Pearson Edexcel IGCSE Biology: Cell Structure and Biological Molecules — Practice Questions (4BI1)

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — Pearson holds copyright in its own papers. Use these alongside the official past papers available through your school or Pearson’s own resources.

Related: Cell structure and biological molecules study guide and revision notes.

Section A

1. Name two structures found in a plant cell but not in an animal cell. [2]

2. State the function of the ribosomes. [1]

3. Name the three biological molecules whose “building-block” units are simple sugars, amino acids, and fatty acids plus glycerol respectively. [3]

Section B

4. A student examines an unlabelled cell under a microscope and observes a cell wall, a large central vacuole, and chloroplasts.

(a) Identify the type of cell, justifying your answer with reference to each structure observed. [3] (b) Explain why the presence of chloroplasts specifically rules out this being an animal cell. [2]

5. An enzyme-catalysed reaction is carried out at four different temperatures: 20°C, 37°C (the enzyme’s optimum), 50°C, and 70°C.

(a) Describe how the rate of reaction would differ between 20°C and 37°C, and explain why. [2] (b) Describe and explain what happens to the enzyme’s activity at 70°C. [3] (c) State whether the change at 70°C is reversible, and explain your answer. [2]

6. A food sample turns blue-black when iodine solution is added.

(a) State which biological molecule this test result indicates is present. [1] (b) Describe how this molecule’s structure relates to the “building-block” pattern. [2]

Worked answers

1. Any two of: cell wall, chloroplasts, large permanent (central) vacuole. [2]

2. Ribosomes are the site of protein synthesis. [1]

3. Starch/glycogen (simple sugar units), protein (amino acid units), lipid (fatty acid and glycerol units). [3]

4. (a) This is a plant cell. The cell wall provides structural support and is not found in animal cells; the large permanent vacuole stores cell sap and maintains turgor and is not found in animal cells; chloroplasts are the site of photosynthesis and are only present in photosynthetic plant cells. [3] (1 mark per correctly justified structure, up to 3.) (b) Chloroplasts are the site of photosynthesis, a process that does not occur in animal cells, so their presence is direct evidence the cell is photosynthetic and therefore plant (or plant-like), not animal. [2]

5. (a) The rate of reaction at 37°C (the optimum) would be higher than at 20°C, because at the lower temperature there is less kinetic energy, resulting in fewer successful collisions between enzyme and substrate per unit time. [2] (b) At 70°C, well above the optimum, the enzyme denatures – the high temperature changes the shape of the active site so the substrate no longer fits, and the reaction rate drops sharply, effectively stopping. [3] (1 mark for “denatures”/active site shape change, 1 for substrate no longer fitting, 1 for sharply reduced/stopped rate.) (c) No, it is not reversible – denaturation at high temperature permanently changes the active site’s shape, unlike the reduced (but recoverable) activity seen at low temperatures. [2]

6. (a) Starch. [1] (b) Starch is a large molecule built from many simple sugar units joined together in a repeating chain, matching the general building-block pattern where a large biological molecule is constructed from smaller repeating units. [2]

Section C

7. A student investigates the effect of pH on the activity of an enzyme by measuring the time taken for a reaction to reach completion at pH 5, pH 7, and pH 9, where the enzyme’s optimum is pH 7.

(a) Predict how the reaction time at pH 5 and pH 9 would compare with pH 7, explaining your reasoning. [3] (b) Explain, in terms of the enzyme’s structure, why both extremes of pH reduce activity by the same underlying mechanism as high temperature. [3]

8. Explain why stem cells are described as having both advantages and disadvantages for medical use, giving one example of each. [4] (Higher tier)

Worked answers (continued)

7. (a) The reaction would take longer to reach completion at both pH 5 and pH 9 compared with pH 7, since the enzyme is furthest from its optimum activity at both extremes, reducing the rate of reaction. [3] (1 mark for correctly predicting longer times at both extremes, 2 for linking this to distance from the optimum.) (b) Both extreme pH values denature the enzyme by altering the shape of its active site, in the same underlying way that high temperature does – once the active site’s shape has changed, the substrate can no longer bind effectively, and the reaction can no longer be catalysed at the normal rate. [3]

8. Advantage: stem cells can differentiate into many different cell types, giving them potential to replace damaged or diseased cells (e.g. treating certain blood disorders by replacing damaged blood-forming cells). Disadvantage: their medical use raises ethical concerns (for example, around the source of embryonic stem cells) and carries practical risks such as the possibility of uncontrolled cell growth if not properly regulated. [4] (2 marks for a valid, explained advantage; 2 marks for a valid, explained disadvantage.)

Why question 5 stays three-part

Question 5 is deliberately split into describing the rate change, explaining the mechanism, and judging reversibility, because the revision notes identify exactly this three-part structure – below optimum, at optimum, well above optimum – as the answer format examiners look for, and flag “describing an enzyme as killed by heat” (rather than explaining denaturation as an active-site shape change) as the most common way marks are lost on this topic.

Official syllabus

Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3 (September 2024) – official specification PDF – the same source cited by the study guide and revision notes.