By Janet Rae-Dupree, Pat DuPree. Think of it as a gatekeeper, guardian, or border guard. Despite being only 6 to 10 nanometers thick and visible only through. 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 spontaneous net movement of solvent molecules through a semi- permeable Water molecules travel through the plasma membrane, tonoplast membrane (vacuole) or protoplast by diffusing across the phospholipid bilayer via. Another example of a harmful osmotic effect is the use of table salt to kill leeches and slugs. Why a Fresh Water Fish dies in Salt Water. The unassisted diffusion of very small or lipid-soluble particles is called simple diffusion. As a result of these factors, in the absence of some countervailing mechanism, the cytosolic solute concentration would increase, causing an osmotic influx of water and eventually cell lysis. Inside, between the two layers, you find hydrophobic or water-fearing nonpolar tails consisting of fatty acid chains. The mechanism responsible for driving osmosis has commonly been represented in biology and chemistry texts as either the dilution of water by solute resulting in lower concentration of water on the higher solute concentration side of the membrane and therefore a diffusion of water along a concentration gradient or by a solute's attraction to water resulting in less free water on the higher solute concentration side of the membrane and therefore net movement of water toward the solute. Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. Toggle navigation Search Submit. The cell cannot get rid of the water fast enough, and the cell swells and lysis. Frog oocytes, which normally do not express aquaporin, were microinjected with erythrocyte mRNA encoding aquaporin. The solvent is the liquid in which a substance is dissolved; water is called the universal solvent because more materials dissolve in it than in any other liquid. In addition, there are secondary active transport processes that are similar to diffusion but instead use imbalances in electrostatic forces to move molecules across the membrane.