OCR GCSE Biology: Cell Level Systems — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Cell Level Systems revision notes and the Cell Level Systems study guide for the full topic explanation.

OCR’s real J247 papers open with a whole-paper Section A of 15 multiple-choice questions (15 marks, four options A–D) before a longer Section B of short-answer and extended questions; the multiple-choice sample below is scoped to this topic (Cell Level Systems) only, and is not a reproduction of a full paper’s Section A.

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Section A — Multiple choice (topic sample)

1. What happens to an enzyme’s activity if the temperature rises well above its optimum? A) Activity increases indefinitely because molecules move faster B) The enzyme is denatured: its active site changes shape and the substrate can no longer bind C) The enzyme becomes more specific to its substrate D) The activation energy needed for the reaction increases [1]

2. Which equation correctly represents anaerobic respiration in yeast (fermentation)? A) glucose → lactic acid B) glucose → ethanol + carbon dioxide C) glucose + oxygen → carbon dioxide + water D) carbon dioxide + water → glucose + oxygen [1]

3. In an investigation of photosynthesis rate against light intensity, the rate rises and then levels off at high light intensity. This is because: A) Chlorophyll is used up as light intensity increases B) Water becomes a product of photosynthesis rather than a reactant C) Another factor, such as carbon dioxide concentration or temperature, becomes limiting D) The plant stops respiring at high light intensity [1]

4. Which cell structure is the main site of aerobic respiration? A) Nucleus B) Mitochondria C) Ribosome D) Chloroplast [1]

5. A sample of double-stranded DNA contains 30% cytosine bases. What percentage of the bases in the sample will be adenine? A) 30% B) 20% C) 40% D) 70% [1]

6. Which of these is a function of the cell membrane? A) Controls what substances enter and leave the cell B) Contains the cell’s genetic material C) Is the main site of protein synthesis D) Is the main site of aerobic respiration [1]

7. Which structure is found in a plant cell but not an animal cell? A) Mitochondrion B) Ribosome C) Cell wall D) Nucleus [1]

8. Which term describes a cell that can differentiate into many different types of specialised cell? A) Stem cell B) Gamete C) Somatic cell D) Meristem tissue [1]

9. What is the main purpose of mitosis in a multicellular organism? A) To produce four genetically different haploid cells for reproduction B) To produce two genetically identical diploid cells for growth and repair C) To halve the chromosome number ready for fertilisation D) To recombine genetic material from two parent cells [1]

10. A root hair cell has a long, thin extension. Which of these best explains why this adaptation helps the cell absorb water and mineral ions? A) It increases the surface area available for absorption B) It allows the cell to photosynthesise more efficiently C) It reduces the distance ions must diffuse across the cell membrane to zero D) It increases the cell’s rate of mitosis [1]

11. The “lock and key” model describes how an enzyme works. What does this model represent? A) The enzyme changes shape to fit around any substrate B) The active site’s shape is complementary to one specific substrate C) The substrate changes shape to fit the enzyme’s active site D) Enzymes work fastest at the lowest possible temperature [1]

12. A student lowers the pH of a solution containing an enzyme well below its optimum. What is the most likely effect on the enzyme’s activity? A) Activity increases, because more hydrogen ions speed up collisions B) Activity is unaffected, since pH only affects the substrate C) The enzyme’s active site changes shape, so it can no longer bind its substrate D) The enzyme becomes permanently more specific to its substrate [1]

13. A student adds a catalase-containing plant extract to hydrogen peroxide solution and observes vigorous bubbling. What gas is being produced? A) Carbon dioxide B) Hydrogen C) Oxygen D) Water vapour [1]

14. Which cell structure contains chlorophyll and is the main site of photosynthesis? A) Mitochondrion B) Chloroplast C) Vacuole D) Nucleus [1]

15. What is meant by an organism’s “genome”? A) A single gene that codes for one protein B) The entire genetic material of that organism C) The proteins produced by all the organism’s cells D) The sequence of bases in one chromosome only [1]

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Section B — Short answer and extended questions

16. State the function of DNA and describe its basic structure. [3]

17. Write the word equation for aerobic respiration and for anaerobic respiration in muscle. [2]

18. Explain how the structure of DNA determines the sequence of amino acids in a protein. [4]

19. Compare aerobic and anaerobic respiration in terms of oxygen, products and energy yield. [4]

20. A student investigates photosynthesis using pondweed and counts bubbles per minute at different light intensities.

(a) Write the word and balanced symbol equation for photosynthesis. [3] (b) Explain why the rate rises then levels off as light intensity increases. [3] (c) State two other factors that could be limiting at the plateau. [2] (d) Explain why counting bubbles is not a fully reliable measure of the rate. [2]

21. Explain why the reactions of respiration and photosynthesis both depend on enzymes. [2]

22. State two structural differences between a prokaryotic cell and a eukaryotic cell. [2]

23. A student photographs a cell under a light microscope. The cell’s actual width is 20 μm, and its width in the photograph is 60 mm.

(a) Calculate the magnification of the photograph. [3] (b) Explain why an electron microscope, rather than a light microscope, would be needed to see the cell’s ribosomes. [2]

24. In an experiment testing the enzyme trypsin, a student times how long a cross drawn under a beaker of cloudy reaction mixture takes to become visible as the mixture clears. The cross becomes visible after 25 seconds. Using rate = 1000 ÷ time, calculate the rate of reaction. [2]

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Answers

1. B) Above the optimum temperature, an enzyme’s active site changes shape (denatures) so the substrate can no longer bind; activity does not keep increasing, and the enzyme’s specificity does not change [1].

2. B) Anaerobic respiration in yeast (fermentation) produces ethanol and carbon dioxide; lactic acid is instead the product of anaerobic respiration in muscle, and the equation with oxygen describes aerobic, not anaerobic, respiration [1].

3. C) At the plateau, light is no longer the limiting factor, so a different factor — carbon dioxide concentration or temperature — must be limiting the rate instead; chlorophyll is not “used up” and the plant continues to respire throughout [1].

4. B) Mitochondria are the main site of aerobic respiration, where most of the energy from glucose is released; the nucleus, ribosomes and chloroplasts have different primary functions [1].

5. B) In double-stranded DNA, C pairs with G, so %G = %C = 30%, giving C + G = 60%. The remaining 40% of bases are A and T in equal amounts (A pairs with T), so %A = 20% [1].

6. A) The cell membrane controls which substances enter and leave the cell; genetic material is in the nucleus, protein synthesis mainly occurs at ribosomes, and aerobic respiration mainly occurs in mitochondria [1].

7. C) Plant cells have a cell wall (made of cellulose), which gives the cell structural support; mitochondria and ribosomes are found in both plant and animal cells, and both have a nucleus [1].

8. A) A stem cell can differentiate into many different types of specialised cell; a gamete is a sex cell, a somatic cell is any ordinary body cell, and meristem tissue is where plant stem cells are found (not a synonym for the cell itself) [1].

9. B) Mitosis produces two genetically identical diploid daughter cells for growth and repair; producing four genetically different haploid cells is the outcome of meiosis, not mitosis [1].

10. A) The long, thin extension of a root hair cell increases its surface area, giving it more membrane across which water and mineral ions can be absorbed; it plays no role in photosynthesis and does not reduce diffusion distance to zero [1].

11. B) In the lock-and-key model, an enzyme’s active site has a shape complementary to one specific substrate, which is why enzymes are specific to particular reactions; it is the substrate, not the enzyme, that fits into a fixed active-site shape [1].

12. C) Far below its optimum pH, an enzyme’s active site changes shape (denatures), so its substrate can no longer bind and activity falls; activity does not increase indefinitely, and denaturation makes the enzyme non-functional rather than more specific [1].

13. C) Catalase breaks down hydrogen peroxide into water and oxygen; the bubbling observed is oxygen gas being released [1].

14. B) Chloroplasts contain chlorophyll and are the main site of photosynthesis; mitochondria are the site of aerobic respiration, and the vacuole and nucleus have different primary functions [1].

15. B) An organism’s genome is its entire genetic material; a single gene coding for one protein, or the sequence of bases in only one chromosome, describes a much smaller part of the genome rather than the whole of it [1].

16. DNA carries the genetic code that determines which proteins a cell makes [1]. It is a double helix of two strands [1] made of nucleotides, each with a sugar, a phosphate and one of four bases, with A pairing with T and C with G [1].

17. Aerobic: glucose + oxygen → carbon dioxide + water (+ energy) [1]. Anaerobic in muscle: glucose → lactic acid (+ energy) [1].

18. The sequence of bases in a gene [1] is read in groups of three, each coding for one amino acid [1]. The gene is copied and the code carried to a ribosome [1], where the amino acids are joined in that order to form the polypeptide [1].

19. Oxygen — aerobic requires it, anaerobic does not [1]. Products — aerobic gives carbon dioxide and water, anaerobic in muscle gives lactic acid [1] (in yeast, ethanol and carbon dioxide) [1]. Energy — aerobic releases much more energy per glucose molecule, because the glucose is completely oxidised [1].

20. (a) carbon dioxide + water → glucose + oxygen [1], in the presence of light and chlorophyll [1]. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ [1]. (b) At low intensity, light is the limiting factor, so more light means a faster rate [1]. At the plateau, light is no longer limiting [1] and another factor such as carbon dioxide concentration or temperature is limiting [1]. (c) Carbon dioxide concentration and temperature [1] [1]. (d) The bubbles may not all be the same size [1], and the gas is not pure oxygen, so the count is only a proxy — measuring the volume of gas collected would be better [1].

21. Both are series of reactions that would otherwise be far too slow at body or ambient temperature [1]; enzymes catalyse each step, lowering the activation energy so the reactions proceed fast enough to sustain life [1].

22. Any two of: a prokaryotic cell has no nucleus, with DNA free in the cytoplasm, while a eukaryotic cell’s DNA is enclosed in a nucleus [1]; a prokaryotic cell is much smaller (around 1 μm vs 10–100 μm) [1]; a prokaryotic cell has only ribosomes as organelles, while a eukaryotic cell has many membrane-bound organelles [1]; a prokaryotic cell often contains plasmids, which eukaryotic cells do not [1].

23. (a) Convert to the same unit first: 60 mm = 60 000 μm [1]. Magnification = image size ÷ actual size = 60 000 ÷ 20 [1] = ×3000 [1]. (b) Electron microscopes have a much higher resolution than light microscopes, because electrons have a far shorter wavelength than light [1], so structures as small as ribosomes can be distinguished as separate rather than appearing as an unresolved blur [1].

24. Rate = 1000 ÷ time = 1000 ÷ 25 [1] = 40 (arbitrary rate units) [1].

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Where marks are usually lost

• Writing the photosynthesis equation without balancing it.
• Saying anaerobic respiration in muscle produces carbon dioxide.
• Explaining the plateau without naming an alternative limiting factor.
• Forgetting that three bases code for one amino acid.
• Mixing up magnification and resolution — a bigger, blurrier image is not “more detail”.
• Forgetting to convert both measurements to the same unit before dividing in a magnification calculation.
• Saying a prokaryotic cell has “no DNA” rather than “no nucleus” — the DNA is present, just not enclosed.
• Giving the rate of a 1000 ÷ time calculation without units, or inventing units that were never specified — state the value as arbitrary rate units unless the question gives real ones.