All living organisms are composed of Cells. Cell is the basic fundamental structural and functional unit of living organism.
1. When Robert Hooke (in 1665), examined a thin slice of cork under the primitive microscope, he observed that cork (from the bark of trees) was made up of small honeycomb-like structures. He called these structures as cells. Basic characteristics of cells are as follows:
(a) Cell can replicate independently.
(b) Cell contain hereditary information.
(c) Cell can perform all the life-sustaining activities on its own.
(d) All cells show similar chemical composition and metabolic activities.
2. Cell Theory
It was given by Schleiden and Schwann. Salient Features of Cell Theory are: –
(a) All animals and plants are made up of cells and cell products. Some organisms are unicellular and some are multi-cellular.
(b) A cell is the structural and functional unit of all living organisms.
(c) New cells come from the divisions of the pre-existing cell. (given by Rudolph Virchow)
3. Cellular Composition in Organisms
On the basis of the number of cells present in different organisms, they are classified into two types:
(a) Unicellular organisms (having a single cell)
(b) Multi-cellular organisms (having many cells)
4. Division of Labour
In multi-cellular organisms, different organs like heart pumps the blood, kidney filters the blood, stomach digests the food. This phenomenon is called as “Division of Labour”.
5. Differences between Unicellular and Multi-cellular Organisms
It is an optical instrument used for viewing very small objects, such as mineral samples or animal or plant cells, typically magnified several hundred times.
7. Shape of Cells
Cells can take a variety of shapes. For example elliptical (e.g. fat cell), spherical (e.g. ovum), spindle-shaped (e.g. smooth muscle cell), knobbed thread (e.g. sperm),· discoidal (e.g. RBC), elongated (e.g. nerve cell), etc.
8. Size of Cells
The size of a cell varies widely from the smallest cell of Mycoplasma (0.1-0.5 m) to very large egg cells of the Ostrich (18 cm). The longest cell in the human body is the nerve cell reaching up to 90 cm.
9. Functions of Cells
Cells carry out the following functions : –
(i) Provide Structure and Support
(ii) Facilitate Growth
(iii) Allow Passive and Active Transport
(iv) Produce Energy
(v) Create Metabolic Reactions
(vi) Aids in Reproduction
[Source: https://sciencing.com/six-main-cell-functions-6891800.html ]