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BI1.1 | Basic Biochemistry — Gate Quiz
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A patient arterial blood gas shows a pH of 7.1. What is the approximate hydrogen ion concentration?
Correct! At pH 7.1, [H+] is approximately 80 nmol/L. pH = -log[H+], so [H+] = 10^-7.1 approximately 79.4 nmol/L.
Normal arterial pH is 7.35-7.45, corresponding to [H+] of 35-45 nmol/L. pH 7.1 indicates severe acidosis. Each 0.3 unit drop in pH doubles [H+].
Incorrect. Use [H+] = 10^-pH. At pH 7.1, [H+] = 10^-7.1 approximately 80 nmol/L.
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The Henderson-Hasselbalch equation describes the relationship between pH and the ratio of which two components of a buffer system?
Correct! pH = pKa + log([A-]/[HA]) — the ratio of conjugate base to conjugate acid.
For the bicarbonate buffer: pH = 6.1 + log([HCO3-]/[H2CO3]). Normal ratio is 20:1, maintaining pH 7.4.
Incorrect. Henderson-Hasselbalch uses pH = pKa + log([conjugate base]/[conjugate acid]).
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A buffer is most effective at resisting pH changes when the pH is:
Correct! A buffer has maximum capacity when pH equals pKa, where concentrations of HA and A- are equal.
Buffer capacity is greatest within 1 pH unit of the pKa. Bicarbonate buffer (pKa 6.1) works in blood (pH 7.4) as an open system replenished by respiration and kidneys.
Incorrect. Buffer capacity is maximal when pH = pKa.
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Which buffer system provides the most important first line of defence against acid-base disturbances in blood?
Correct! The bicarbonate/carbonic acid system is the most important extracellular buffer, regulated by both lungs (CO2) and kidneys (HCO3-).
Though haemoglobin is quantitatively important inside RBCs, the bicarbonate system dominates extracellular fluid buffering. The open system compensates for its pKa being away from blood pH.
Incorrect. The bicarbonate buffer is the primary extracellular buffer, uniquely regulated by two organ systems.
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Which property of water is most responsible for its ability to dissolve ionic compounds like NaCl?
Correct! Water high dielectric constant (80) reduces electrostatic attraction between Na+ and Cl- ions, allowing them to separate and dissolve.
Dielectric constant measures a solvent ability to reduce electrostatic forces. Water value (80 vs. 2 for hexane) makes it excellent for ionic and polar compounds, crucial for biochemical reactions.
Incorrect. The high dielectric constant reduces the force between oppositely charged ions, enabling dissolution.
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A biochemist is preparing an acetate buffer (pKa = 4.76) for an enzyme assay at pH 4.76. What molar ratio of sodium acetate to acetic acid should be used?
Correct! At pH = pKa, log([A-]/[HA]) = 0, so [A-]/[HA] = 1:1.
When pH equals pKa, the ratio of conjugate base to acid is exactly 1. This is also where buffer capacity is maximum.
Incorrect. Henderson-Hasselbalch: when pH = pKa, log ratio = 0, so ratio = 1:1.
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The unusually high boiling point of water compared to other molecules of similar molecular weight is primarily due to:
Correct! Hydrogen bonds between water molecules require significant energy to break, giving water an exceptionally high boiling point (100°C vs. -61°C for H2S).
Each water molecule can form up to 4 hydrogen bonds (2 as donor, 2 as acceptor). These bonds collectively make water properties unique and essential for life.
Incorrect. Hydrogen bonds between water molecules are responsible for the high boiling point.
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Bile salts spontaneously form micelles in aqueous solution because they are:
Correct! Bile salts are amphipathic — steroid nucleus is hydrophobic while hydroxyl groups and charged side chain are hydrophilic. This drives micelle formation.
Amphipathic molecules self-assemble to minimize hydrophobic exposure to water. In micelles, the hydrophobic core sequesters lipids while the hydrophilic exterior interacts with water — essential for lipid digestion.
Incorrect. Micelle formation requires amphipathic molecules with both hydrophilic and hydrophobic domains.
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A 45-year-old diabetic man presents with pH 7.25, pCO2 28 mmHg, HCO3- 12 mEq/L. Which buffer system is compensating by lowering pCO2?
Correct! In metabolic acidosis, the respiratory system compensates by increasing ventilation to blow off CO2 (pCO2 28 mmHg is below normal 40 mmHg).
Diabetic ketoacidosis causes metabolic acidosis (low HCO3-). Respiratory compensation (Kussmaul breathing) rapidly lowers pCO2. The bicarbonate buffer operates as an open system regulated by lungs and kidneys.
Incorrect. The low pCO2 (28 mmHg) indicates respiratory compensation through hyperventilation.
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The ion product of water (Kw) at 25 degrees C is 10^-14. This means pure water has a pH of:
Correct! In pure water, [H+] = [OH-] = 10^-7 M, so pH = 7.
Water auto-ionises: H2O = H+ + OH-. Kw = 10^-14 at 25 degrees C. Pure water is neutral (pH 7) because [H+] = [OH-]. Kw increases with temperature, so neutral pH is slightly below 7 at body temperature.
Incorrect. In pure water [H+] = 10^-7 M, pH = 7.
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