Cell - Structure, and Functions
The basic structural unit of an organ, which is the cell. Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism.
Discovery of the Cell
Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Cork is a part of the bark of a tree. He took thin slices of cork and observed them under a microscope. He noticed partitioned boxes or compartments in the cork slice. These boxes appeared like honeycomb. He also noticed that one box was separated from the other by a wall or partition. Hooke coined the term ‘cell’ for each box. What Hooke observed as boxes or cells in the cork were actually dead cells.
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| Cork Cell |
The Cell
A Cell can be defined as the basic structural and functional unit of all living organisms.
Both, bricks in a building and cells in living organisms, are basic structural units. The buildings, though built of similar bricks, have different designs, shapes, and sizes. Similarly, in the living world, organisms differ from one another but all are made up of cells.
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| Brick wall & Onion peel |
➤ The egg of a hen represents a single cell and is big enough to be seen by the unaided eye.
Organisms show Variety in (1) Cell Number, (2) Shape, and (3) Size.
1) Number of Cells
The human body has trillions of cells that vary in shapes and sizes.
Unicellular Organism
The single-celled organisms are called unicellular (uni: one; cellular: cell) organisms.
A single-celled organism performs all the necessary functions that multicellular organisms perform.
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| Amoeba & Paramecium |
➤ A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows, and reproduces. Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.
Multicellular Organism
Organisms made of more than one cell are called multicellular (multi: many; cellular: cell) organisms.
You will be surprised to know that an organism with billions of cells begins life as a single cell which is the fertilised egg. The fertilised egg cell multiplies and the number of cells increases as development proceeds.
Cell Shape
The shape of cells differs not only in different organisms but also in different organs of the same organism. They may be oval, spherical, cuboidal, fibre-like or polygonal. These differences in shapes are due to their location and function in the tissue.
For example, a nerve cell has to transmit nerve impulses to organs located in different parts of the body. Hence they possess a long fibre-like structure.
Cell Size
The size of cells in living organisms may be as small as a millionth of a metre (micrometre or micron) or may be as large as a few centimetres. However, most of the cells are microscopic in size and are not visible to the unaided eye. They need to be enlarged or magnified by a microscope. The smallest cell is 0.1 to 0.5 micrometre in bacteria. The smallest cell PPLO (Pleuropneumonia-like organism), also called mycoplasma is about 0.1 micron (denoted as `μ`) in diameter `(1μ = 10^{-6}m)` The largest cell measuring 170 mm in diameter, is the egg of an ostrich.
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