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2. Water, pH and Buffers — Biochemistry MCQs

KEMU · 24 MCQs with clinical pearls · Prof Jariya

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Sample MCQs with clinical pearls

The most important extracellular buffer in the body is:
A) Ammonia buffer [5th]
B) Bicarbonate buffer ✔
C) Hemoglobin buffer system
D) Phosphate buffer
E) Protein buffer
The BICARBONATE system is the most important EXTRACELLULAR buffer, despite a pKa of 6.1 that is far from blood pH - which should make it a poor buffer. Its power comes from being an OPEN system: carbon dioxide is continuously removed by the lungs and bicarbonate regenerated by the kidneys, so neither component is exhausted. MEMORY: 'A mediocre buffer made excellent by having both ends open.'
Which of the following is the principal buffer in interstitial fluid?
A) Bicarbonate buffer system [5th] ✔
B) H2PO4
C) Other proteins
D) Phosphate buffer
E) Carbonic acid
BICARBONATE is also the principal buffer of INTERSTITIAL fluid, which is essentially plasma without the proteins - so the protein buffering available in plasma is absent and bicarbonate dominates even more completely. Interstitial fluid is the largest extracellular compartment, making this quantitatively important. MEMORY: 'No protein in the interstitium, so bicarbonate does the work alone.'
If the H+ concentration of a solution is 10⁻³ mol/L, the pH is equal to:
A) -3
B) 1.3
C) 3 ✔
D) 3.1
E) 7.4
pH is the NEGATIVE LOGARITHM of hydrogen ion concentration, so 10 to the minus 3 mol/L gives a pH of 3. Each pH unit is a TENFOLD change in concentration - which is why a fall from 7.4 to 7.0 represents a near trebling of hydrogen ion concentration and is life-threatening. Normal arterial pH is 7.35 to 7.45, equivalent to about 40 nanomoles per litre. MEMORY: 'The pH is the exponent without its minus sign.'

24 MCQs are arranged to the learning outcomes of this kemu.

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