All the chemicals and structures that make up a cell are contained within the cell membrane.  This is good!  The membrane keeps all the elements that the cell needs together, and forms a boundary between the living cell and the mixture of substances in the cell's environment.   The membrane forms a protective barrier that  keeps unwanted substances out of the cell.
                      
 However, a cell is alive, and it cannot live in a sealed container.  It needs to be able to have water, oxygen, and carbon dioxide come to the inside of the cell and move out again.  The cell membrane is porous with tiny openings.  The cell needs to get materials from the environment and it needs to get rid of wastes.

     Think about this for a minute.  Look at the back of your hand.  Can you see the little pores in the skin?  What purpose do they serve?  If you look at the underside of a leaf you will see that it has little pores too -- pores called stomata.  Plants breathe through their stomata.  Look carefully at an egg and you will see tiny openings, so that a developing chick will have access to oxygen.  Cells are very tiny, and they they also need tiny openings in their membranes to  function properly.

     We say that the cell membrane is permeable.  This means that tiny atoms and molecules can go through the holes in the membrane.   Actually, a cell is so tiny that its membrane is really only semi-permeable.  It can let small molecules in through the membrane, such as molecules of oxygen, water, carbon dioxide, and glucose, a very simple sugar.  However, big molecules cannot get through the tiny holes.

     The movement of water or carbon dioxide or oxygen through the cell membrane is called diffusion.   Tiny molecules float through the cell membrane with no problems. If the cell uses up oxygen inside itself, more oxygen just moves in.  In a cell has too much carbon dioxide, some of the carbon dioxide molecules float out through the membrane.  

The cell tries to stay in harmonious balance with its environment.  

However, sometimes big molecules create problems for the cell because only small molecules can diffuse through the cell wall.  What happens then is called osmosis.