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.
The block of wood does not stop moving at any time as shown on the chart, so the answer can't be A. or C. and the speed is not increasing so it can't be B.
