All living things on Earth, like plants, animals and human beings, are made up of cells. Our entire body, bones, muscles, skin, blood, hair, and nerves, contain more than 1012 cells. Each part of the body is made up of its own kind of cells and each kind performs a special function. The largest cells are the nerve cells that may be about a metre long but very thin. The smallest cells are the red blood cells, which are less than thousandth of a centimetre across. The red blood cells are so tiny that 5,000,000 of them would fit inside a drop of blood. Some cells are so tiny that even the most modern electron microscope, which magnifies 200,000 times and can enlarge an ant to a length of 3 or 4 kilometres cannot show details of these cells. The cells may be oval, spherical, flat, elongated or cylindrical in shape. A soft and elastic membrane called plasma membrane surrounds every cell. It acts as a semi-permeable membrane. As such, it plays an important role in regulating the passage of some substances through it. The cell membrane is filled with a semitransparent, jelly-like fluid called ‘protoplasm’. Protoplasm consists of nucleus, ribosome, mitochondria, golgi bodies and centrosomes, etc., which perform various functions in the body. Protoplasm is a complicated chemical substance, which carries on all the processes necessary for life. It takes in food and oxygen, changes some of the food into living matter, gives off waste, repairs its worn-out parts and reproduces itself. Cells combine together to form different tissues, which in turn make different organs of the body.