Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life. Hence, cell is the fundamental structural and functional unit of all living organisms.


Aristotle proposed that all animals and plants, however complicated but are constituted of a few elements which are repeated in each of them.

The simple microscope was invented by Galileo.

The first compound microscope was made by Robert Hooke (1665).

He examined thin slices of cork under his microscope and observed the honey comb like structures composed of box like compartments which were termed as the cellulae (cells). 

His work was published in his book "Micrographia". Cells were observed prior to Hooke by Malpighi (1661), who called them saccules and utricles.

Leeuwenhoek observed few living cells capable of moving, such as bacteria. protozoa, spermatozoa and red blood corpuscles under his own designed microscope.

Lamarck remarked that "no living being can have life if its constituent parts are not formed by cells". 
Dutrochet concluded that plants and animals were made up of globular cells and the cells are held together by cohesion. 

In 1831, Robert Brown discovered the presence of nucleus in the cells of orchid root. 
Fontana discovered nucleolus in the skin cell of Eel. 

The term nucleolus was given by Bowman.

Colloidal theory of protoplasm explains the nature of protoplasm in the best manner. It is most acceptable theory.

It was proposed by Fischer, according to which conversion of solution into gel and vice versa is due to colloidal nature of cytoplasm.

Presently, this can be better explained as "Multiphasic colloidal system of life".

Protoplasm theory was proposed by Max Shultze (1861).

According to it "cell is an accumulation of living substances which is limited by an outer membrane & possesses a nucleus".

M.J. Schleiden, a German botanist in 1838 stated that "All plants are formed of one or more cells".

Theodore Schwann, a German Zoologist in 1839 stated that "All animals are formed of cells, have nuclei and are enclosed by thin cell membrane instead of thick cell wall as found in plant cells".

Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and their products.

Rudolf Virchow in 1858 observed that new cells arise from pre-existing cells by division i.e., Omnis cellula e cellula.