Topic 4 of IGCSE Biology (Cambridge 0610) is biological molecules. You need to know the structure and functions of carbohydrates, fats (lipids), and proteins, and the food tests for reducing sugar (Benedict’s), starch (iodine), and protein (Biuret). This revision guide covers each group of molecules, how to describe each test and its results, and how to link structure to function so you can answer “state”, “describe”, and “explain” questions for full marks.

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Carbohydrates: structure and function

Carbohydrates contain the elements carbon (C), hydrogen (H), and oxygen (O). They include sugars (e.g. glucose, fructose, sucrose) and starch. Sugars are a source of energy for respiration; starch is a storage molecule in plants (e.g. in seeds, tubers). Starch is a polysaccharide: many glucose units joined together. It is insoluble in water, so it does not affect the water potential of the cell and is suitable for storage. When energy is needed, starch can be broken down to glucose.

You may be asked to state the elements in carbohydrates (C, H, O), one function of sugars (energy) and one of starch (storage in plants), and why starch is suitable for storage (insoluble; compact).

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Fats (lipids): structure and function

Fats, or lipids, are made of fatty acids and glycerol. They also contain C, H, and O (and sometimes other elements). Fats are used for energy storage (e.g. under the skin, around organs), insulation (reducing heat loss), and as a major component of cell membranes. They contain more energy per gram than carbohydrates but are broken down when glucose is in short supply.

In exams you may need to state the elements (C, H, O), the building blocks (fatty acids and glycerol), and one or two functions (energy storage; insulation; cell membranes).

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Proteins: structure and function

Proteins are made of long chains of amino acids. They contain C, H, O, and nitrogen (N) (and often sulfur). Proteins have many roles: growth and repair of tissues, enzymes (catalyse reactions), antibodies (fight infection), hormones (e.g. insulin), and structural components (e.g. muscle, skin). The sequence of amino acids determines the shape and function of each protein.

You may be asked to state the elements (C, H, O, N), the building blocks (amino acids), and one or two functions (e.g. growth and repair; enzymes; antibodies).

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Food test: Benedict’s test for reducing sugar

Benedict’s test is used to detect reducing sugars (e.g. glucose, fructose, maltose). You add Benedict’s solution to the food sample (liquid or crushed in water) and heat the mixture in a water bath (e.g. 2–3 minutes). A positive result is a colour change from blue through green, yellow, orange, to brick-red (the more reducing sugar, the more red). A negative result is no change (stays blue).

Always say the test is for reducing sugar, not just “sugar” (sucrose is a non-reducing sugar and gives a negative result unless first hydrolysed). You must state that the mixture is heated; without heating, no colour change occurs even if reducing sugar is present.

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Food test: iodine test for starch

The iodine test for starch: add iodine solution (dissolved in potassium iodide) to the food sample. A positive result is a colour change to blue-black. A negative result is no change (stays brown/orange). No heating is needed. This test is often used on leaves to show that photosynthesis has produced starch (e.g. after destarching and then exposing part of the leaf to light).

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Food test: Biuret test for protein

The Biuret test for protein: add sodium hydroxide solution, then copper(II) sulfate solution (or use ready-made Biuret reagent), to the food sample. Do not heat. A positive result is a colour change to lilac (purple). A negative result is no change (stays blue).

Do not confuse Biuret (protein, lilac, no heating) with Benedict’s (reducing sugar, green/yellow/red on heating). Remember: Biuret = protein = lilac.

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Describing the tests in exams

When you describe a test, state: (1) what you add (e.g. Benedict’s solution), (2) what you do (e.g. heat in a water bath), (3) the positive result (e.g. colour change to brick-red). For a negative result, say “no change” or “stays blue” (for Benedict’s). You may be asked to identify the type of molecule from a test result (e.g. blue-black with iodine → starch; lilac with Biuret → protein).

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Exam tips and command words

• Describe the test: What you add, what you do (e.g. heat for Benedict’s), and the positive result.
• State the result: The colour change for a positive result (and “no change” for negative if asked).
• Explain why: Link structure to function (e.g. “Starch is insoluble so it does not affect water potential in storage cells.”).
• Identify: Name the type of molecule from a test result or description.

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Common mistakes to avoid

• Saying Benedict’s tests for “sugar” without specifying reducing sugar; or forgetting that the solution must be heated.
• Confusing Biuret (protein, lilac) with Benedict’s (reducing sugar, colour change on heating).
• Not stating that a negative result for Benedict’s is “no change” or “stays blue”.
• Forgetting the elements in each type of molecule (C, H, O for carbs and fats; C, H, O, N for proteins).

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Revision checklist

• List the elements in carbohydrates, fats, and proteins; state the building blocks (e.g. amino acids for proteins).
• Describe Benedict’s, iodine, and Biuret tests (what you add, what you do, positive result).
• State one function each of carbohydrates, fats, and proteins in the body.
• Explain why starch is suitable for storage in plants (insoluble; compact; does not affect water potential).

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Next steps

Book a free trial with an IGCSE Biology tutor to practise food tests and molecule questions, or explore Tutopiya’s learning portal for more revision resources.