How substances enter and leave cells

Chemicals travel into and out of cells through the cell membrane. They move by one of three processes; 

Diffusion; Substances move from an area of higher concentration to an area of lower concentration

Osmosis; This is a special term for the diffusion of water molecules through the cell membrane.

Active transport; This is the 'pumping' of substances from an area of lower concentration to an area of higher concentration using energy from the cell

Molecules move randomly. However, because there is more 'room' to move in areas of lower concentration, their natural tendency is to spread out from areas of higher concentration to areas of lower concentration.

The membrane is selectively permeable, which means it lets certain substances through but stops others. Water, oxygen and carbon dioxide are three important substances that can diffuse in and out of cells.

Sometimes a cell needs to move up substance up a concentration gradient. This will not happen naturally, because diffusion works in the opposite direction. In order to move chemicals in this way, the cell has to use energy to 'pump' the substance up a gradient. This process is clled active transport. It is usually used to move substances into the cell rather than out of it.

Osmosis is the diffusion of water through a selectively permeable membrane. The net movement is from a more dilute solution to a more concentrated solution.

In experiments visking tubing is used to model osmosis because it acts like a selectively permeable membrane

The molecules are moving all the time and will come into contact with the membrane. If a molecule hits a 'pore', it will go through if it is a water molecule, but not if it is a solute molecule. As there are more water molecules in the dilute solution, more molecules will go through from that side than in the reserve direction. Although water moves in both directions, the net movement of water is from the more dilute solution to the more concentrated solution