Organisms show variety in cell number, shape and size

• Organisms are of different shapes and sizes. Their organs also vary in shape, size and number of cells.

Number of Cells:

• Depending on number of cells, organisms could be multicellular or unicellular.

Multicellular organisms:

• Organisms made up of more than one cell are called multicellular (multi: many; cellular: cell) organisms.
• The number runs into billions and trillions.
•  For example: Human beings, plants, ducks, dogs, insects, fishes etc.

• These organisms begin their life as a single cell which is the fertilized egg i.e. a zygote.
• The fertilized egg cell multiplies and the number of cells increases as development proceeds.

Unicellular organisms:

• The single-celled organisms are called unicellular (union: one; cellular: cell) organisms.
• For example: Bacteria, plasmodium, blue-green algae, euglena, amoeba, paramecium etc.

• A single-celled organism performs all the necessary functions that multicellular organisms perform.

Shape of Cells:

• Cells are round, spherical or elongated.
• Some cells are long and pointed at both ends i.e. spindle shape
• Cells sometimes are quite long. Some are branched like the nerve cell or a neuron.
• Some cells also shows irregular pattern.
• For example: white blood cell (WBC) in human blood and amoeba can change their shape.

Size of Cells:

• 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.

• The smallest cell is 0.1 to 0.5 micrometer in bacteria (Mycoplasma).

Smallest cell - Mycoplasma

• The largest cell measuring 170 mm × 130 mm, is the egg of an ostrich.

Largest cell- ostrich egg

• The size of the cells has no relation with the size of the body of the animal or plant.
• The size of the cell is related to its function.