Monosaccharides and disaccharidesAQA A-Level Biology: Revision notes
Section 1
Monosaccharides
Monosaccharides are the monomers from which larger carbohydrates are made. The common ones are glucose, galactose and fructose. All three have the molecular formula C₆H₁₂O₆, but their atoms are arranged differently, so they are isomers of each other.
Section 2
Alpha- and beta-glucose
Glucose has two isomers, α-glucose and β-glucose. They differ only in the position of the –OH group on carbon 1: it is below the ring in α-glucose and above the ring in β-glucose.
This small difference matters later, because α-glucose builds starch and glycogen while β-glucose builds cellulose.
Remember: alpha is 'down', beta is 'up', for the OH group on carbon 1.
Section 3
Forming disaccharides
Two monosaccharides join in a condensation reaction, which eliminates a molecule of water and forms a glycosidic bond. The product is a disaccharide.
- Maltose = glucose + glucose
- Sucrose = glucose + fructose
- Lactose = glucose + galactose
Hydrolysis (adding water) breaks the glycosidic bond and releases the monosaccharides.
Section 4
Worked example: hydrolysing maltose
Maltose (Mr 342) hydrolyses to two glucose molecules (Mr 180 each). Mass balance: 342 + 18 (water) = 360 = 2 × 180.
How much glucose from 3.42 g maltose? Moles maltose = 3.42 ÷ 342 = 0.0100 mol. Moles glucose = 2 × 0.0100 = 0.0200 mol. Mass glucose = 0.0200 × 180 = 3.60 g.
Section 5
Benedict's test for reducing sugars
All monosaccharides and the disaccharides maltose and lactose are reducing sugars.
- Add Benedict's reagent (blue) to the sample.
- Heat in a boiling water bath.
- A positive result passes through green, yellow and orange to a brick-red precipitate. Blue means no reducing sugar.
The reducing sugar reduces soluble copper(II) ions to insoluble copper(I) oxide. More reducing sugar gives a redder, denser precipitate, so the test can be semi-quantitative.
Section 6
Testing for non-reducing sugars
Sucrose is a non-reducing sugar, so it gives no change with Benedict's reagent (it stays blue).
To test for it:
- First do the Benedict's test and confirm it stays blue.
- Boil a fresh sample with dilute hydrochloric acid, which hydrolyses the glycosidic bond.
- Neutralise with sodium hydrogencarbonate (Benedict's reagent needs alkaline conditions).
- Heat with Benedict's reagent. A brick-red precipitate now shows that glucose and fructose were released.
Do not forget to neutralise after boiling with acid. Without it the Benedict's test can fail even though a sugar is present.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Monosaccharides and disaccharides
- Carbohydrates in the human diet include the monosaccharides glucose, fructose and galactose and the disaccharides maltose, sucrose and lactose.Name the bond that joins two monosaccharides in a disaccharide and name the small molecule that is released when it forms.2 marks
- A technician has a clear, colourless solution that is either sucrose or maltose. She heats a sample with Benedict's reagent in a boiling water bath and the mixture stays blue.The repeated test gives a brick-red precipitate. Explain this result.2 marks
- Glucose, galactose and fructose all have the molecular formula C₆H₁₂O₆. Glucose exists as two isomers, α-glucose and β-glucose. Maltose is a disaccharide made from two α-glucose molecules. Relative formula masses: maltose 342, glucose 180.Explain what is meant by isomers, and describe how α-glucose differs from β-glucose.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).