Chemistry of biomolecules sample question 1

glycerol and the sodium salts of three fatty acids

MCAT · Biological and Biochemical Foundations · General and Organic Chemistry

Chemistry of biomolecules

Chemistry of biomolecules is part of General and Organic Chemistry in the MCAT Biological and Biochemical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.

Question types you will see here include sugar stereochemistry and anomers, glycosidic bonds and reducing sugars, fatty acid and triglyceride properties and amphipathic molecules and micelles.

Common traps: confusing epimers with enantiomers; mixing alpha and beta by the OH orientation relative to the CH2OH group; reading mutarotation as a change in chemical identity.

Sample questions

Question 1
Heating a triacylglycerol with aqueous NaOH produces:
- A.
  glycerol and three fatty acids in their protonated form
- B.
  glycerol and the sodium salts of three fatty acids
- C.
  one glycerol and three molecules of acetyl-CoA
- D.
  a phospholipid and one sodium salt of a fatty acid
Answer: B. glycerol and the sodium salts of three fatty acids
This is saponification: hydroxide hydrolyzes the three ester bonds, releasing glycerol and three fatty acid carboxylates, which pair with Na⁺ as soap. In strongly basic solution the fatty acids exist as deprotonated carboxylate salts, not as protonated acids.
Question 2
The imidazole side chain of a histidine residue has a pKa of 6.0. At pH 7.0, what is the approximate ratio of the neutral (deprotonated) form to the positively charged (protonated) form?
- A.
  1:1
- B.
  100:1
- C.
  10:1
- D.
  1:10
Answer: C. 10:1
By the Henderson–Hasselbalch equation, log([A]/[HA]) = pH − pKa = 1, so the neutral form outnumbers the protonated form by 10 to 1. Getting 1:10 comes from reversing the equation; the side chain is mostly protonated only when the pH is below its pKa.

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