- The cell is the basic structural and functional unit of life. All living organisms are made up of cells. Cells make
tissues, tissues make organs, organs make organ systems and organ systems make a living organism. Thus, the cell is the building block, or the structural unit of the living body.
- Cells were discovered for the first time by Robert Hooke in 1665. He observed a piece of cork under his microscope.
He saw that the cork was made up of many small boxes. He called these little boxes cells. These box-like structures were actually dead cells.
- Living cells were observed by Anton Von Leeuwenhoek for the first time using improved microscopes.
Later in 1838, Mathias Schleiden and Theodor Schwann put forward the cell theory of life.
According to this theory:
- All living organisms are made up of cells.
- New cells arise from pre-existing cells.
- All cells are similar in structure and function but are not identical.
- The function of an organism depends on the way the cells work or the functions they perform.
Shapes and sizes
- Depending on their function, the cells show a variation in their shapes and sizes.
- They may be oval, spindle-shaped, spherical etc.
- Sizes of cells are measured in micrometer or microns. (1micrometer = 1×10-6m)
- Most cells range in size from 5 micrometer to 15 micrometer.
- Bacterial cells are the smallest cells.
- The longest cells in our body are the nerve cells.
- The egg of ostrich is the largest cell- it is a single cell.
According to the number of cells, organisms are broadly classified into two groups- unicellular and multicellular.
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Unicellular: Organisms made up of a single cell are called unicellular organisms. The single cell performs all the functions needed for sustaining life- growth, movement, digestion. Amoeba, Paramecium and bacteria are examples of unicellular organisms.
Multicellular: Organisms made up of many cells are called multicellular organisms. The cells of such organisms usually perform specialized functions. Most plants and animals are examples of multicellular organisms.