1.1.3Cell Level Systems

Electron Microscopes

5 min read 9 parts Lesson 4 of 13 1 quiz

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Section 1 of 9The need for greater resolving power

Plant Cell — Light vs Electron Microscope

Plant cell under light microscope vs electron microscope

The light microscope is limited by the wavelength of visible light, so it cannot resolve objects smaller than about 0.2 micrometres.

Structures such as ribosomes, the internal membranes of mitochondria and the details of the nucleus are smaller than this.

Electron microscope

In the 1930s the electron microscope was invented.

It uses a beam of electrons instead of light.

Electrons have a much shorter wavelength than visible light,

which allows far greater resolution, and magnifications of more than ×1 000 000 are possible.

Section 2 of 9Two types

Coloured TEM of a plant cell with organelles labelled

Electron micrograph of plant cells with labels

Chloroplast electron micrograph (coloured)

A transmission electron microscope (TEM) passes electrons through a very thin slice of specimen.

Two types

Denser parts absorb or scatter more electrons and appear darker,

giving a 2-D image of the internal structure of cells, such as the details of organelles.

Scanning electron microscope (SEM)

A scanning electron microscope (SEM) bounces electrons from the surface of the specimen, giving a detailed 3-D view of its surface.

In both cases, the electrons are focused using electromagnets, and the image is displayed on a screen or photographed,

producing a black and white image (an electron micrograph) that may be coloured by computer afterwards.

Section 3 of 9Comparison with the light microscope

Comparison of electron and light microscopes

Examples of Microscope Specimen Stains — Simulation

Organelles: light vs electron microscope

Microscope Slide Preparation — Step-by-Step Simulation

Feature	Light microscope	Electron microscope
Radiation used	Visible light	Beam of electrons
Maximum magnification	About ×1500	More than ×1 000 000
Resolution	About 0.2 micrometres	About 0.1 nanometres (TEM)
Specimen	Can be living	Dead, in a vacuum
Image	Colour (with stains)	Black and white
Cost and size	Cheap, portable	Very expensive, large, needs training

The specimen must be dead for an electron microscope because the electrons only travel through a vacuum, and the preparation, which includes fixing, dehydrating and staining with heavy metals, can create artefacts: structures in the image that were not present in the living cell.

Section 4 of 9What electron microscopy revealed

Organelles visible under light vs electron microscope

Key Structures of the Respiratory System

Identifying cell structures under a microscope

Electron microscopes have allowed biologists to see sub-cellular structures in detail.

What electron microscopy revealed

They showed the internal folds of mitochondria (cristae) where respiration takes place, the stacks of membranes in chloroplasts, ribosomes, the nuclear envelope with its pores,

and the structure of bacteria and viruses.

They also showed the surface of structures such as pollen grains, insect eyes and cilia.

This understanding of the organelles helped to explain how each cell works, for instance how the folded inner membrane of a mitochondrion gives a large surface area for respiration.

Section 5 of 9Calculations with scale

For electron micrographs, the units are often micrometres or nanometres. 1 mm = 1000 µm and 1 µm = 1000 nm.

Calculations with scale

When a scale bar is given, measure the bar in the image, work out its real length,

and use it to calculate the magnification: magnification = measured length of scale bar / real length it represents.

Then use image size / magnification to find actual sizes.

Always write the units, and standard form is useful for very small values, for example 0.0000025 m is 2.5 × 10^-6 m.

Section 6 of 9Worked example: reading a micrograph

Mitochondrion electron micrograph (coloured)

Suppose a TEM image shows a chloroplast and a scale bar of 20 mm that represents 2 micrometres.

Worked example: reading a micrograph

The magnification is 20 mm / 0.002 mm = ×10 000.

If the chloroplast in the image is 50 mm long, then its actual length is 50 / 10 000 = 0.005 mm = 5 micrometres.

This kind of two-step calculation, finding the magnification from the scale bar and then using it, appears often.

When describing what you can see on a micrograph, name the organelles by their features: a mitochondrion appears as a small oval with folded inner membranes; a nucleus is a large round structure with a double membrane and pores; ribosomes are tiny dark dots; chloroplasts have stacks of membranes; and a cell wall appears as a thick line outside the membrane in plant cells.

Applications also extend beyond cellsSEM images are used to study the structure of insects, pollen and fibres, while TEM is used to study viruses and the details of the cell membrane.

Worked example: reading a micrograph

Both are used in medical diagnosis, for instance identifying a virus in a patient sample.

The limits to bear in mind are cost, specimen preparation and the risk of artefacts,

which is why electron microscopy results are checked against other methods.

Section 7 of 9Exam-style questions

Scanning electron micrograph of a spiracle surrounded by hairs

An electron micrograph shows a mitochondrion that measures 45 mm long. The magnification is ×30 000. Calculate the actual length in micrometres. [3]3 marks
Explain why electron microscopes have been able to show more detail of cell structures than light microscopes. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Electron microscope	A microscope that uses a beam of electrons to give high magnification and resolution.
TEM	Transmission electron microscope, which shows thin sections in 2-D.
SEM	Scanning electron microscope, which shows the surface in 3-D.
Resolution	The ability to see two close points as separate.
Wavelength	The distance between two successive peaks of a wave.
Artefact	A feature on an image created by the preparation, not present in the living specimen.
Nanometre	One billionth of a metre (1 nm = 0.001 micrometre).

Section 9 of 9Exam technique

Saying electron microscopes have greater magnification but forgetting to say that the key advantage is resolution.

Saying electron microscopes can view living specimens. The specimen must be in a vacuum, so is dead.

Forgetting to convert units (mm to micrometres) in magnification questions.

Saying electron micrographs show true colours; they are black and white unless computer-coloured.

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.