The development of microscopes

Throughout their development, the magnification of light microscopes has increased, but very high magnifications are not possible. The maximum magnification with a light microscope is around ×1,500. This means that the microscopes can make the image look 1,500 times bigger than the actual object.

The limits of the light microscope

The magnification of a microscope is not the only factor that's important when viewing cells. The detail that can be seen is also important.

The ability to see greater detail in an image depends on the resolution or resolving power. This is the ability to see two points as two points, rather than merged into one.

Think about a digital photo. It can be enlarged, but over a certain size, you won't be able to see any more detail. It will just become blurry.

The resolution of a light microscope is around 200 nm. This means that it cannot distinguish two points closer than 200 nm. One nm, or nanometre, is one billionth of a metre.

The electron microscope

Electron microscopes use a beam of electrons instead of light rays.

There are two types of electron microscope:

  1. The scanning electron microscope (SEM) has a large depth of field so can be used to examine the surface structure of specimens. SEMs are often used at lower magnifications.
  2. The transmission electron microscope (TEM) is used to examine thin slices or sections of cells or tissues.

TEMs have a maximum magnification of around ×1,000,000, but images can be enlarged beyond that photographically. The limit of resolution of the TEM is now less than 1 nm.

The TEM has revealed structures in cells that are not visible with the light microscope.

Visualising sub-cellular structures

Structures that are found inside a cell, such as the nucleus, are called sub-cellular structures. If the sub-cellular structure has a membrane surrounding it then it is also known as an organelle. Some sub-cellular structures can be studied easily using a light microscope because they are quite large, for example the nucleus inside an animal or plant cell. However, there are many more sub-cellular structures in animal and plant cells and most of these cannot be viewed using a light microscope because the magnification and resolution of the light microscope is too low.

Once the electron microscope had been developed, scientists could find out much more about sub-cellular structures. Often, the more scientists know about the structure of something, the more they can work out about how that thing functions. The development of the electron microscopes therefore helped scientists to learn about the sub-cellular structures involved in aerobic respiration called mitochondria. The scientists developed their explanations about how the structure of the mitochondria allowed it to efficiently carry out aerobic respiration.

Another organelle that has been studied in much more detail since the development of the electron microscope is the chloroplast. Chloroplasts are found in plant cells and are used by the plant to carry out photosynthesis so that the plant can make its own source of glucose to be used in respiration. Scientists have much more knowledge and understanding of how a chloroplast can carry out photosynthesis because they have been able to use electron microscopes to look at them in lots of detail due to the high magnifying power of an electron microscope.

Basic plant cell as seen under two different types of microscope