Substances can move into and out of cells through the cell membrane. The three main types of movement are diffusion, osmosis and active transport. Osmosis is the diffusion of water across a cell membrane. It is an essential process in cell membrane functioning. Whether or not a cell contains a rigid cellular. Despite being only 6 to 10 nanometers thick and visible only through an electron microscope, the cell membrane keeps the cell's cytoplasm in place and lets.