Now we have established the general features of both animal and plant cells we must also remember that many cells do look different and this is because they have their own special jobs to do. These cells display specialised features that make them suited to carry out their specific function.

As we know each cell is surrounded by a cell membrane. This membrane has tiny holes in it which allow small molecules to pass through but not large ones. Due to this nature the cell membrane is described as being partially (or selectively) permeable.

Osmosis is a special kind of diffusion involving water molecules. It occurs when two solutions are separated by a partially permeable membrane. The definition of osmosis is as follows:


Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.



Water is actually moving both ways to try to even up the concentration, however more water molecules are moving from the pure water towards the sugar solution because there is a higher concentration of water molecules in the pure water solution than the sugar solution. The result is a net transfer of molecules down a concentration gradient from the water to the sugar solution. Eventually the level of water on the lower water concentration side rises while that on the more concentrated side falls. When the concentration of water is the same on both sides the movement of water will be the same in both directions at this point the net exchange of water is zero and the system is in equilibrium.