B
Two carboxylic acid residues and one phosphate group
C
One carboxylic acid residue and two phosphate groups
D
Three phosphate groups
The answer is B.
Solution
Glycerol has three –OH groups. In a triglyceride (fat), all three –OH groups are esterified with fatty acids (carboxylic acids). In a phospholipid, two –OH groups are esterified with fatty acids and the third with phosphoric acid (a phosphate group), which is often further linked to a nitrogenous base such as choline (e.g. lecithin). This gives the molecule a hydrophobic tail (two fatty acid chains) and a hydrophilic head (phosphate), which is why phospholipids form cell membranes. So phospholipids are esters of glycerol with two carboxylic acid residues and one phosphate group.
ConceptStructure of phospholipids
Biomolecules in past papers
67 questions from this chapter have appeared across 25 exam years.
