The permease encoded by the lac operon of E. coli that transports lactose into the cell. A growing number of human diseases have been discovered to be caused by inherited mutations in genes encoding channels. Osmosis Osmosis is a special term used for the diffusion of water through cell membranes. Although water is a polar molecule, it is able to pass through the lipid bilayer of the plasma membrane. Transmembrane proteins that form hydrophilic channels accelerate the process, but even without these, water is still able to get through.
Water passes by diffusion from a region of higher to a region of lower concentration.
Note that this refers to the concentration of water, NOT the concentration of any solutes present in the water. However, the concentration of water can be altered by the active transport of solutes and in this way the movement of water in and out of the cell can be controlled. If the concentration of water in the medium surrounding a cell is greater than that of the cytosol, the medium is said to be hypotonic . Water enters the cell by osmosis. Plant cells and bacterial cells avoid bursting in hypotonic surroundings by their strong cell walls.