Cell is the unit that it becomes a basic structure and activity of all living things. Cell differ in shape and size even within an organism. Human nerve cells that need to transmit a stimulus quickly look like a long electric cords, and are connected to each other. There are also cells floating in the blood in a round shape, such as white blood cells or red blood cells.

Each cell has a fixed lifespan so it dies after a certain period of time, and new cells are created. This will be repeated countless times as long as the organism is alive. In a way, Cells are also the smallest unit of life in an organism.