Cambridge IGCSE Biology
Cambridge IGCSE Biology 4 Biological Molecules Exam Questions
Free Cambridge IGCSE Biology revision for 4 Biological Molecules: what you need to know, common mistakes and exam-style questions with answers. Covers the elements in carbohydrates, fats and proteins, their building blocks, food tests and the structure of DNA.
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About Cambridge IGCSE Biology 4 Biological Molecules
Biological molecules is a short but heavily examined topic in Cambridge IGCSE Biology. You must know the chemical elements in carbohydrates, fats and proteins, the smaller molecules each is made from, and the food tests for starch, reducing sugars, proteins, fats and oils and vitamin C. Extended candidates also describe the structure of DNA: two strands coiled into a double helix, held together by bonds between complementary bases. The food tests are a favourite in Paper 5 and Paper 6, where you must give the reagent, method and positive result. This topic links directly to enzymes, human nutrition and inheritance.
What you need to know
- Elements Carbohydrates contain carbon, hydrogen and oxygen. Fats and oils also contain carbon, hydrogen and oxygen. Proteins contain carbon, hydrogen, oxygen and nitrogen (and some contain sulfur).
- Building blocks Starch, glycogen and cellulose are made from glucose. Proteins are made from amino acids. Fats and oils are made from fatty acids and glycerol.
- Starch and reducing sugar tests Starch: add iodine solution, colour changes from brown (orange-brown) to blue-black. Reducing sugars such as glucose: add Benedict's solution and heat in a water bath, colour changes from blue to green, yellow, orange or brick red depending on the amount.
- Protein, fat and vitamin C tests Protein: biuret solution turns from blue to purple (lilac). Fats and oils: ethanol emulsion test, shake with ethanol then add to water, a cloudy white emulsion forms. Vitamin C: DCPIP turns from blue to colourless.
- DNA structure (Extended) DNA has two strands coiled together to form a double helix. Each strand contains chemicals called bases, and bonds between pairs of bases hold the strands together. Bases always pair A with T and C with G.
Common mistakes in 4 Biological Molecules
- Saying iodine turns black with starch without the starting colour. State brown or orange-brown to blue-black.
- Forgetting to heat Benedict's solution. A water bath at around 80 degrees Celsius or above is needed for the colour change.
- Saying proteins are made of glucose or fats of amino acids. Learn the correct building block for each molecule.
- Pairing A with G or C with T in DNA. Complementary pairs are A with T and C with G.
- Writing 'goes clear' for the emulsion test. A positive result is a cloudy white emulsion.
Try three 4 Biological Molecules exam-style questions
1. Name the chemical elements found in proteins but not in carbohydrates. (1 mark)
Show the mark scheme answer
Nitrogen (and sulfur in some proteins).
2. Describe how you would test a sample of food for reducing sugar, including the positive result. (3 marks)
Show the mark scheme answer
Add Benedict's solution to the food sample (in solution); heat in a water bath (around 80 degrees Celsius or above) for a few minutes; positive result is a colour change from blue to brick red (or green, yellow, orange).
3. One strand of DNA has the base sequence ATTGCA. State the sequence on the complementary strand. (1 mark)
Show the mark scheme answer
TAACGT.
4 Biological Molecules FAQs
What does Cambridge IGCSE Biology 4 Biological Molecules cover?
It covers the elements and building blocks of carbohydrates, fats and proteins, the food tests for each, the DCPIP test for vitamin C and, for Extended candidates, the structure of DNA.
Which paper is 4 Biological Molecules examined in?
It is examined in the theory and multiple choice papers: Papers 1 and 3 for Core candidates, or Papers 2 and 4 for Extended candidates. Practical skills linked to it can also appear in Paper 5 (Practical Test) or Paper 6 (Alternative to Practical).
Which food tests do I need to learn?
Learn five: iodine for starch, Benedict's for reducing sugars, biuret for protein, the ethanol emulsion test for fats and oils, and DCPIP for vitamin C. For each, know the reagent, whether heating is needed, and the colour change from start to finish.
