A signal cell may constitute a whole organism such as bacteria, amoeba and Chlamydomonas. The bodies of fungi, plants and animals are made of millions of cells packed together. The shape and size of cells, in fact , are related to specific function they perform, while number (of cells) is to increase the size of the organ/body Thus small organisms have limited number of cells; larger ones such as elephant, have a countless number of cells.
In man, the number of cells is estimated to be about 100 trillion (10 ). Based on the observations of Robert Hooke and their own observations, two German biologists M Scheiden and T Schwann proposed the cell theory in 1838-39. They postulated that all organisms were made up of cells and that the cell was the basic unit of structure and function of all organisms.
The cell is made up of “life” giving substance called protoplasm. Protoplasm is an aggregate of molecules of various chemicals. Most of these are organic molecules such as proteins, carbohydrates, fats, nucleic acids, etc