Seeing is believing

Cells were first seen over 300 years ago

No one knew that cells existed before the invention of the microscope. In 1665, Robert Hooke saw spaces in dead sections of cork that he called ‘cells’, and the Dutch pioneer Anthonie van Leeuwenhoek was astonished by living cells 20 years later. The insides of cells were observed much later, with more powerful microscopes.

When talking about how ‘powerful’ a microscope is, scientists usually talk about the magnification and resolution. But what do these mean? The magnification describes the ability of the microscope to make things look bigger. The resolution describes how well the image created allows distinction between two objects, so a higher resolution means we can see a greater amount of detail.

There are two main types of microscope used today to look at cells and their contents. Light microscopes are the simplest form of microscope, and they work by focusing light before passing it through the specimen and onto an objective lens. This magnifies the sample, which can then be viewed through an eyepiece. Specimens can be living or dead, and are often stained with colourful dyes to make the whole specimen – or certain components – visible.

Light microscopes typically have up to x1000 magnification, although some can go up to x2000, and a resolution of 200 nm. This means light microscopes can typically visualise cells and some larger bacteria.

To visualise smaller things, scientists use electron microscopes. Instead of light, these microscopes use beams of electrons that are focused using electromagnets and detected using a fluorescent screen or photographic film. Unlike with light microscopy, the specimens must be dead: the electron beam needs to be inside a vacuum and the specimens need to be stained with a heavy, electron-dense element, such as gold or a heavy metal.

Within electron microscopy, there are two ways of visualising samples. Transmission electron microscopy (TEM) uses the electron beam to pass through the specimen and creates a high-resolution 2D image – allowing visualisation of the inside of a cell. Scanning electron microscopy (SEM), as its name suggests, passes its electron beam over the surface of the specimen. The beam is reflected off the heavy-metal coating and gives a 3D image, which is usually lower in resolution than a TEM image.

Electron microscopes can be used to visualise all the way down to the atomic level, so they are often preferred over light microscopes when detailed information about the cell and its components is needed.

Size matters

Many people around the world use different units for different measurements, which gets a bit confusing. An international system of units (Système International d’Unités, or SI units) was developed so all scientists could record their data using the same units. All SI units are based on seven base units, each for a different measurement, and use prefixes to indicate multiplication or division by a power of ten.

For length, we use ‘metre’, and we have a range of prefixes to describe units smaller than this. In cell microscopy, most measurements are going to be far smaller than metres. These prefixes indicate a thousand-fold decrease in size: 1 millimetre (mm) is equal to one-thousandth of a metre (written as 10–3 m); 1 micrometre (μm) is equal to 1 millimetre divided by 1,000 (10–3 mm, or 10–6 m); and a nanometre (nm) is equal to 1 micrometre divided by 1,000 (10–3 nm, or 10–9 m). This makes it much easier to describe the very small parts of a cell. If we are talking about the width of the cell membrane, we can say it is 10 nm, rather than 0.00000001 m.

Lead image:

Robert Hooke, ‘Micrographia’, detail: microscope, 1665.

Wellcome Library, London

Further reading

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About this resource

This resource was first published in ‘The Cell’ in January 2011 and reviewed and updated in September 2015.

Cell biology, Medicine, History, Health, infection and disease
The Cell
Education levels:
16–19, Continuing professional development