Cambridge International AS & A Level Biology 9700: The microscope in cell studies – Study Guide

This guide teaches sub-topic 1.1 The microscope in cell studies, part of topic 1 Cell structure in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It is AS Level content. Papers 1 and 2 are based on the AS content, Paper 4 assumes it, and the light-microscope skills it builds are assessed hands-on in Paper 3 (Advanced Practical Skills). There are no tiers in 9700: everything here is for every candidate.

The topic-level Cell structure study guide covers all of topic 1; this page goes deeper into 1.1 only. Afterwards, condense it with the revision notes and test yourself with the practice questions.

What this sub-topic covers

Outcome	What you must be able to do
1.1.1	Make temporary preparations of cellular material for viewing with a light microscope
1.1.2	Draw cells from microscope slides and photomicrographs
1.1.3	Calculate magnifications of images and actual sizes of specimens from drawings, photomicrographs and electron micrographs (scanning and transmission)
1.1.4	Use an eyepiece graticule and stage micrometer scale to make measurements, using mm, µm and nm
1.1.5	Define resolution and magnification and explain the differences between them, with reference to light and electron microscopy

The syllabus calls light microscopy a fundamental skill that later topics (transport in plants and mammals, gas exchange) rely on.

1.1.1 Making a temporary preparation

A temporary preparation is a layer of cells thin enough for light to pass through, in a drop of liquid under a coverslip, with structures made visible by a stain. Standard examples use stains on the syllabus materials list: iodine in potassium iodide solution or methylene blue.

Method for onion epidermis

1. Cut a piece from one fleshy scale leaf of an onion bulb. Peel the thin, transparent epidermis from the inner (concave) surface with blunt forceps.
2. Place a drop of iodine in potassium iodide solution in the centre of a clean slide.
3. Lay a small square of epidermis flat in the drop. Use a mounted needle to unfold it, because folded tissue gives overlapping layers that cannot be focused.
4. Touch one edge of a coverslip to the slide beside the drop. Support the opposite edge with the mounted needle and lower it slowly. This pushes air out ahead of the liquid, so you avoid air bubbles, which show as thick black-ringed circles.
5. Blot excess stain from around the coverslip with a paper towel.
6. Place the slide on the stage. Focus with the low-power (×10) objective first, then centre the area you want and turn to the high-power (×40) objective, using only the fine focus.

Unstained cells are almost colourless; the stain makes nuclei and cell outlines stand out. Low power gives a wider field of view, so you find the cells quickly. Total magnification = eyepiece × objective, so the syllabus microscope (×10 eyepiece; ×10 and ×40 objectives) gives ×100 and ×400.

1.1.2 Drawing cells and tissues

The practical section of the syllabus sets fixed drawing conventions. There are two kinds of drawing.

A plan diagram shows the distribution of tissues in a specimen, such as a section of a leaf or stem. No individual cells are drawn. The outlines of the tissue layers must be in the correct proportions to each other.

A high-power drawing shows a small number of individual cells. It must show:

• the correct shapes of the cells;
• the thickness of cell walls where present, drawn with two lines, or three lines where two cells touch (the middle line is the boundary between neighbouring walls);
• the relative sizes and proportions of the cells;
• observable contents only. Draw the nucleus if you can see it. Do not add organelles from a textbook diagram that you cannot see on the slide.

Both need:

• A sharp pencil, with clear, continuous, unbroken lines and no shading.
• Use most of the space provided.
• Ruled label lines that touch the structure and do not cross (a standard marking convention).

The same rules apply when drawing from a photomicrograph.

1.1.3 Magnification and actual size

Magnification is how many times larger the image is than the real object:

magnification = image size / actual size
actual size   = image size / magnification
image size    = actual size x magnification

Magnification has no unit and is written with a times sign, such as ×4000. Image size and actual size must be in the same unit before you divide: images are measured in mm, cells and organelles quoted in µm or nm.

Units

1 mm = 1000 µm        1 µm = 1000 nm        1 mm = 1 000 000 nm

To go to a smaller unit, multiply by 1000. To go to a larger unit, divide by 1000. So 0.0035 mm = 3.5 µm = 3500 nm.

Worked example 1 – finding the magnification

On a transmission electron micrograph, a mitochondrion’s image is 54 mm long. Its actual length is 1.8 µm. Calculate the magnification.

image size  = 54 mm = 54 x 1000 = 54 000 µm
magnification = 54 000 / 1.8 = 30 000

Magnification = ×30 000.

Worked example 2 – using a scale bar

Many micrographs show a scale bar instead of a magnification. A photomicrograph of leaf cells has a scale bar labelled 5 µm. The bar measures 20 mm with a ruler. A chloroplast in the image is 38 mm long.

Step 1 – magnification from the scale bar:

magnification = 20 mm / 5 µm = 20 000 µm / 5 µm = 4000

Step 2 – find the actual size:

actual length = 38 000 µm / 4000 = 9.5 µm

Actual length = 9.5 µm. A faster check uses proportion: the chloroplast is 38/20 = 1.9 times as long as the bar, and 1.9 × 5 µm = 9.5 µm.

Worked example 3 – scanning electron micrograph

A scanning electron micrograph of a pollen grain is labelled ×1200. The grain’s image is 66 mm across.

actual diameter = 66 000 µm / 1200 = 55 µm

Actual diameter = 55 µm. The method is identical for SEM, TEM, photomicrographs and drawings.

Worked example 4 – magnification of your own drawing

You measure a cell with a graticule as 32 µm wide and draw it 80 mm wide.

magnification of drawing = 80 000 µm / 32 µm = 2500

Drawing magnification = ×2500, not the microscope’s ×400: a drawing has its own magnification.

Significant figures

A calculated value should have the same number of significant figures as, or one more than, the smallest number in the data used. A 2 s.f. measurement such as 38 mm supports a 2 or 3 s.f. answer, so 9.5 µm is right.

1.1.4 Eyepiece graticule and stage micrometer

An eyepiece graticule is a scale of equal divisions inside the eyepiece. The divisions (“eyepiece units”) have no fixed value: what one unit covers depends on the objective in use.

A stage micrometer is a slide with a scale of known length engraved on it. For example, a 1 mm line divided into 100 divisions makes each division 0.01 mm = 10 µm. Always read the value given in the question.

Calibrating

1. Put the stage micrometer on the stage and focus on its scale.
2. Rotate the eyepiece so the two scales lie side by side, parallel.
3. Line up the zero of the graticule with a line on the micrometer.
4. Find a point further along where a graticule line and a micrometer line coincide exactly (a long stretch reduces reading error).
5. Calculate the length of one eyepiece unit.
6. Remove the stage micrometer and replace it with the specimen. Measure the specimen in eyepiece units and convert.

Recalibrate for every objective. The graticule sits in the eyepiece, so its divisions stay the same size in your view while the specimen is magnified more or less.

Worked example 5 – calibrating at two magnifications

Stage micrometer divisions are each 10 µm.

Low power (×100): 20 eyepiece units coincide with 20 micrometer divisions.

20 divisions = 20 x 10 = 200 µm
1 eyepiece unit = 200 / 20 = 10 µm

An onion epidermal cell is 22 eyepiece units long, so its length is 22 × 10 = 220 µm.

High power (×400): 32 eyepiece units coincide with 8 micrometer divisions.

8 divisions = 8 x 10 = 80 µm
1 eyepiece unit = 80 / 32 = 2.5 µm

A cheek cell spans 24 eyepiece units, so its diameter is 24 × 2.5 = 60 µm.

Check: magnification rose four times, and one eyepiece unit fell to a quarter (10 µm to 2.5 µm), as expected.

Measure several cells and calculate a mean, as one cell may not be typical. Choose the unit that gives a sensible number: µm for whole cells, nm for ultrastructure such as ribosomes, mm for tissue layers at low power.

1.1.5 Resolution and magnification

Magnification: the number of times larger an image is than the actual size of the object.

Resolution: the ability to distinguish two separate points as separate. It is often given as the smallest distance between two points that can still be seen as two.

Magnification makes an image bigger; resolution decides how much detail it contains. Magnifying beyond the limit of resolution gives a larger image that is no sharper: two points closer than the limit still appear as one blur.

Why the light microscope has a limit

Resolution is limited by the wavelength of the radiation used. Visible light has wavelengths of about 400-700 nm. The best resolution of a light microscope is about 200 nm (0.2 µm). Objects smaller than this, or closer together than this, cannot be distinguished.

That explains what you can and cannot see:

• Nuclei, chloroplasts and whole cells are well above 200 nm, so they are visible.
• Mitochondria are about 0.5-1 µm wide. They can show as small dots, but their internal membranes (cristae) are far too close together to resolve.
• Ribosomes (about 20-30 nm) and the cell surface membrane (well under 200 nm thick) are below the limit, so the light microscope cannot show them.

Electron microscopes

An electron beam has a much shorter wavelength than light, so electron microscopes have a much better resolution and can show the ultrastructure of cells, including ribosomes and membranes. The beam is focused by electromagnets and travels in a vacuum, because air molecules would scatter the electrons, so specimens must be dead and processed. Images are black and white; any colour is added afterwards.

	Light microscope	Transmission EM	Scanning EM
Radiation	Light	Electrons	Electrons
Resolution	About 200 nm	Best of the three	Better than light, not as good as TEM
Specimens	Living or dead	Dead, very thin sections	Dead, whole surface
Image	Colour, 2-D	2-D section; internal structure	3-D appearance of surfaces

In a TEM, electrons pass through a very thin section; denser, stained parts appear darker. In an SEM, the beam scans the surface, giving a three-dimensional view of structures such as pollen grains. The electron microscope’s real advantage is higher resolution; extra magnification alone would show no extra detail.

Common errors

• Dividing mm by µm without converting first. Convert both to one unit before you divide.
• Giving magnification a unit (writing “×4000 µm”).
• Quoting the microscope’s magnification for a drawing.
• Using a graticule calibration from one objective after switching to another.
• Saying an electron microscope is better “because it magnifies more”. Link it to resolution and wavelength.
• Shading, sketchy double lines, or drawing organelles that are not visible on the slide.
• Drawing cells inside a plan diagram.

Where next

• Revision notes for 1.1 for a short summary and quick self-test.
• Practice questions for 1.1 with fully marked answers.
• Cell structure revision notes and practice questions for all of topic 1.
• The 9700 course hub, the printable 9700 checklist and the free 9700 AS diagnostic.

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.