Some organisms are one-celled organisms, like amoebas and bacteria. The CELL is defined as the basic living structural and functional unit of all living organisms. You can think of cells as the structural units of the body, as bricks are the structural units of a house. The cell is also the functional unit of the living organism. All functions that occur within tissues and organs take place at the cellular level.
Cells are microscopic in size and are measured in metric units called micrometers.
To observe a cell, you must use a light microscope, which magnifies objects up to 1000 times their size. Electron microscopes magnify objects up to 1 million times and are used to observe cellular structures. Several electron micrographs are in this textbook. As you can see in Fig. No cell is exactly like all others, but cells do have many common structural and functional features.