7.1.1Practical Skills

Practical: Using Light Microscopes to View Cells

5 min read 8 parts Lesson 2 of 11 1 quiz

0%read

Section 1 of 8Why we use light microscopes

Plant cell under light microscope vs electron microscope

Plant Cell — Light vs Electron Microscope

Organelles: light vs electron microscope

Examples of Microscope Specimen Stains — Simulation

Plant cell under light vs electron microscope

Most cells are far too small to see with the naked eye.

Light microscope

A light microscope passes light through a thin specimen and uses glass lenses to magnify the image.

It lets us see the nucleus, cytoplasm and cell wall of plant cells, and the chloroplasts and large vacuole in them.

Resolution

It cannot show fine detail such as ribosomes or the internal folds of mitochondria, because its resolution is limited.

Resolution is the smallest distance between two points that can still be seen as separate.

Electron microscopes have much better resolution, but they cannot be used on living specimens and are far more expensive.

Magnification and resolution are different ideas. Making an image bigger (magnification) does not make it clearer unless the resolution is high enough to show more detail.

Section 2 of 8Parts of the microscope and what they do

Part	Job
Eyepiece lens	Lens you look through; usually x10
Objective lenses	Lenses of different power (for example x4, x10, x40) on a rotating nosepiece
Stage and clips	Hold the slide still
Light source or mirror	Shines light up through the specimen
Coarse focus wheel	Moves the stage or lens a large distance to find the image
Fine focus wheel	Small movements to sharpen the image

Total magnification = eyepiece magnification x objective magnification. A x10 eyepiece with a x40 objective gives x400.

Section 3 of 8Method: preparing and viewing a slide

A typical specimen is a thin layer of onion epidermis stained with iodine solution.

Place a drop of water or stain on the centre of a clean slide.

Use forceps to peel a very thin, single-cell layer of tissue from the inside of an onion scale leaf and lay it flat in the drop.

Lower a coverslip at an angle with a mounting needle. This pushes out air bubbles that would block the view.

Blot away excess liquid with paper towel.

Lowest-power objective

Clip the slide on the stage and select the lowest-power objective.

Looking from the side, turn the coarse focus so the objective is close to the slide but not touching it.

Then look through the eyepiece and focus slowly, moving the lens away from the slide.

Sharpen with the fine focus. Move up to a higher-power objective only once the image is clear, and refocus using the fine focus only.

Starting on low power gives the widest field of view, so the specimen is easy to find. Focusing by moving the lens away from the slide avoids cracking the coverslip. Staining adds contrast because most cells are almost transparent.

Common problems

air bubbles look like dark-edged circles and should not be drawn;
- a specimen that is too thick lets too little light through, so the cells overlap and the image is blurred;
- and a dry slide shrinks the cells.
Add a drop of water if the mount dries out.

Section 4 of 8Drawing the specimen

A good biological drawing is a scientific record, not a work of art.

Use a sharp pencil and draw clear, continuous lines. No shading and no colouring in.

Draw only what you see, in proportion, and make it fill at least half the space.

Label with straight lines that do not cross, ruled from the structure to the label.

Give the drawing a title and record the total magnification used.

Draw a few cells carefully rather than a whole field of view.

Section 5 of 8Calculating magnification and actual size

Eyepiece Graticule Calibration — Worked Example

Magnification calculation: leaf cross-section

Measuring onion epidermal cells using a calibrated eyepiece graticule

Eyepiece graticule and stage micrometer; calibration example: 100 µm divided by 40 divisions gives 2.5 µm per division

Actual size equals image size divided by magnification.

The key equation links three quantities:

magnification = image size / actual size

magnification = image size / actual size

Rearranged: actual size = image size / magnification, and image size = magnification x actual size.

Always measure with a ruler in millimetres and convert to the unit you need.
Units of length1 mm = 1000 micrometres so multiply by 1000 to go from mm to um.

Worked idea: a cell measures 30 mm across on your drawing, and the magnification of the drawing is x400.

The actual width is 30 / 400 = 0.075 mm, which is 75 um.
A frequent exam skill is working with scale bars: measure the bar, work out what each mm represents, then measure the structure.

This practical links to cell structure (animal and plant cells) and to why specialised cells have particular features,

because microscopes are how those features are observed.

Section 6 of 8Exam-style questions

A student views onion cells using a x10 eyepiece lens and a x40 objective lens. (a) Calculate the total magnification. [1] (b) The image of one cell is 24 mm long in the student's drawing, which has the same magnification. Calculate the actual length of the cell in micrometres. [3]3 marks
Describe how you would use a light microscope to focus on a prepared slide, and explain why you begin with the lowest-power objective. [4]4 marks
Clip the slide on the stage and rotate the nosepiece to the lowest-power objective [1].
Exam-style questions
Use the coarse focus to bring the image into view, then the fine focus to make it sharp [1].
Only then switch to a higher-power objective and refocus with the fine focus [1].
The low-power lens has a larger field of view,
so the specimen is easier to find, and the lens is further from the slide so there is less risk of damaging the slide or lens [1].
A student's drawing of a plant cell was criticised for three reasons. Suggest three improvements. [3]3 marks

Section 7 of 8Key terms

Term	Meaning
Magnification	How many times larger an image appears than the real object.
Resolution	The smallest distance between two points that can still be seen as separate.
Field of view	The area of the specimen that can be seen through the microscope at one time.
Objective lens	One of the lenses on the nosepiece, nearest the specimen, with different magnifying powers.
Eyepiece lens	The lens at the top of the microscope that you look through.
Coverslip	A thin square of glass or plastic placed over the specimen to flatten it and protect the lens.
Stain	A dye added to a specimen to make structures show up more clearly.
Micrometre (um)	A unit of length equal to one thousandth of a millimetre.

Section 8 of 8Exam technique

Confusing magnification with resolutiona higher magnification does not always give a clearer picture, because resolution limits the detail visible.

Forgetting to multiply the eyepiece and objective magnifications; the total magnification is both together, not just the objective.

Mixing units in the magnification equation. Convert mm to um (x1000) before or after, and state the final unit.

Using the coarse focus on high power, which can break the slide and scratch the lens; use only the fine focus once on higher power.

Shading or colouring a biological drawing and drawing label lines that cross each other.

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.