INTRODUCTION
Each of the three natural sciences has a basic unit which is the fundamental unit with which each science is concerned.
Thus all thing in the physical sphere are made up of atoms. All things in the chemical sphere are made of molecules, and all things in the biological sphere are made of cells. This does not mean that each sphere does not examine the components of their units. For instance, physics not only considers objects made of atoms but also considers the interior structure of atoms such as protons, neutrons and electrons. Chemistry considers the electronic structure of molecules (which can also include single atoms). And biology is also concerned with the macromolecules that exist in the structures that exist within cells.
CELLS
All living things are made of cells. A biological cell is thus the basic unit of all life. It is bound by a membrane and contains all the fundamental molecules of life. Life consists of both unicellular organisms which are single cells, and also groups of cells which form a multicellular organisms. Examples of unicellular organisms are bacteria, achaea and yeasts. Examples of multicellular organisms include plants and animals.
Most cells can only be seen under a microscope. They were first seen by Robert Hooke in 1665. A theory of cells however, was not developed until 1839 by Matthias Schleiden and Theodor Schwann. Their theory stated that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all organisms, and that all cells come from pre-existing cells. This thus separates life sciences from physicals sciences.
Cell sizes range from about 2 to 200 microns (micrometres). Bacteria are at the lower end of the scale whereas plant and animal cells range from about 10 to 100 microns.
When an animal starts to grow all the cells are stem cells. These cells then differentiate into cells with specialist functions to make the various system that will eventually comprise the animal.
Adult humans consist of around 50 trillion (5 x 1013) cells and these can be broken down into about 200 different types of cells. The smallest cells are the spermatazoa and the largest are the ova.
Australian Space Academy