Osmosis is the diffusion of water molecules (1 mark) from a region of high particle concentration (1 mark) to a region of low particle concentration (1 mark), through a partially permeable membrane (1 mark). Water moves in and out of cells (cell membrane-partially permeable membrane) through osmosis. (1 mark) It also moves in and out of animal cells through osmosis (membrane -partially permeable and no cell wall so cell changes shape as it acquires or looses water).