Cells are the basic structural and functional unit of life. Cells provide structure and stability to an organism. Cells provide energy and a means of reproduction for an organism. Organisms are made up of cells. Organisms containing only one cell in their body are called unicellular organism. Example Amoeba (animal), Chlorella (plant).

Organisms containing many cells in their body are called multi cellular organisms. Example: rabbit, mango tree. Scientists estimate that our bodies contain anywhere from 75 to 100 trillion cells. Cytology is the study of cells. Cells are very small in size. Cells range in size from 1 to 100 micrometers. Microscope is needed to study the cells. But an ordinary compound microscope is not enough to study the details of a cell. With the help of Scanning Electron Microscope and Transmission Electron Microscope, cell biologists are able to obtain detailed images of the smallest of cell structures.

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