OCR GCSE Biology: Cell Level Systems — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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.

---

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

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

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

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

5. 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]

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

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

8. 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]

9. 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]

---

Answers

1. 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].

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

3. 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].

4. 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].

5. (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].

6. 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].

7. 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].

8. (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].

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

---

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.