Simple diffusion and osmosis

Here is an example of oxygen diffusing into a cell and carbon dioxide diffusing

Osmosis

The larger particles for instance Sodium chloride common salt will not pass through the channelproteins but the smaller water molecules can pas through. The water molecules are randomly moving in all directions. The chances of water molecules moving from the low solute side of the membrane are much greater because there are far more water molecules in motion than on the high solute concentration side. So the net movement of molecules is from the low solute(high water)concentration to the high solute (low water) concentration.

Low solute concentration

High solute concentrations