A Level Biology: Cell Structure — 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 Structure revision notes

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Section A

1. Give four differences between a prokaryotic and a eukaryotic cell. [4]

2. Distinguish between magnification and resolution. [2]

3. State one reason why a virus is not classified as a living cell. [1]

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Section B

4. A protein is synthesised and secreted from a cell.

(a) Describe the pathway, naming the organelles in order. [5]

(b) Explain why a cell that secretes large amounts of protein has many mitochondria. [2]

5. An image of a mitochondrion measures 42 mm in a micrograph taken at ×15 000 magnification.

(a) Calculate the actual length in micrometres. [3]

(b) A student writes the answer as 0.0028 mm. Explain whether this is equivalent, and state which unit is more appropriate. [2]

6. A student calibrates an eyepiece graticule against a stage micrometre before measuring a cell.

(a) Explain why the eyepiece graticule must be calibrated against a stage micrometre for each objective lens used. [2]

(b) At ×400 magnification, 25 stage micrometre divisions (each representing 10 μm) align exactly with 40 eyepiece graticule divisions. Calculate the value of one eyepiece graticule division at this magnification, in micrometres. [3]

(c) A cell measured at this magnification spans 18 eyepiece graticule divisions. Calculate its actual diameter, in micrometres. [3]

7. A plant cell and an animal cell are compared under a light microscope.

(a) State one structure present in the plant cell that is absent from the animal cell. [1]

(b) Explain the function of this structure in the plant cell. [2]

8. Explain why electron microscopes achieve higher resolution than light microscopes, and state one major limitation this brings compared with light microscopy. [3]

9. Distinguish between free ribosomes and ribosomes bound to the rough endoplasmic reticulum, in terms of the proteins each produces. [2]

10. Compare the cell wall composition of a prokaryotic cell, a fungal cell and a plant cell. [3]

11. A student states that prokaryotic cells contain no additional DNA beyond their single circular chromosome. Explain why this is incorrect. [1]

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Answers

1. Any four: prokaryotes have no nucleus (DNA free in cytoplasm) whereas eukaryotes have a membrane-bound nucleus [1]; prokaryotic DNA is circular and not associated with histones [1]; prokaryotes have 70S ribosomes, eukaryotes 80S [1]; prokaryotes have no membrane-bound organelles [1]; prokaryotes are smaller (1–5 μm vs 10–100 μm); prokaryotes may have plasmids.

2. Magnification is how many times larger the image is than the object [1]; resolution is the ability to distinguish two points as separate [1].

3. Any one: a virus has no cytoplasm, ribosomes or metabolism of its own [1]; it cannot respire, grow or reproduce independently — it can only replicate by infecting a host cell and using the host’s ribosomes and enzymes [1].

4. (a) DNA in the nucleus is transcribed to mRNA [1], which leaves through a nuclear pore [1]. A ribosome on the rough endoplasmic reticulum synthesises the protein [1]. A vesicle carries it to the Golgi apparatus, which modifies and packages it [1]. A secretory vesicle fuses with the cell surface membrane and the protein leaves by exocytosis [1].

(b) Protein synthesis, vesicle transport and exocytosis all require ATP [1], and mitochondria are the site of aerobic respiration, which produces it [1].

5. (a) actual = image ÷ magnification = 42 ÷ 15 000 [1] = 0.0028 mm [1] = 0.0028 × 1000 = 2.8 μm [1].

(b) It is numerically equivalent [1], but micrometres are more appropriate because the value is then a convenient number rather than a small decimal [1].

6. (a) The value of one graticule division is arbitrary and changes with magnification [1], so it is different for every objective lens and must be recalibrated whenever the objective is changed [1].

(b) 25 stage micrometre divisions = 25 × 10 μm = 250 μm [1], which spans 40 eyepiece divisions [1], so one eyepiece division = 250 ÷ 40 = 6.25 μm [1].

(c) 18 × 6.25 μm [1] = 112.5 μm [1], with correct unit and working shown [1].

7. (a) Any one: cell wall (cellulose) / chloroplast / large permanent vacuole [1].

(b) For a cell wall: provides structural support and prevents the cell bursting by osmotic uptake of water, maintaining turgor [1], allowing the cell to support the plant [1]. (Equivalent reasoning credited for chloroplast — photosynthesis — or vacuole — turgor and storage.)

8. Electron microscopes use a beam of electrons, which has a much shorter effective wavelength than light, and resolution is fundamentally limited by wavelength — a shorter wavelength distinguishes points that are closer together [1] [1]. Limitation: samples must be dead, dehydrated and viewed in a vacuum, so living, moving processes cannot be observed in real time as they can with light microscopy [1].

9. Free ribosomes (in the cytoplasm) produce proteins used within the cell itself [1]. Ribosomes bound to the rough endoplasmic reticulum produce proteins destined for secretion or for use in membranes, since the rough ER processes and packages the protein for export [1].

10. A prokaryotic cell wall is built from peptidoglycan (murein) [1]; a fungal cell wall is built from chitin [1]; a plant cell wall is built from cellulose [1] — the three differ entirely in composition despite all providing structural support.

11. Prokaryotic cells may also carry plasmids — small, additional circular DNA molecules separate from the main chromosome — which eukaryotic cells do not have [1].

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Where marks are usually lost

• Saying prokaryotes have “no organelles” — they have ribosomes.
• Failing to convert units in a magnification calculation.
• Forgetting that a virus has no metabolism of its own and cannot replicate outside a host cell.
• Saying the rough ER modifies proteins — that is the Golgi’s role.
• Confusing magnification with resolution.
• Stating electron microscopy has “better resolution” without linking this to the shorter wavelength of electrons.
• Describing an organelle’s appearance without linking its structure to its function.