Carbohydrate

In the syllabus and the exam

Carbohydrates open the syllabus's chapter on nutrition and biomolecules, and candidates are expected to know their elemental composition (carbon, hydrogen and oxygen), their general role as the body's primary and most readily available energy source, and food tests used to detect them, particularly Benedict's test for reducing sugars and the iodine test for starch, both of which appear regularly in Paper 3 and in structured food-test questions in Paper 2. A well-prepared candidate can also classify carbohydrates into monosaccharides (such as glucose), disaccharides (such as sucrose) and polysaccharides (such as starch, glycogen and cellulose), and explain that larger carbohydrate molecules are built from repeating smaller sugar units joined together, a structural idea the syllabus revisits when comparing carbohydrates with proteins and lipids. A common error is confusing the colour changes of the two main food tests, particularly forgetting that a positive Benedict's test requires heating and produces a brick-red precipitate, while a positive iodine test needs no heating and produces a blue-black colour; mixing up these observations is a frequent source of lost marks in practical-based questions. Some candidates also incorrectly state that carbohydrates are used for growth and repair, a role that belongs primarily to protein, rather than for energy. Structured questions sometimes give a table of food-test results for an unknown sample and expect candidates to work backward, deducing that a sample is rich in carbohydrate purely from a positive iodine or Benedict's result combined with a negative Biuret and ethanol-emulsion result. Candidates should also be able to state that fibre, such as cellulose from plant cell walls, is a type of carbohydrate the human digestive system cannot break down, yet it still plays an important role in digestive health.

Example

Rice, bread and potatoes are all rich in carbohydrate, mainly in the form of starch, which the body breaks down into glucose to release energy.

Related terms

ProteinLipid

Source:SRC-DSKP-EN