THE CELL
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Organisms differ from one another structurally and functionally but all are made up of cells. All living organisms carry out certain basic functions.
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The basic structural and functional unit of an organ is the cell.
Discovery of the Cell:
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Robert Hooke in 1665 observed slices of cork under a simple magnifying device. He noticed partitioned boxes or compartments in the cork slice. These boxes appeared like a honeycomb. Some box was separated from the other by a wall Hooke coined the term ‘cell’ for each box.
Dead cork cells as observed by Sir Robert Hooke under microscope
Organisms show Variety in Cell Number, Shape and Size:
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Organisms are of different shapes and sizes. Their organs also vary in shape, size and number of cells.
Number of Cells:
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Depending on number of cells, organisms could be multicellular or unicellular.
Multicellular Organisms:
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Organisms made up of more than one cell are called multicellular (multi: many; cellular: cell) organisms.
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The number runs into billions and trillions.
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For example: Human beings, dogs, plants, insects, fishes etc.
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These organisms begin their life as a single cell which is the fertilized egg i.e. a zygote.
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The fertilized egg cell multiplies and the number of cells increases as development proceeds.
Unicellular organisms:
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The single-celled organisms are called unicellular (union: one; cellular: cell) organisms.
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For example: Bacteria, plasmodium blue-green algae, euglena, amoeba etc.
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A single-celled organism performs all the necessary functions that multicellular organisms perform.
Shape of Cells:
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Cells are round, spherical or elongated.
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Some cells are long and pointed at both ends i.e. spindle shape
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Cells sometimes are quite long. Some are branched like the nerve cell or a neuron.
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Some cells also show irregular pattern.
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For example: white blood cell (WBC) in human blood and amoeba can change their shape.
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Sr. No.
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Shape
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Example
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Image
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(1)
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Round/ spherical
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Red blood cell
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(2)
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Spindle
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Muscle cell
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(3)
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Branched
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Nerve cell
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(4)
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irregular
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Amoeba
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Size of Cells:
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The size of cells in living organisms may be as small as a millionth of a metre (micrometer or micron) or may be as large as a few centimetres.
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The smallest cell is 0.1 to 0.5 micrometer in bacteria (Mycoplasma).
Smallest cell - Mycoplasma
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The largest cell measuring 170 mm × 130 mm, is the egg of an ostrich.
Largest cell- ostrich egg
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The size of the cells has no relation with the size of the body of the animal or plant.
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The size of the cell is related to its function.
