● What makes an organism `color{violet}"living"`, or that which an `color{violet}"inanimate thing"` does not have which a living thing has is the presence of the `color{brown}"basic unit of life"` – the cell in all living organisms.

● All organisms are `color{violet}"composed of cells"`.

● Some are composed of a `color{violet}"lsingle cell"` and are called `color{violet}"unicellular organisms"` while others, like us, composed
of many cells, are called `color{violet}"multicellular organisms"`.

● `color{violet}"Unicellular organisms"` are capable of

(i) `color{violet}"Independent existence"`
(ii) Performing the `color{violet}"essential functions"` of life.

● Anything less than a `color{violet}"complete structure"` of a cell does not ensure `color{violet}"independent living"`.

● Hence, cell is the `color{violet}"fundamental"` `color{brown}"structural and functional unit"` of all living organisms.

● `color{brown}"Anton Von Leeuwenhoek"` first saw and described a `color{violet}"live cell"`.

● `color{brown}"Robert Brown"` later discovered the `color{violet}"nucleus."`

● The invention of the `color{violet}"microscope"` and its improvement leading to the `color{violet}"electron microscope"` revealed all the `color{violet}"structural details"` of the cell.

● In `color{brown}"1838, Malthias Schleiden"`, a German botanist, examined a large number of plants and observed that `color{violet}"all plants"` are composed of `color{violet}"different kinds of cells"` which form the tissues of the plant.

● At about the same time, `color{brown}"Theodore Schwann (1839)"`, a British Zoologist, studied different types of `color{violet}"animal cells"` and reported that cells had a thin outer layer which is today known as the `color{violet}"plasma membrane"`.

● He also concluded, based on his studies on plant tissues, that the presence of `color{violet}"cell wall"` is `color{violet}"a unique character"` of the plant cells.

● On the basis of this, `color{violet}"Schwann"` proposed the hypothesis that the bodies of `color{violet}"animals and plants"` are composed of `color{violet}"cells and products of cell"`s.

● `color{violet}"Schleiden and Schwann"` together formulated the `color{brown}"cell theory"`.

● This theory however, did not explain as to `color{violet}"how new cells"` were formed.

● `color{brown}"Rudolf Virchow (1855)"` first explained that `color{violet}"cells divided"` and new cells are formed from `color{violet}"pre-existing cells"` (`color{brown}"Omnis cellula-e cellula"`).

● He modified the hypothesis of `color{violet}"Schleiden and Schwann"` to give the cell theory a `color{violet}"final shape"`.

● `color{brown}"Cell theory"` as understood today is:

(i) `color{violet}"All living organisms"` `color{violet}"are composed of cells"` `color{violet}"and products of cells"`.

(ii) `color{violet}"All cells arise from"` `color{violet}"pre-existing cells"`.