Cambridge International AS & A Level Biology 9700: The microscope in cell studies – Revision Notes

These notes condense sub-topic 1.1 The microscope in cell studies from 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, examined on Papers 1 and 2 and assumed on Paper 4; the light-microscope skills also appear in the Paper 3 practical. There are no tiers in 9700. For full explanations and worked examples, use the 1.1 study guide.

For the rest of topic 1 (organelles, prokaryotes, viruses), see the Cell structure revision notes. When you are ready, try the 1.1 practice questions.

The five learning outcomes in one line each

1. 1.1.1 Make a temporary preparation (stained wet mount) for the light microscope.
2. 1.1.2 Draw cells from slides and photomicrographs.
3. 1.1.3 Calculate magnification and actual size from drawings, photomicrographs, TEM and SEM micrographs.
4. 1.1.4 Calibrate and use an eyepiece graticule with a stage micrometer; use mm, µm and nm.
5. 1.1.5 Define resolution and magnification; explain the difference for light and electron microscopes.

Definitions to learn word for word

Term	Definition
Magnification	How many times larger the image is than the actual size of the object
Resolution	The ability to distinguish two points that are close together as separate
Eyepiece graticule	A scale in the eyepiece with equal divisions of no fixed value
Stage micrometer	A slide carrying a scale of known length, used to calibrate the graticule
Photomicrograph	A photograph taken through a light microscope
Electron micrograph	An image from an electron microscope (TEM or SEM)

Formulas and units

You want	Use
Magnification	image size ÷ actual size
Actual size	image size ÷ magnification
Image size	actual size × magnification
Magnification from a scale bar	measured length of bar ÷ length written on bar
Value of one eyepiece unit	(micrometer divisions × value of one division) ÷ eyepiece units that match
Total magnification of microscope	eyepiece × objective

1 mm = 1000 µm      1 µm = 1000 nm      1 mm = 1 000 000 nm
smaller unit: multiply by 1000      larger unit: divide by 1000

Magnification has no unit. Write it as ×8000, not “8000 µm”.

Method boxes

Magnification or actual size

1. Measure the image with a ruler in mm.
2. Convert to the unit of the other quantity (usually µm). Multiply mm by 1000.
3. Substitute into the formula.
4. Give the answer in a sensible unit, to the right significant figures.

Reminder: a nucleus on a TEM micrograph labelled ×14 000 measures 84 mm. 84 mm = 84 000 µm. 84 000 ÷ 14 000 = 6 µm.

Scale bar

1. Measure the scale bar in mm and convert to the bar’s unit.
2. Magnification = measured bar ÷ bar value.
3. Measure the object and divide by that magnification.
4. Shortcut: actual size = (object length ÷ bar length) × bar value.

Graticule calibration

1. Stage micrometer on the stage; line up the zeros of the two scales.
2. Find where two lines coincide exactly, as far along as possible.
3. One eyepiece unit = total micrometer length ÷ number of eyepiece units.
4. Swap in the specimen, count eyepiece units, multiply.
5. Recalibrate after changing the objective. Quadruple the magnification and one eyepiece unit covers a quarter of the distance.

Temporary preparation

1. Thin layer of tissue (for example, onion epidermis) flat in a drop of stain on a slide.
2. Iodine in potassium iodide solution or methylene blue makes nuclei and cell outlines visible.
3. Lower the coverslip at an angle with a mounted needle to avoid air bubbles.
4. Blot off excess stain.
5. Focus at low power first, then high power with fine focus only.

Drawing checklist

Plan diagram (low power): outlines of tissue layers only, no cells, correct proportions of the layers.

High-power drawing (cells):

• correct shapes and relative sizes;
• cell walls with two lines, or three lines where two cells touch;
• only visible contents, such as a nucleus you can actually see;
• sharp pencil, continuous lines, no shading, most of the space used;
• ruled label lines that do not cross.

A drawing has its own magnification: drawing size ÷ actual size. It is not the ×100 or ×400 of the microscope.

Light versus electron microscopes

	Light	TEM	SEM
Radiation	Visible light (about 400-700 nm)	Electron beam	Electron beam
Best resolution	About 200 nm	Highest	High, below TEM
Specimen	Living or dead	Dead, ultra-thin section, in a vacuum	Dead, surface, in a vacuum
Image	Colour; 2-D	Black and white; 2-D internal detail	Black and white; 3-D surface
Shows ribosomes inside cells?	No	Yes	No (surfaces only)

Why electrons resolve more: resolution is limited by wavelength. The electron beam has a much shorter wavelength than light, so points much closer together can be distinguished.

Why a vacuum: air molecules would scatter the electrons. So no living specimens.

Empty magnification: enlarging beyond the resolution limit gives a bigger, equally blurred image with no new detail.

Must-know distinctions

• Magnification vs resolution. One is size; the other is detail. You can have high magnification with poor resolution.
• Eyepiece graticule vs stage micrometer. The graticule stays in the eyepiece and is used for measuring. The micrometer is a known scale used only for calibrating, then removed.
• Microscope magnification vs drawing magnification. The first is set by the lenses; the second by how big you draw.
• TEM vs SEM. TEM: beam passes through a thin section, internal structure. SEM: beam scans the surface, 3-D appearance.
• Plan diagram vs cell drawing. Plan: tissues, no cells. Cell drawing: individual cells and visible contents.

Size scale for sense-checking answers

• Typical plant and animal cells: tens of µm (onion epidermal cells are longer, up to a few hundred µm).
• Nucleus: a few µm to around 10 µm.
• Chloroplast: a few µm long.
• Mitochondrion: about 0.5-1 µm wide.
• Ribosome: about 20-30 nm.
• Light microscope limit: about 200 nm.

If your answer says a mitochondrion is 1.8 mm long, you have missed a conversion.

Quick self-test

1. Convert 0.42 mm to µm.
2. Convert 7.5 µm to nm.
3. An image 27 mm long is at ×9000. Calculate the actual length in µm.
4. A vesicle 250 nm across has an image 10 mm across. Calculate the magnification.
5. A scale bar labelled 2 µm measures 16 mm. A structure on the same image measures 44 mm. Find the magnification and the actual length.
6. 50 eyepiece units line up with 12 stage micrometer divisions of 10 µm each. Find the value of one eyepiece unit, and the length of a cell spanning 15 eyepiece units.
7. Define resolution.
8. Explain why ribosomes cannot be seen with a light microscope.
9. A microscope has a ×10 eyepiece and a ×40 objective. What is the total magnification?
10. Which type of microscope gives a three-dimensional image of a surface?
11. State why a temporary preparation is stained.
12. State two features that must not appear in a plan diagram.

Answers

1. 0.42 × 1000 = 420 µm.
2. 7.5 × 1000 = 7500 nm.
3. 27 mm = 27 000 µm; 27 000 ÷ 9000 = 3 µm.
4. 10 mm = 10 000 000 nm; 10 000 000 ÷ 250 = ×40 000.
5. 16 mm = 16 000 µm; 16 000 ÷ 2 = ×8000. 44 000 ÷ 8000 = 5.5 µm (check: 44 ÷ 16 × 2 = 5.5).
6. 12 × 10 = 120 µm; 120 ÷ 50 = 2.4 µm per eyepiece unit. 15 × 2.4 = 36 µm.
7. The ability to distinguish two points close together as separate.
8. Ribosomes (about 20-30 nm) are smaller than the light microscope’s resolution limit of about 200 nm, which is set by the wavelength of light.
9. 10 × 40 = ×400.
10. The scanning electron microscope.
11. Most cell structures are nearly colourless; the stain makes nuclei and cell outlines visible.
12. Individual cells; shading (also: organelles).

Where marks are usually lost

• Dividing a measurement in mm by one in µm without converting.
• Adding a unit to magnification.
• Measuring the scale bar or image carelessly, for example from the wrong end of the ruler.
• Using a low-power calibration after switching to the ×40 objective.
• Saying electron microscopes are better because they “magnify more”, with no mention of resolution or wavelength.
• Defining resolution as “how clear the image is” instead of distinguishing two points as separate.
• Drawing cells in a plan diagram, shading, or adding textbook organelles that cannot be seen.
• Single-line cell walls in a plant tissue drawing.
• Giving eight significant figures copied from the calculator. Use the same number as the data, or one more.
• Forgetting that TEM and SEM specimens must be dead.

Next steps

• Full teaching and worked examples: 1.1 study guide.
• Marked practice: 1.1 practice questions and the topic-level Cell structure practice questions.
• The 9700 course hub, the printable 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.