List various types of variations observed …

• Organisms show variety in Cell Number, Shape and Size:

There are millions of living organisms having variety in shapes and sizes. Their organs also show variety in shapes and sizes and also the number of cells in them.

1. Numbers of cells – All living organisms are made up of cells but the number of cells varies from organism to organism. Some organisms are made up of just one cell and are called unicellular organisms. Example – Amoeba, Paramecium, Euglena, bacteria, etc. Some organisms are made up of more than one cell or many cells are called multicellular organisms. Example – plants, animals, etc. The number of cells less or more, does not affect the functioning in the body of any organism. A single-celled organism performs all the necessary functions that multicellular organisms perform. 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 tissues which group together to form organs.
2. Shape of cells – There are many types of cells in the bodies of multicellular organisms which differ in shapes. Example – an amoeba appears irregular in shape and a nerve cell is long and branched whereas a muscle cell is pointed at both ends and has a spindle shape.
3. Size of cells – The cells are of many different sizes. Most of the cells are extremely small in size and hence cannot be seen with the naked eyes. The smallest cell is 0.1 to 0.5 micrometre in bacteria.  The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich. The size of the cell is related to its function. For example, nerve cells, both in the elephant and rat, are long and branched. They perform the function of transferring messages.