- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient as a result of their random movement.
- e.g. Diffusion in the alveoli as gaseous exchange
- A Visking tubing bag full of starch solution is placed into a beaker of iodine solution.
- Iodine solution molecules go into the bag as there is a higher concentration outside the bag and they are smaller molecules.
- The starch molecules cannot get out of the bag as they are too big.
- The Visking tubing slowly turns an orangey colour as the iodine enters it.
- Partially permeable membrane: membranes which only let some solute molecules through.