Biological Molecules: Question 1
Syllabus 2.2
Two molecules of α-glucose react together to form a molecule of the disaccharide maltose.
Which statement correctly describes this reaction and the bond formed between the two glucose units?
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Worked solution
Condensation reactions in carbohydrate synthesis
When two monosaccharides join together, a condensation reaction takes place: a hydroxyl (-OH) group on one glucose molecule reacts with a hydroxyl group on the other, and a molecule of water is released as the new bond forms. This is the general pattern used to build up larger biological molecules (carbohydrates, lipids and proteins) from smaller subunits.
The bond formed between two glucose units
The covalent bond that links two monosaccharides together is called a glycosidic bond. When two molecules of α-glucose react in this way, the product is the disaccharide maltose:
α-glucose + α-glucose → maltose + water
This is the reverse of hydrolysis, in which a molecule of water is used to break a glycosidic bond apart, splitting a disaccharide (or polysaccharide) back into its monosaccharide units.
Why the other options are wrong
- B wrongly labels bond formation as hydrolysis; hydrolysis breaks bonds by using water, it does not form them.
- C correctly identifies the reaction as condensation but wrongly names the bond a “peptide bond”, peptide bonds link amino acids together in proteins, not monosaccharides.
- D wrongly labels the reaction as hydrolysis and wrongly names the bond an “ester bond”, ester bonds link fatty acids to glycerol in lipids, not monosaccharides.
Final answer
A, a condensation reaction releases one molecule of water and forms a glycosidic bond between the two α-glucose units.