Question 1

Which of the following statements best explains why noncovalent interactions are crucial for the structure and function of biomolecules in living systems?

Accepted Answer

Noncovalent interactions are weak and reversible, allowing biomolecules to dynamically interact and respond to environmental changes.

Question 2

A salt bridge in a protein is an example of which type of noncovalent interaction, and what law governs the force between the charged groups?

Accepted Answer

Charge–charge interaction; governed by Coulomb’s Law: $$F = \frac{q_1$ q_2$}{4 \pi \$$varepsilon_0$$ r^2$}.$$

Question 3

Water has a high dielectric constant. What is the biological significance of this property?

Accepted Answer

It screens electrostatic interactions, reducing the force between charged particles and allowing ions and polar molecules to dissolve easily.

Question 4

Which of the following best describes the hydrophobic effect and its role in protein folding?

Accepted Answer

Hydrophobic effect is the tendency of nonpolar side chains to aggregate, minimizing the ordering of water molecules and stabilizing protein structure.

Question 5

A molecule with both hydrophilic and hydrophobic regions is called amphipathic. Which structure can amphipathic molecules form in water, and what is its biological relevance?

Accepted Answer

Bilayers, which are the main component of cellular membranes.

Question 6

Which of the following equations correctly relates the pH of a solution to the concentration of hydrogen ions?

Accepted Answer

$$\mathrm{pH} = -\log_{10}[\mathrm{H}^+]$$

Question 7

A buffer solution is prepared using acetic acid and its conjugate base. If the pKa of acetic acid is 4.76, what will be the pH of the solution when the concentrations of acetic acid and acetate are equal?

Accepted Answer

pH = 4.76

Question 8

Which equation is used to calculate the pH of a buffer solution containing a weak acid and its conjugate base?

Accepted Answer

$$\mathrm{pH} = \mathrm{p}K_a + \log_{10}\left(\frac{[\mathrm{A}^-]}{[\mathrm{HA}]}\right)$$

Question 9

Glycine is an ampholyte with two ionizable groups. If its pKa values are 2.34 (carboxyl) and 9.60 (amino), what is the isoelectric point (pI) of glycine?

Accepted Answer

$$\mathrm{pI} = \frac{2.34 + 9.60}{2} = 5.97$$

Question 10

A protein is most soluble at which pH relative to its isoelectric point (pI), and why?

Accepted Answer

Far from its pI, because the net charge is high and repulsion between molecules prevents aggregation.

Question 11

Which of the following best describes the principle behind isoelectric focusing in protein separation?

Accepted Answer

Proteins migrate in an electric field until they reach the pH where their net charge is zero (their pI).

Question 12

Which of the following statements about hydrogen bonds in biological molecules is correct?

Accepted Answer

Hydrogen bonds are interactions between a hydrogen atom covalently bonded to O or N and a lone pair on another O or N atom.

Question 13

Which of the following is a consequence of water’s ability to form four hydrogen bonds per molecule?

Accepted Answer

Water has a high heat capacity and forms a molecular lattice in its solid state.

Question 14

A solution contains 0.1 M acetic acid and 0.1 M acetate. If 0.01 mol of HCl is added to 1 L of this buffer, what will happen to the pH?

Accepted Answer

The pH will decrease slightly, as the buffer resists drastic changes within about 1 pH unit of the pKa.

Question 15

Which of the following best explains why proteins and nucleic acids are classified as macroions?

Accepted Answer

They are large molecules with multiple ionizable groups, resulting in significant net charge at certain pH values.

Question 16

Which type of gel electrophoresis is most appropriate for separating DNA fragments by size?

Accepted Answer

Agarose gel electrophoresis.

Question 17

Which of the following interactions is responsible for the formation of clathrate structures around hydrophobic molecules in water?

Accepted Answer

Ordering of water molecules around nonpolar surfaces, decreasing the entropy of the solvent.

Question 18

A weak acid has a pKa of 5.0. If the ratio of conjugate base to acid is 10:1, what is the pH of the solution? (Use the Henderson–Hasselbalch equation.)

Accepted Answer

$$\mathrm{pH} = 5.0 + \log_{10}(10) = 6.0$$

Chapter 2: The Chemical Foundation of Life – Weak Interactions in an Aqueous Environment

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