Cambridge International AS & A Level Biology 9700: The microscope in cell studies – 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 sub-topic 1.1 The microscope in cell studies, from topic 1 Cell structure of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It is AS Level content, examined on Papers 1 and 2 and assumed on Paper 4, and the microscope skills are also assessed practically on Paper 3. There are no tiers in 9700. The syllabus allows calculators in the examination, so use one for the calculations here, but show every step.

Revise first with the 1.1 study guide and the 1.1 revision notes. The topic-level Cell structure practice questions cover organelles, prokaryotes and viruses; the questions below are different and go deeper into 1.1. The 9700 course hub and the printable checklist show where this fits.

Questions

1. Convert: (i) 0.068 mm to µm; (ii) 2.4 µm to nm; (iii) 15 000 nm to mm. [3]

2. On a transmission electron micrograph labelled ×12 000, the image of a nucleus is 96 mm across. Calculate the actual diameter of the nucleus in µm. Show your working. [2]

3. Describe how you would make a temporary preparation of onion epidermis, stained with iodine in potassium iodide solution, and bring the cells into focus at ×400. [5]

4. A scanning electron micrograph of a pollen grain has a scale bar labelled 20 µm. The scale bar measures 30 mm. The pollen grain’s image is 63 mm across.

(a) Calculate the magnification of the micrograph. [2] (b) Calculate the actual diameter of the pollen grain. [2]

5. A student uses a microscope with a ×10 eyepiece containing a graticule. Each stage micrometer division is 10 µm. With the ×40 objective, 45 eyepiece units coincide exactly with 9 stage micrometer divisions.

(a) Calculate the length represented by one eyepiece unit with the ×40 objective. [2] (b) The student measures five cells from a root tip. Their widths in eyepiece units are 16, 17, 15, 18 and 19. Calculate the mean width of the cells in µm. [3] (c) The student switches to the ×10 objective. Predict the length represented by one eyepiece unit, and explain your prediction. [2]

6. This question is about drawing from a microscope slide.

(a) State two ways in which a low-power plan diagram differs from a high-power drawing of cells. [2] (b) A student’s high-power drawing of leaf epidermis cells has: shaded nuclei; cell walls drawn as a single line; and chloroplasts drawn in every cell, although none were visible on the slide. For each fault, state what the student should have done. [3]

7. A student draws a guard cell 72 mm long. The actual length of the guard cell, measured with a calibrated graticule, is 45 µm.

(a) Calculate the magnification of the drawing. [2] (b) The student wants to add a 10 µm scale bar to the drawing. Calculate how long the bar should be. [1]

8. A photomicrograph of a liver cell is taken at ×1500. Mitochondria show only as small dots. A student enlarges the photograph four times, hoping to see the cristae inside the mitochondria.

(a) Explain why the enlarged photograph will not show the cristae. [3] (b) Name the type of microscope that would show the cristae. [1]

9. For each investigation, state the most suitable type of microscope (light, transmission electron or scanning electron) and give a reason.

(i) watching chloroplasts move in living pondweed leaf cells; (ii) studying the surface sculpturing of a pollen grain; (iii) studying the internal membranes of a chloroplast. [3]

10. A transmission electron micrograph of a pancreatic cell is labelled ×25 000.

(a) A mitochondrion’s image is 45 mm long. Calculate its actual length in µm. [2] (b) Ribosomes on the micrograph are dots 0.6 mm across. Calculate the actual diameter of a ribosome in nm. [3] (c) Use your answer to (b) to explain why ribosomes are not visible with a light microscope. [2] (d) Explain why the cell could not have been alive when this image was made. [1]

11. A student studies palisade cells in a leaf section. With the ×40 objective, 40 eyepiece units coincide with 12 stage micrometer divisions, each 10 µm.

(a) Calculate the length of one eyepiece unit. [2] (b) A palisade cell is 25 eyepiece units long. Calculate its actual length. [1] (c) The student draws this cell 90 mm long. Calculate the magnification of the drawing. [2] (d) A photomicrograph of the same leaf, labelled ×500, shows a palisade cell 39 mm long. Calculate its actual length, to an appropriate number of significant figures. [2] (e) Suggest why the answers to (b) and (d) are not identical. [1]

Answers

1. (i) 0.068 × 1000 = 68 µm [1] (ii) 2.4 × 1000 = 2400 nm [1] (iii) 15 000 ÷ 1 000 000 = 0.015 mm [1] Examiner insight: Each conversion is marked on its own, so one slip costs one mark; check direction by asking whether a smaller unit should give a bigger number.

2. 96 mm = 96 000 µm; actual = 96 000 ÷ 12 000 [1] = 8 µm [1] Examiner insight: A bare “8” with no unit, or 0.008 given with “µm”, does not score the answer mark; the unit must match the value.

3. Peel a thin piece of epidermis from the inner surface of an onion scale leaf with forceps [1]. Place it flat in a drop of iodine in potassium iodide solution on a slide, unfolding it with a mounted needle [1]. Lower a coverslip at an angle, supported by the mounted needle, to avoid trapping air bubbles [1]. Blot away excess stain with a paper towel [1]. Focus first with the ×10 (low-power) objective, centre the cells, then turn to the ×40 objective and use only the fine focus [1]. Examiner insight: Each mark needs a specific action; “put it on a slide and look at it” earns nothing, while the reason for lowering the coverslip at an angle (no air bubbles) is a separate creditworthy point.

4. (a) 30 mm = 30 000 µm [1]; magnification = 30 000 ÷ 20 = ×1500 [1] (b) 63 mm = 63 000 µm; 63 000 ÷ 1500 [1] = 42 µm [1] Examiner insight: Allow error carried forward in (b) from a wrong magnification in (a), but only if the working is shown; a magnification written with a unit loses the answer mark.

5. (a) 9 × 10 = 90 µm [1]; 90 ÷ 45 = 2.0 µm per eyepiece unit [1] (b) mean = (16 + 17 + 15 + 18 + 19) ÷ 5 = 17 eyepiece units [1]; 17 × 2.0 [1] = 34 µm [1] (c) About 8 µm per eyepiece unit [1]; the objective magnification falls four times, so the same graticule division covers four times as much of the specimen [1]. Examiner insight: In (c) the prediction alone earns one mark; the second mark needs the link between lower magnification and each division covering a greater distance, and “it changes with magnification” is too vague.

6. (a) Any two: a plan diagram shows no individual cells [1]; it shows the outlines and distribution of tissue layers in correct proportions, whereas a high-power drawing shows the shapes and contents of a few individual cells [1]. (b) Draw nuclei as clear outlines with no shading [1]; draw cell walls with two lines, or three where two cells touch [1]; draw only the structures that are visible on the slide, so leave out the chloroplasts [1]. Examiner insight: The three faults in (b) are marked separately; restating the fault (“the walls were single lines”) without saying what to do instead does not score.

7. (a) 72 mm = 72 000 µm; 72 000 ÷ 45 [1] = ×1600 [1] (b) 10 µm × 1600 = 16 000 µm = 16 mm [1] Examiner insight: The drawing’s magnification is set by the drawing, so an answer of ×400 (the microscope) scores zero in (a).

8. (a) Enlarging the photograph increases magnification only, not resolution [1]. The resolution of a light microscope is limited to about 200 nm by the wavelength of light [1]. The cristae are closer together than this, so they cannot be distinguished as separate structures; the enlarged image is bigger but just as blurred [1]. (b) Transmission electron microscope [1] Examiner insight: “Not enough magnification” is the wrong idea and scores nothing; the answer must say resolution is the limit and link it to wavelength.

9. (i) Light microscope: the cells must be alive, and electron microscope specimens are dead [1] (ii) Scanning electron microscope: it gives a three-dimensional image of the surface [1] (iii) Transmission electron microscope: it has the highest resolution and shows internal structure in thin sections [1] Examiner insight: Each mark needs both the correct microscope and a valid reason; a correct name with no reason does not score.

10. (a) 45 mm = 45 000 µm; 45 000 ÷ 25 000 [1] = 1.8 µm [1] (b) 0.6 mm = 600 µm [1]; 600 ÷ 25 000 = 0.024 µm [1]; = 24 nm [1] (c) The ribosome (24 nm) is smaller than the resolution limit of the light microscope, about 200 nm [1]; light’s wavelength is too long to separate points this close, so ribosomes cannot be distinguished [1]. (d) The specimen is held in a vacuum (and must be fixed, sectioned and stained) because air would scatter the electron beam [1]. Examiner insight: In (c) a comparison with a stated value for the light microscope limit is needed; “too small to see” without reference to resolution is not enough for both marks.

11. (a) 12 × 10 = 120 µm [1]; 120 ÷ 40 = 3.0 µm per eyepiece unit [1] (b) 25 × 3.0 = 75 µm [1] (c) 90 mm = 90 000 µm; 90 000 ÷ 75 [1] = ×1200 [1] (d) 39 mm = 39 000 µm; 39 000 ÷ 500 [1] = 78 µm (2 significant figures, matching the data) [1] (e) Any one: palisade cells vary in length, so two different cells were measured; or each measurement has its own error (lining up graticule divisions, measuring with a ruler) [1]. Examiner insight: For (d) the answer should have the same number of significant figures as the least precise data, or one more; 78 µm or 78.0 µm is acceptable, but a long string of calculator digits is not.

Where marks are usually lost

• Leaving one measurement in mm and the other in µm before dividing.
• Writing a unit after a magnification, such as “×1500 µm”.
• Calculating one eyepiece unit as eyepiece units ÷ micrometer length, the wrong way up.
• Keeping a ×40 calibration after switching to the ×10 objective.
• Explaining electron microscopes by “more magnification” instead of better resolution from a shorter wavelength.
• Describing a slide preparation with no stain, no coverslip technique or no low-power focusing step.
• Drawing cells in a plan diagram, shading, or drawing organelles that were not visible.
• Not showing the working, so no method mark can be given when the final answer is wrong.
• Too many significant figures in the final answer.

Next steps

• 1.1 revision notes
• 1.1 study guide
• Cell structure study guide for all of topic 1
• 9700 course hub
• Printable 9700 checklist
• Free 9700 AS diagnostic
• Book a free trial class

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027 (Version 1, published September 2022), Cambridge International Education: topic 1 Cell structure, sub-topic 1.1 The microscope in cell studies, and the Practical assessment section.