Carbohydrates

Classes of carbohydrates

• Monosaccharides, the simplest sugars, made of a single sugar unit, such as glucose, fructose and galactose; they cannot be broken down further into simpler sugars.
• Disaccharides, formed when two monosaccharides join, such as maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose).
• Polysaccharides, large molecules made of many monosaccharide units joined together, such as starch and glycogen (energy storage) and cellulose (structural).

How carbohydrates form and break apart

A disaccharide or polysaccharide is built by condensation reaction, in which two sugar molecules join together with the removal of a water molecule, forming a glycosidic bond. The reverse process, hydrolysis, breaks a glycosidic bond by adding a water molecule, splitting a larger carbohydrate back into smaller sugar units, this is essentially what happens during digestion.

Functions of carbohydrates

Testing for carbohydrates

Benedict's test detects reducing sugars, such as glucose: on heating a sample with Benedict's solution, a brick-red precipitate forms if a reducing sugar is present, while the solution stays blue if it is absent. The iodine test detects starch: adding iodine solution to a sample turns it blue-black if starch is present, and it remains orange-brown if starch is absent.

How it is examined

Exam questions often ask you to classify a named carbohydrate as a monosaccharide, disaccharide or polysaccharide, to describe the condensation or hydrolysis reaction linking sugar units, or to state the expected colour change of Benedict's test or the iodine test and explain what it shows. You may also be asked to relate a carbohydrate's structure to its role as an energy source or structural material.

Reducing and non-reducing sugars

Benedict's test detects reducing sugars, which include all the monosaccharides (glucose, fructose, galactose) and some disaccharides (maltose and lactose). These give a brick-red precipitate directly when heated with Benedict's solution.

Sucrose is a non-reducing sugar, so it gives no colour change with Benedict's solution on its own. To detect it, the sample is first boiled with dilute hydrochloric acid to hydrolyse the sucrose into glucose and fructose, then neutralised with an alkali, and finally re-tested with Benedict's solution, which now gives a brick-red precipitate.

Worked exam-style question

Question. A student tests three food samples. Sample 1 gives a brick-red precipitate with Benedict's solution on heating.

Sample 2 turns blue-black with iodine solution. Sample 3 gives no change with Benedict's solution until it is boiled with dilute hydrochloric acid, neutralised, and re-tested, when it gives a brick-red precipitate.

(a) What type of carbohydrate does Sample 1 contain? (b) What does Sample 2 contain?

(c) Explain the result for Sample 3. (d) Name the reaction that broke down the substance in Sample 3 when it was boiled with acid.

Model answer. (a) A reducing sugar, because the brick-red precipitate with Benedict's solution is a positive result. (b) Starch, because the blue-black colour is a positive iodine test.

(c) Sample 3 contains a non-reducing sugar such as sucrose; it does not react with Benedict's solution until it is hydrolysed by boiling with acid into reducing sugars (glucose and fructose), which then give the brick-red precipitate. (d) Hydrolysis.

Practice question

Try this. (a) Name the two monosaccharides produced when sucrose is hydrolysed. (b) State the type of bond broken during this hydrolysis.

(c) A polysaccharide is made of many glucose units joined together. Name two polysaccharides that store energy and one that provides structural support.

Exam tip

Key terms

• Carbohydrate, an energy or structural molecule built from sugar units.
• Protein, a molecule built from amino acids joined by peptide bonds.
• Lipid, a fat or oil built from glycerol and fatty acids.
• Enzyme, a biological catalyst that speeds up reactions such as digestion.
• Substrate, the substance an enzyme acts on, such as a carbohydrate.

Source:SRC-DSKP-EN

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