Historical understanding of biological building blocks of living things : Cell Theory

How did the modern cell theory develop?

Today we take it for granted that living things are made of cells. Billions of cells in the case of human beings, just one cell in the case of organisms like amoeba. There is only one cell in Amoeba and in Vorticella.

It seems strange that the idea of organisms being made from cells is a relatively recent idea. Only a few hundred years ago, cells had not been discovered. Their discovery had to wait for the development of reliable microscopes that could magnify sufficiently to show the cellular structure of living organisms. Many biologists and other scientists contributed to the discovery of cells and the statement of the very fist cell theory. Th timeline below shows some of the major contributors.

A timeline for the development of the cell theory

1665 : Robert Hooke, with one of the earliest compound microscopes, makes drawings of cork and sees tiny structures that he calls ‘cells’. However, although his microscope is a compound microscope, the lenses are not very good and magnifications of more than 30× are very blurred and do not show much detail. Also, Hooke saw only dead cells.

1674 : Anton van Leeuwenhoek sees living, moving unicellular organisms (protoctistans) in a drop of water. He is using a simple microscope with only one lens. It is really little more than a magnifying glass with a mount for the specimens. However, van Leeuwenhoek is very skilled at grinding lenses and so his microscope can achieve magnifications of 300×. He calls the moving organisms ‘animalcules’. He also sees bacteria (from his teeth), which he also calls ‘tiny animalcules’.

A compound microscope is the sort of microscope we use in biology today. It has two lenses – the eyepiece and the objective lens – that combine to produce the final image. Because two lenses are used, compound microscopes are capable of higher magnifications than simple microscopes, which use only one lens. Th second lens (the eyepiece) magnifies the already magnified image produced by the objective lens. However, it also magnifies any ‘aberrations’ or faults in the image. So if the lenses are not well made, the final image, at high magnifications, will be blurred. Th fist compound microscope was made in 1595 by the Dutch scientist, Zaccharias Jansen.

1824 : The French biologist Rene Dutrochet concludes that all organisms are composed of cells. This follows many years work in which he also discovers:

• the stomata in the epidermis of leaves

• the process of osmosis

• chlorophyll is needed for photosynthesis to occur

• respiration occurs in both animals and plants

In many ways, Dutrochet is the man who fist states the cell theory by recognizing that all organisms are made of cells and that ‘all growth occurs because of the increase in volume of cells or by the addition of more little cells’.

1839 : Matthias Schleiden and Thodor Schwann put forward the fist clearly stated cell theory. It states that: • the cell is the unit of structure, physiology and organization in living things

• the cell retains a dual existence as: – a distinct entity, and – a ‘building block’ in the formation of organisms

• cells form by free-cell formation (spontaneous generation)

Although we still accept the fist two ideas, the final idea of spontaneous generation has now been proved false.

1858 : Rudolf Virchow, a German doctor who develops many surgical techniques and promotes several fields of modern medicine, declares that: ‘Omnis cellula e cellula’, which means that a cell can only arise from another cell like it. With this he completes the fist accepted version of the cell theory:

• all organisms are made up of one or more cells

• all cells come from per-existing cells

• the cell is the unit of structure, physiology and organization in living things

• the cell retains a dual existence as a distinct entity and a building block in the construction of organisms Today, this has been modified and extended in the light of our increased knowledge of genetics and cell biology and now reads:

• all known living things are made up of cells

• the cell is a structural and functional unit of all living things

• all cells come from per-existing cells by division (there is no spontaneous generation of cells)

• cells contain hereditary information which is passed from cell to cell during cell division

• all cells have basically the same chemical composition

• all energy flow (the metabolism and biochemistry of life) occurs within cells

Besides these major steps in the development of a cell theory, there have been other developments in the study of cell biology. Some of these are listed below

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