How does α-glucose differ from β-glucose?

The OH group on carbon 1 is below the ring in α-glucose and above the ring in β-glucose.

What bond joins two monosaccharides?

A glycosidic bond, formed in a condensation reaction.

How do you test for a reducing sugar?

Add Benedict's reagent and heat in a boiling water bath; a green to brick-red precipitate is positive.

How do you test for a non-reducing sugar?

Boil with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, then heat with Benedict's reagent; a brick-red precipitate is positive.

Why must the sample be neutralised before the second Benedict's test?

Benedict's reagent needs alkaline conditions, and the acid would stop the reaction.

Monosaccharides and disaccharidesAQA A-Level Biology: Flashcards

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Name three common monosaccharides.

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Name three common monosaccharides.: Glucose, galactose and fructose.
What is the molecular formula of glucose, galactose and fructose?: C₆H₁₂O₆.
What are the two isomers of glucose?: α-glucose and β-glucose.
How does α-glucose differ from β-glucose?: The OH group on carbon 1 is below the ring in α-glucose and above the ring in β-glucose.
What bond joins two monosaccharides?: A glycosidic bond, formed in a condensation reaction.
What is maltose made from?: Two glucose molecules.
What is sucrose made from?: Glucose and fructose.
What is lactose made from?: Glucose and galactose.
How do you test for a reducing sugar?: Add Benedict's reagent and heat in a boiling water bath; a green to brick-red precipitate is positive.
What colour is a negative Benedict's test?: It stays blue.
Which of maltose, sucrose and lactose is a non-reducing sugar?: Sucrose.
How do you test for a non-reducing sugar?: Boil with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, then heat with Benedict's reagent; a brick-red precipitate is positive.
Why must the sample be neutralised before the second Benedict's test?: Benedict's reagent needs alkaline conditions, and the acid would stop the reaction.

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

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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).