Biomolecules Overview Questions and Answers

Q: A solution is made by mixing 50 ml of 2 M K2HPO4 and 25 ml of 2.0 M KH2PO4. The solution is diluted to a final volume of 250 ml. What is the pH of the final solution ? (PK = 6.82)

The pH is 7.12.K2HPO4 is the conjugate base and KH2PO4 the acid. Moles of each:\[ \begin{aligned} \text{base} &= 0.050 \times 2 = 0.10\ \text{mol} \\ \text{acid} &= 0.025 \times 2.0 = 0.05\ \text{mol} \end{aligned} \]By the Henderson-Hasselbalch equation (dilution to 250 ml changes both equally, so the ratio is unchanged):\[ \begin{aligned} \text{pH} &= 6.82 + \log \frac{0.10}{0.05} \\ &= 6.82 + 0.30 = 7.12 \end{aligned} \]6.82 would need equal amounts of acid and base, and 6.52 comes from inverting the ratio.

Q: In which of the following the hydrogen bond is NOT formed ?

No hydrogen bond forms between methyl group of alanine and water.A hydrogen bond needs a hydrogen attached to a strongly electronegative atom (nitrogen, oxygen or fluorine) and a lone pair on another electronegative atom. The C-H bonds of a methyl group are almost non-polar, so the methyl side chain of alanine cannot donate a hydrogen bond, and it has no lone pair to accept one. It is hydrophobic, which is why such side chains cluster inside folded proteins.The other pairs do hydrogen bond: the N-H of an amino group with water, the backbone C=O and N-H groups of proteins (the bonds that hold alpha helices and beta sheets), and the O-H of an alcohol with water.

Q: Hydrogen bonds are present in water structure. Presence of hydrogen bonds gives a characteristic property to water, which is :

Hydrogen bonding gives water high melting and high boiling points.Each water molecule can form up to four hydrogen bonds with its neighbours. Extra energy is needed to break this network, so water melts at 0°C and boils at 100°C, far higher than the hydrides of its neighbours in the periodic table: H2S, with a heavier molecule but weak hydrogen bonding, boils at about -60°C.Hydrogen bonding raises both transition temperatures together, because both melting and boiling require hydrogen bonds to be broken. So the options pairing a high value with a low one, or giving low values for both, run against the effect.

Q: Which among the following is functionally more diverse ?

The most functionally diverse molecules are proteins.Proteins are built from 20 different amino acids, which allows an enormous range of three-dimensional shapes and chemical surfaces. They act as enzymes, structural elements, transporters, receptors, hormones, antibodies, motors and regulators of gene expression.DNA is almost entirely a store of information. RNA is more versatile than DNA, serving as messenger, adaptor, structural RNA and ribozyme, but with only four bases its range is much narrower than that of proteins. Lipids serve mainly as membranes, energy stores and some signals.

Q: What is the resultant pH of a phosphate buffer made by mixing 0.2 M NaH2PO4 and 0.2 M Na2HPO4 ? (pKa = 6.86)

The pH is 6.86.By the Henderson-Hasselbalch equation, with the weak acid \(\text{H}_{2}\text{PO}_{4}^{-}\) and its conjugate base \(\text{HPO}_{4}^{2-}\) at equal concentrations:\[ \begin{aligned} pH &= pK_a + \log \frac{[\text{HPO}_4^{2-}]}{[\text{H}_2\text{PO}_4^{-}]} \\ &= 6.86 + \log \frac{0.2}{0.2} \\ &= 6.86 + 0 = 6.86 \end{aligned} \]A buffer has a pH equal to its pKa when acid and base are equimolar. 5.86 and 7.86 would need a tenfold excess of acid or base, and 7.00 assumes neutrality.

Q: Which of the following two statements are not true with respect to the properties of water molecules ?(i) Density of water is maximum at 4°C(ii) Water molecules dissociate very strongly(iii) Water has high heat of vaporization(iv) Water is a non-polar molecule

The two untrue statements are (ii) and (iv).Water dissociates only very weakly: at 25 °C the ion product is about \(10^{-14}\), so only about one molecule in 550 million is ionised. And water is strongly polar, because oxygen is more electronegative than hydrogen and the molecule is bent, which is what lets it form hydrogen bonds and dissolve ions.Statements (i) and (iii) are true. Liquid water is densest at about 4 °C, which is why ice floats and lakes freeze from the top. Its heat of vaporisation is high because hydrogen bonds must be broken, which makes sweating an effective coolant. Options including (i) or (iii) are wrong.

Q: A drug inhibits dehydration synthesis in cells. Which process would be directly affected ?

The process directly affected is the formation of polymers from monomers.Dehydration synthesis (condensation) joins two monomers by removing a molecule of water, forming a peptide, glycosidic or ester bond. It is how cells build proteins, polysaccharides, nucleic acids and fats, so blocking it stops polymer assembly.Breakdown of polymers is the opposite reaction, hydrolysis, which adds water. ATP hydrolysis is also a water-adding reaction. Diffusion across a membrane is a physical process involving no bond formation at all.

11.

A solution is made by mixing 50 ml of 2 M K₂HPO₄ and 25 ml of 2.0 M KH₂PO₄. The solution is diluted to a final volume of 250 ml. What is the pH of the final solution ? (PK = 6.82)

A.6.82
B.7.12
C.6.52
D.7.51

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The pH is 7.12.

K₂HPO₄ is the conjugate base and KH₂PO₄ the acid. Moles of each:

\[ \begin{aligned} \text{base} &= 0.050 \times 2 = 0.10\ \text{mol} \\ \text{acid} &= 0.025 \times 2.0 = 0.05\ \text{mol} \end{aligned} \]

By the Henderson-Hasselbalch equation (dilution to 250 ml changes both equally, so the ratio is unchanged):

\[ \begin{aligned} \text{pH} &= 6.82 + \log \frac{0.10}{0.05} \\ &= 6.82 + 0.30 = 7.12 \end{aligned} \]

6.82 would need equal amounts of acid and base, and 6.52 comes from inverting the ratio.

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12.

In which of the following the hydrogen bond is NOT formed ?

A.between water and amino group of an amino acid
B.between carbonyl group of protein and amino group of protein
C.between hydroxy group of alcohol and water
D.between methyl group of alanine and water

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

No hydrogen bond forms between methyl group of alanine and water.

A hydrogen bond needs a hydrogen attached to a strongly electronegative atom (nitrogen, oxygen or fluorine) and a lone pair on another electronegative atom. The C-H bonds of a methyl group are almost non-polar, so the methyl side chain of alanine cannot donate a hydrogen bond, and it has no lone pair to accept one. It is hydrophobic, which is why such side chains cluster inside folded proteins.

The other pairs do hydrogen bond: the N-H of an amino group with water, the backbone C=O and N-H groups of proteins (the bonds that hold alpha helices and beta sheets), and the O-H of an alcohol with water.

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13.

Hydrogen bonds are present in water structure. Presence of hydrogen bonds gives a characteristic property to water, which is :

A.High melting and high boiling points
B.High melting and low boiling points
C.Low melting and high boiling points
D.Low melting and low boiling points

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

Hydrogen bonding gives water high melting and high boiling points.

Each water molecule can form up to four hydrogen bonds with its neighbours. Extra energy is needed to break this network, so water melts at 0°C and boils at 100°C, far higher than the hydrides of its neighbours in the periodic table: H₂S, with a heavier molecule but weak hydrogen bonding, boils at about -60°C.

Hydrogen bonding raises both transition temperatures together, because both melting and boiling require hydrogen bonds to be broken. So the options pairing a high value with a low one, or giving low values for both, run against the effect.

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14.

Which among the following is functionally more diverse ?

A.DNA
B.RNA
C.Protein
D.Lipid

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

The most functionally diverse molecules are proteins.

Proteins are built from 20 different amino acids, which allows an enormous range of three-dimensional shapes and chemical surfaces. They act as enzymes, structural elements, transporters, receptors, hormones, antibodies, motors and regulators of gene expression.

DNA is almost entirely a store of information. RNA is more versatile than DNA, serving as messenger, adaptor, structural RNA and ribozyme, but with only four bases its range is much narrower than that of proteins. Lipids serve mainly as membranes, energy stores and some signals.

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15.

What is the resultant pH of a phosphate buffer made by mixing 0.2 M NaH₂PO₄ and 0.2 M Na₂HPO₄ ? (pKa = 6.86)

A.5.86
B.6.86
C.7.86
D.7.00

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: B

The pH is 6.86.

By the Henderson-Hasselbalch equation, with the weak acid \(\text{H}_{2}\text{PO}_{4}^{-}\) and its conjugate base \(\text{HPO}_{4}^{2-}\) at equal concentrations:

\[ \begin{aligned} pH &= pK_a + \log \frac{[\text{HPO}_4^{2-}]}{[\text{H}_2\text{PO}_4^{-}]} \\ &= 6.86 + \log \frac{0.2}{0.2} \\ &= 6.86 + 0 = 6.86 \end{aligned} \]

A buffer has a pH equal to its pKa when acid and base are equimolar. 5.86 and 7.86 would need a tenfold excess of acid or base, and 7.00 assumes neutrality.

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16.

Which of the following two statements are not true with respect to the properties of water molecules ?

(i) Density of water is maximum at 4°C
(ii) Water molecules dissociate very strongly
(iii) Water has high heat of vaporization
(iv) Water is a non-polar molecule

A.(i) and (ii)
B.(ii) and (iii)
C.(i) and (iv)
D.(ii) and (iv)

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

The two untrue statements are (ii) and (iv).

Water dissociates only very weakly: at 25 °C the ion product is about \(10^{-14}\), so only about one molecule in 550 million is ionised. And water is strongly polar, because oxygen is more electronegative than hydrogen and the molecule is bent, which is what lets it form hydrogen bonds and dissolve ions.

Statements (i) and (iii) are true. Liquid water is densest at about 4 °C, which is why ice floats and lakes freeze from the top. Its heat of vaporisation is high because hydrogen bonds must be broken, which makes sweating an effective coolant. Options including (i) or (iii) are wrong.

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17.

A drug inhibits dehydration synthesis in cells. Which process would be directly affected ?

A.Breakdown of polymers
B.ATP hydrolysis
C.Diffusion across membrane
D.Formation of polymers from monomers

Exam Year(s)

Tripura Public Service Commission - 2026,

Answer: D

The process directly affected is the formation of polymers from monomers.

Dehydration synthesis (condensation) joins two monomers by removing a molecule of water, forming a peptide, glycosidic or ester bond. It is how cells build proteins, polysaccharides, nucleic acids and fats, so blocking it stops polymer assembly.

Breakdown of polymers is the opposite reaction, hydrolysis, which adds water. ATP hydrolysis is also a water-adding reaction. Diffusion across a membrane is a physical process involving no bond formation at all.

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