Reason: DNA contains cytosine and thymine as pyrimidine bases. 
Answer: b

Question: Assertion: Glucose and fructose are reducing sugars.
Reason: Glucose and fructose contain a free aldehydic and ketonic group adjacent to a > CHOH group respectively.
Answer: a

Question: Assertion: b-pleated sheet structure of protein shows maximum extension.
Reason: Intermolecular hydrogen bonding is present in them.
Answer: b

Question: Assertion: Alpha (a) amino acids exist as internal salt in solution as they have amino and carboxylic acid groups in near vicinity.
Reason: H+ ion given by carboxylic group (–COOH) is captured by amino group (–NH₂) having lone pair of electrons.
Answer: a

Question: Assertion: Haemoglobin is a globular protein.
Reason: Globular proteins are insoluble in water.
Answer: c

A statement of assertion is followed by a statement of reason. Mark the correct choice from the options given below:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.

Question: Assertion : Purine bases present in DNA are adenine and guanine.
Reason : The base thymine is present in RNA while base uracil is present in DNA.
Answer: c

Question: Assertion : D – glucose is dextrorotatory whereas L – glucose is levorotatory.
Reason : D – compounds are always dextro and L – compounds are always laevo.
Answer: b

Question: Assertion : Keratin is a globular protein.
Reason : Globular shape is due to secondary structure of proteins.
Answer: c

Question: Assertion : The two strands of DNA are complementary.
Reason : Cytosine always pairs with guanine and thymine pairs with adenine.
Answer: a

Question: Assertion : α – Amino acids are the building blocks of proteins.
Reason : Natural amino acids are mostly α – amino acids.
Answer: b

Case Based Questions

Carbohydrates are polyhydroxy aldehydes and ketones and those compounds which on hydrolysis give such compounds are also carbohydrates. The carbohydrates which are not hydrolysed are called monosaccharides. Monosaccharides with aldehydic group are called aldose and those which free ketonic groups are called ketose. Carbohydrates are optically active. Number of optical isomers = 2ⁿ
…[where n = number of asymmetric carbons Carbohydrates are mainly synthesised by plants during photosynthesis. The monosaccharides give the characteristic reactions of alcohols and carbonyl group (aldenydes and ketones). It has been found that these monosaccharides exist in the form of cyclic structures. In cyclization the –OH groups (generally C5 or C4 in aldohexoses and C5 or C6 in ketohexoses) combine with the aldehyde or keto group. As a result, cyclic structure of five or six membered rings containing one oxygen atom are formed, e.g., glucose forms a ring structure. Glucose contains one aldehyde group, one 1º alcholic group and four 2º alcoholic group in its open chain structure.

Answer the following MCQs by choosing the most appropriate options:

Question: First member of ketose sugar is
a) Ketotriose
b) Ketotetrose
c) Ketopentose
d) Ketohexose
Answer: a

Question: Some statements are given below:
I. Glucose is aldohexose.
II. Naturally occurring glucose is dextrorotatory.
III. Glucose contains three chiral centres.
IV. Glucose contains one 1º alcoholic group and four 2º alcoholic groups.
Among the above, correct statements are
a) I and II only
b) III and IV only
c) I, II and IV only
d) I, II, III and IV
Answer: c

Question: Which of the following reactions of glucose can be explained only by its cyclic structure?
a) Glucose forms cyanohydrin with HCN.
b) Glucose reacts with hydroxylamine to form an oxime.
c) Pentacetate of glucose does not react with hydroxylamine.
d) Glucose is oxidised by nitric acid to gluconic acid.
Answer: c

Question: Two hexoses form the same osazone, find the correct statement about these hexoses.
a) Both of them must be aldoses.
b) They are epimers at C-3
c) The carbon atoms 1 and 2 in both have the same configuration.
d) The carbon atoms 3, 4 and 5 in both have the same configuration.
Answer: d

When a protein in its native form, is subjected to physical changes like change in temperature or chemical changes like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. The denaturation causes changed in secondary and tertiary structures but primary structures remains intact. Examples of denaturation of protein are coagulation of egg white on boiling, curdling of milk, formation of cheese when an acid is added to milk.

Answer the following MCQs by choosing the most appropriate options:

Question: Which structure(s) of proteins remain(s) intact during denaturation process?
a) Both secondary and tertiary structure
b) Primary structure only
c) Secondary structure only
d) Tertiary structure only
Answer: b

Question: Mark the wrong statement about denaturation of proteins.
a) The primary structure of the protein does not change.
b) Globular proteins are converted into fibrous proteins.
c) Fibrous proteins are converted into globular proteins.
d) The biological activity of the protein is destroyed.
Answer: c

Question: Secondary structure of protein refers to
a) Mainly denatured proteins and structure of prosthetic groups.
b) Three-dimensional structure, especially the bond between amino acid residues that are distant from each other in the polypeptide chain.
c) Linear sequence of amino acid residues in the polypeptide chain.
d) Regular folding patterns of continuous portions of the polypeptide chain.
Answer: d

Question: α-helix and β-pleated structures of proteins are classified as
a) Primary structure
b) Secondary structure
c) Tertiary structure
d) Quaternary structure
Answer: b

Question: Cheese is a
a) Globular protein
b) Conjugated protein
c) Denatured protein
d) Derived protein
Answer: c

Very Short Answer Type Questions

Question: What are the expected products of hydrolysis of lactose ? 
Answer. On hydrolysis, lactose gives β-D-galactose and β-D-glucose.

Question: Where does the water present in the egg go after boiling the egg? 
Answer. Denaturation of proteins is a process that changes the physical and biological properties of proteins without affecting the chemical composition of protein. In an egg, denaturation of protein is the coagulation of albumin present in the white of an egg. When an egg is boiled in water, the globular proteins present in it change to a rubber like insoluble mass which absorbs all the water present in the egg by making hydrogen bond with it.

Question: Name one water soluble vitamin which is a powerful antioxidant. Give its one natural source.
Answer. Water soluble vitamin : Vitamin C Natural source : Amla

Question: What are three types of RNA molecules which perform different functions? 
Answer. m–RNA, t-RNA, r-RNA

Question: What is a glycosidic linkage?
Answer. The two monosaccharide units are joined together through an etheral or oxide linkage formed by loss of a molecule of water. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage.

Short Answer Type Questions-I

Question:Which nucleic acid is responsible for carrying out protein synthesis in the cell?
Answer. RNA

Question: How would you explain the amphoteric behavior of amino acids?
Answer. Amino acids are amphoteric due to the presence of both acidic and basic functional groups.

Question: How many asymmetric carbon atoms are present in D (+) glucose?
Answer. 4

Question: Write the Zwitter ion form of amino acetic acid. (H₂NCH₂COOH).
Answer. NH₃⁺CH₂COO^–

Question: Give the significance of prefix ‘D’ in the name D- (+)-glucose.
Answer. ‘D’ Signifies that –OH group on C-5 is on the right-hand side

Question: Mention the number of hydrogen bonds between adenine and thymine.
Answer. Two

Question: Give the significance of (+)-sign in the name D- (+)-glucose.
Answer. (+) sign indicates dextrorotatory nature of glucose.

Question:The two strands in DNA are not identical but complementary. Explain.
Answer. complementary bases are prepared.

Question: Name the linkage present in proteins.
Answer. Peptide linkage

Question: What are essential and non-essential amino acids in human food? Give one example of each type.
Answer. Essential amino acids : Amino acids which the body cannot synthesize are called essential amino acids.
Example : Valine, leucine etc. Therefore they must be supplied in diet.
Non-essential amino acids : Amino acids which the body can synthesize are called non-essential amino acids.Therefore, they may or may not be present in diet.
Example : Glycine, alanine etc.

Question: (i) Write the structural difference between starch and cellulose.
(ii) What type of linkage is present in Nucleic acids?
(iii) Give one example each for fibrous protein and globular protein.
Answer. (i) Starch contains the α-D-glucose as its monomer units while cellulose contains β-Dglucose as its monomer units.
(ii) Phosphodiester linkages are present in Nucleic Acids
(iii) Globular protein : All enzymes and hormones like insulin.
Fibrous protein : Keratin in skin.

Question: What is difference between reducing and non‐reducing sugars or carbohydrates?
Answer. All those carbohydrates which contain aldehydic and ketonic group in the hemiacetal or hemiketal form and reduce Tollen’s reagent or Fehling’s solution are called reducing carbohydrates while others which do not reduce these reagents are called non‐reducing sugars.

Question: What are essential and non-essential amino acids? Give two examples of each. 
Answer. Essential amino acids : Amino acids which the body cannot synthesize are called essential amino acids.
Example : Valine, leucine etc. Therefore they must be supplied in diet. Non-essential amino acids : Amino acids which the body can synthesize are called non-essential amino acids. Therefore, they may or may not be present in diet.
Example : Glycine, alanine etc.