OCR A Level Biology A: Cell structure, microscopy, prokaryotes and eukaryotes (H420) – 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 from your board or school.

These questions cover section 2.1.1 (a) to (k), Cell structure, of the OCR A Level Biology A specification (H420, Version 4.1, April 2026). Work through the study guide first if the topic is new, or use the revision notes to refresh. Course pages: the OCR A Level Biology hub and the printable checklist.

Questions

1. Distinguish between magnification and resolution. Explain why images from a transmission electron microscope contain finer detail than those from a light microscope. [3]

2. A student calibrates an eyepiece graticule. A 120 µm length on a stage micrometer lines up with 32 graticule divisions at one objective lens.

(a) Calculate the length of one graticule division. [2] (b) A nucleus measures 10 graticule divisions. Calculate its diameter in µm. [1] (c) Explain why the graticule has to be calibrated again if the objective lens is changed. [2]

3. A TEM micrograph shows a chloroplast 46 mm long at a magnification of ×12 500.

(a) Work out the actual length of the chloroplast, in µm. [3] (b) Calculate the length in mm of a scale bar on this micrograph that represents 1 µm. [2]

4. Compare the images produced by a transmission electron microscope and a scanning electron microscope. [4]

5. Describe how you would prepare a temporary slide of a thin piece of onion epidermis stained with iodine solution for examination with a light microscope. [4]

6. A mixed sample of bacteria is treated with a differential stain. Under the light microscope some cells appear purple and others appear red.

(a) State what is meant by differential staining. [1] (b) Name this stain and explain what the two colours show about the cells. [2] (c) Suggest one advantage of using a light microscope rather than a TEM for this investigation. [1]

7. Name the eukaryotic cell component described in each case.

(a) An organelle with two surrounding membranes; the inner membrane has shelf-like folds (cristae). [1] (b) A pair of short cylinders made of microtubules, lying at right angles. [1] (c) A single-membrane sac containing hydrolytic enzymes. [1] (d) The dark-staining area within the nucleus where rRNA is synthesised. [1] (e) A stack of flattened sacs that modifies proteins and packages them into vesicles. [1]

8. A TEM micrograph of a cell from an enzyme-secreting gland shows a nucleus near the base. Most of the cytoplasm is filled with flattened sacs covered in small dots. Close to the nucleus is a stack of smooth curved sacs with small vesicles budding from the edges. Many structures with a double membrane and internal folds lie between the sacs. Vesicles are packed near the cell surface.

(a) State one feature that shows that the image is a TEM image and not an SEM image. [1] (b) Identify the structures covered in dots, and the stack of curved sacs. [2] (c) Explain why this cell has large amounts of the dotted sacs, the stack of curved sacs and structures with a double membrane. [3]

9. Cells that secrete a protein were given radioactively labelled amino acids for two minutes. They were then moved to a medium without label. The percentage of labelled protein in each location over time is shown.

Time / min	Rough ER	Golgi apparatus	Secretory vesicles	Outside the cell	Other
2	80	10	5	0	5
10	35	50	10	0	5
30	10	30	45	10	5
60	5	10	25	55	5

(a) Suggest why labelled amino acids were used. [1] (b) State the order in which the protein passes through the locations, using the data. [2] (c) Calculate the percentage decrease in labelled protein in the Golgi apparatus between 10 and 30 minutes. [2] (d) Explain why the labelled protein in the rough ER falls between 2 and 30 minutes. [2] (e) A drug stops mitochondria making ATP. Predict the effect on the amount of protein outside the cell at 60 minutes and explain your answer. [2]

10. The cytoskeleton is important in eukaryotic cells.

(a) State three functions of the cytoskeleton. [3] (b) A drug breaks down microtubules in a secretory cell. Suggest the effect on secretion and explain it. [2]

11. (a) Give four differences in structure between a bacterial cell and a plant cell. [4] (b) State two structures that both cell types have. [2]

12. A TEM micrograph of a nucleus has a scale bar 30 mm long, labelled 2 µm. The nucleus is 105 mm across in the micrograph. The nucleolus is 21 mm across.

(a) Find the magnification of the micrograph. [2] (b) Calculate the diameter of the nucleus in µm. [2] (c) Calculate the diameter of the nucleolus in µm. [1] (d) Calculate the diameter of the nucleolus as a percentage of the diameter of the nucleus. [1] (e) State the function of the nucleolus. [1]

Answers

1. Magnification is image size divided by real size [1]. Resolution is the minimum gap that lets two points be seen apart [1]. The electron beam has a shorter wavelength than light, so the TEM separates points that lie closer together [1]. [3] Examiner insight: Definitions of resolution that only say “clarity” are vague. Refer to distinguishing two points as separate, and give the cause (wavelength).

2. (a) 120 ÷ 32 [1] = 3.75 µm [1] (b) 10 × 3.75 = 37.5 µm [1] (c) A different objective changes the magnification, so the image of the scale is larger or smaller [1]. One graticule division then represents a different real length and must be recalibrated against the stage micrometer [1]. [5] Examiner insight: In a calibration, show the division of real length by the number of divisions. Answers that say only “it is different” do not explain why.

3. (a) Real length = image ÷ magnification [1] = 46 ÷ 12 500 = 0.00368 mm [1] = 3.68 µm [1] (b) 1 µm × 12 500 [1] = 12 500 µm = 12.5 mm [1] [5] Examiner insight: Say which unit your answer is in and show the conversion from mm to µm. Rounding early can move the final value.

4. Any four from: TEM passes electrons through the specimen, SEM scans electrons over the surface [1]; TEM shows internal ultrastructure, SEM shows surface structure [1]; TEM image is 2D, SEM gives a 3D appearance [1]; TEM needs ultra-thin sections, SEM does not need sectioning [1]. Also creditworthy (within the four): TEM has higher resolution; both use a vacuum and cannot show living cells. [4] Examiner insight: A comparison needs both microscopes in each point. Writing “TEM shows internal detail” alone leaves the comparison unfinished.

5. Cut or peel a very thin piece of epidermis so light can pass through [1]. Place it flat on a clean slide in a drop of iodine solution [1]. Lower a cover slip slowly at an angle using a mounted needle to avoid trapping air bubbles [1]. Blot excess liquid with filter paper [1]. [4] Examiner insight: Give the reason beside each step, such as “thin so light passes through” and “angle to avoid bubbles”. Stating the purpose of each step makes an answer stronger than listing actions alone.

6. (a) Using stains so that different structures or cell types take up different colours and can be told apart [1]. (b) The Gram stain [1]. Purple cells are Gram-positive and red cells are Gram-negative, which shows a difference in the cell wall [1]. (c) The light microscope needs simple preparation and shows colour, so the stain result is seen [1]. [4] Examiner insight: Colour is the clue. A TEM image is monochrome, so any answer on stain colour must refer to light microscopy.

7. (a) Mitochondrion [1]. (b) Centrioles [1]. (c) Lysosome [1]. (d) Nucleolus [1]. (e) Golgi apparatus [1]. [5] Examiner insight: Use the exact term. Spell and use the standard names, and do not swap similar terms such as centriole and nucleolus, or lysosome and ribosome.

8. (a) Any one: monochrome/grey scale image; internal structure of the cell visible; membranes seen in section [1]. (b) Dotted sacs: rough endoplasmic reticulum [1]. Stack of smooth curved sacs: Golgi apparatus [1]. (c) Rough ER with ribosomes makes and folds large amounts of protein for secretion [1]. Golgi modifies and packages the protein into vesicles for secretion [1]. Many mitochondria make ATP by aerobic respiration to supply energy for protein synthesis and transport [1]. [6] Examiner insight: Link each organelle to the cell’s function (secretion). Naming the structures without explaining why there are many of them does not answer “explain”.

9. (a) The label allows the protein made to be followed through the cell [1]. (b) Rough ER, then Golgi apparatus [1], then secretory vesicles, then outside the cell [1]. (c) 50 − 30 = 20 [1]; 20 ÷ 50 × 100 = 40% decrease [1]. (d) The protein moves from the rough ER in vesicles to the Golgi apparatus [1] and no more labelled amino acid is available, so no new labelled protein is made [1]. (e) The amount outside would be lower [1], because the protein synthesis, vesicle movement and exocytosis need ATP [1]. [9] Examiner insight: Use the figures. In (c), say whether the answer is a percentage decrease or a change in percentage points; they are not the same.

10. (a) Mechanical strength / support of the cell [1]; transport within the cell [1]; cell movement [1]. (b) Secretion would decrease [1], because vesicles carrying protein would not be moved along the microtubules to the Golgi and plasma membrane [1]. [5] Examiner insight: A “suggest” answer needs the consequence and the reason. Name the type of movement that is lost, such as vesicles moving along tracks.

11. (a) Any four from: bacterium has no nucleus, plant has a nucleus [1]; bacterial DNA is circular, plant DNA is linear [1]; bacterial DNA is not with histones, plant DNA is [1]; bacterial ribosomes are 70S, plant ribosomes are 80S [1]. Also creditworthy (within the four): no membrane-bound organelles in the bacterium; peptidoglycan wall against cellulose wall; plasmids. [4] (b) Any two from plasma membrane, cytoplasm, ribosomes, cell wall, DNA [1] [1]. [2] Examiner insight: Differences need both cell types in the same statement. “Has no nucleus” alone does not say what the other cell has.

12. (a) 30 mm = 30 000 µm [1]; 30 000 ÷ 2 = ×15 000 [1] (b) 105 ÷ 15 000 = 0.007 mm [1] = 7 µm [1] (c) 21 ÷ 15 000 = 0.0014 mm = 1.4 µm [1] (d) 1.4 ÷ 7 × 100 = 20% [1] (e) Makes ribosomal RNA and assembles ribosome subunits [1]. [7] Examiner insight: A scale bar gives the magnification. Show that you converted the bar to the same unit as its label before dividing.

Where marks are usually lost

• Using magnification and resolution interchangeably.
• Leaving out the conversion between mm and µm.
• Describing SEM as showing internal structure, or TEM as 3D.
• Describing a slide method without saying why each step is done.
• Naming an organelle without linking it to the function of the cell in the question.
• Confusing percentage decrease with percentage points.
• Giving only one side of a prokaryote and eukaryote comparison.
• Saying prokaryotes have no DNA instead of no nucleus.

Next steps

• Read the revision notes and the study guide.
• Return to the OCR A Level Biology hub and tick off the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OCR, A Level in Biology A (H420), specification Version 4.1 (April 2026), section 2.1.1 Cell structure.