Microscopy

This lesson covers microscopy as required by AQA GCSE Biology specification 4.1.1, including the Required Practical: Using a light microscope to observe, draw, and label a selection of plant and animal cells. You need to understand the different types of microscope, calculate magnification, and describe how to prepare and observe microscope slides.

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Types of Microscope

There are two main types of microscope you need to know about: light microscopes and electron microscopes.

Light Microscopes

Light microscopes use visible light and glass lenses to magnify specimens. They were first developed in the 1590s by Hans and Zacharias Janssen, and have been refined over centuries. Robert Hooke used a light microscope to observe cells for the first time in 1665.

Feature	Detail
Maximum magnification	About x1500
Maximum resolution	About 200 nm (0.2 um)
Type of image	Can view living or dead specimens, in colour
Specimen preparation	Thin sections, often stained with dyes
Cost	Relatively inexpensive
Size	Portable — can be used in classrooms and field work

Electron Microscopes

Electron microscopes use beams of electrons instead of light to form an image. They were invented in the 1930s and have much higher magnification and resolution than light microscopes.

There are two types of electron microscope:

Type	Description
Transmission Electron Microscope (TEM)	Passes electrons through a thin specimen. Produces a 2D image with extremely high resolution. Used to see internal cell structures.
Scanning Electron Microscope (SEM)	Bounces electrons off the surface of a specimen. Produces a 3D image of the surface. Lower resolution than TEM but shows surface detail.

Comparison Table: Light vs Electron Microscopes

Feature	Light Microscope	Electron Microscope
Maximum magnification	x1500	x2,000,000 (TEM)
Maximum resolution	200 nm	0.1 nm (TEM)
Radiation used	Visible light	Beams of electrons
Specimen	Living or dead	Dead only (vacuum required)
Image colour	Colour possible	Black and white (can be artificially coloured)
Cost	Low	Very expensive
Size	Small, portable	Large, requires dedicated rooms
Preparation time	Quick	Time-consuming

Exam Tip: The key advantage of electron microscopes is their much higher resolution (not just magnification). Resolution is the ability to distinguish between two points that are close together. Higher resolution means you can see finer detail. This is why electron microscopes allowed scientists to discover sub-cellular structures like ribosomes and the internal structure of mitochondria.

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Resolution vs Magnification

These two terms are often confused but mean different things:

• Magnification — how many times larger the image appears compared to the actual object.
• Resolution — the ability to distinguish between two separate points that are very close together. The smaller the distance that can be resolved, the higher the resolution.

Increasing magnification without improving resolution just makes a blurry image bigger. You do not gain more detail.

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The Magnification Formula

You must be able to use the magnification formula:

magnification = image size / actual size

This can be rearranged:

actual size = image size / magnification

image size = actual size x magnification

Worked Example 1

A cell has an actual diameter of 50 um. Under the microscope, the image of the cell is 25 mm across. What is the magnification?

Step 1: Convert to the same units. 25 mm = 25,000 um

Step 2: magnification = image size / actual size = 25,000 / 50 = x500

Worked Example 2

A drawing of a cell is 60 mm long. The magnification is x400. What is the actual size?

actual size = image size / magnification = 60 mm / 400 = 0.15 mm = 150 um

Exam Tip: Always check your units. Convert everything to the same unit (usually um) BEFORE doing the calculation. The most common mistake is dividing mm by um without converting first.

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Required Practical: Using a Light Microscope

This is one of the AQA required practicals. You must be able to describe the method, explain how to produce a clear image, and make a biological drawing.

Equipment Needed

• Light microscope
• Glass microscope slides and coverslips
• Dropping pipette
• Forceps (tweezers)
• Iodine solution (stain for plant cells — stains starch blue-black and cell walls yellow-brown)
• Methylene blue (stain for animal cells — stains the nucleus blue)
• Mounted needle
• Filter paper / blotting paper

Method: Preparing an Onion Epidermal Cell Slide

1. Peel a thin layer of epidermis (skin) from the inner surface of an onion scale using forceps.
2. Place the epidermis flat onto a clean glass microscope slide.
3. Add a drop of iodine solution using a dropping pipette. This stains the cells so they are visible.
4. Carefully lower a coverslip at an angle (using a mounted needle) to avoid trapping air bubbles.
5. Use filter paper to blot away any excess stain from around the edges of the coverslip.

Method: Viewing the Slide

1. Place the prepared slide on the stage and secure it with the stage clips.
2. Select the lowest power objective lens (e.g. x4).
3. Use the coarse focus wheel to bring the image roughly into focus. Always start by moving the lens away from the slide to avoid cracking it.
4. Once roughly focused, switch to a higher power objective lens (e.g. x10, then x40).
5. Use the fine focus wheel to sharpen the image.
6. Adjust the light source or mirror to ensure the specimen is well-lit.

flowchart TD
    A["Prepare specimen on slide"] --> B["Add stain (iodine or methylene blue)"]
    B --> C["Lower coverslip at an angle"]
    C --> D["Place slide on microscope stage"]
    D --> E["Start with lowest power objective"]
    E --> F["Use coarse focus to find specimen"]
    F --> G["Switch to higher power objective"]
    G --> H["Use fine focus for sharp image"]
    H --> I["Draw and label what you observe"]

    style A fill:#3498db,color:#fff
    style E fill:#e67e22,color:#fff
    style I fill:#27ae60,color:#fff

Calculating Total Magnification

The total magnification of a microscope is calculated by multiplying the eyepiece lens magnification by the objective lens magnification.

total magnification = eyepiece lens x objective lens

Eyepiece Lens	Objective Lens	Total Magnification
x10	x4	x40
x10	x10	x100
x10	x40	x400

Exam Tip: In the required practical, always start with the lowest power objective lens. This gives you a wider field of view to find the specimen. Then switch to higher magnification for more detail. If you start on high power, you may not be able to find the specimen at all.

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Making Biological Drawings

When drawing cells from a microscope, you must follow specific rules: