A cell can be thought of as a bag in which the chemistry of life is allowed to occur, partially separated from the environment outside the cell.
The thin membrane which surrounds all cells is essential in controlling exchange between the cell and its environment. It is a very effective barrier, but also allows a controlled traffic of materials across it in both directions.
The membrane is therefore described as partially permeable. If it were freely permeable, life could not exist, because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion.
The study of cells has given rise to an important branch of biology known as cell biology. Cells can now be studied by many different methods, but scientists began simply by looking at them, using various types of microscope.
There are two fundamentally different types of microscope now in use: the light microscope and the electron microscope. Both use a form of radiation in order to create an image of the specimen being examined.
The light microscope uses light as a source of radiation, while the electron microscope uses electrons, for reasons which are discussed later.
The golden age' of light microscopy could be said to be the 19th century. Microscopes had been available since the beginning of the 17th century but, when dramatic improvements were made in the quality of glass lenses in the early 19th century, interest among scientists became widespread.
The fascination of the microscopic world that opened up in biology inspired rapid progress both in microscope design and, equally importantly, in preparing material for examination with microscopes. This branch of biology is known as cytology. Figure 1.3 shows how the light microscope works.
By 1900, all the structures shown in Figures 1.4 and 1.5 had been discovered. Figure 1.4 shows the structure of a generalised animal cell and Figure 1.5 the structure of a generalised plant cell as seen with a light microscope.
(A generalised cell shows all the structures that are typically found in a cell.) Figure 1.6 shows some actual human cells and Figure 1.7 shows an actual plant cell taken from a leaf.