In passive diffusion, the small water molecules can move across the phospholipid bilayer, seen in pink. This layer acts as a <em>semi-permeable membrane</em>; its hydrophilic heads are attracted to water (seen facing outwards) while its water-repellent hydrophobic tails face towards each other- allowing molecules of water to diffuse across the membrane <em>down the concentration gradient. </em>
Thus the water will move from an area of high concentration (more blue dots) to an area of low concentration (i.e. from B to A), until the system reaches a steady state called equilibrium- after which, there will be no net movement of water. Similarly via osmosis, the water passes through the membrane due to the difference in <em>osmotic pressure </em>on either side of the phospholipid by layer this means that the water moves from regions of high osmotic pressure/concentration to regions of low pressure/ concentration to a steady state.
