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TRANSPORT ACROSS PLASMA MEMBRANES By Johnny McKinstry A phospholipid bilayer makes a good barrier, particularly against the movement of water-soluble molecules. The water contents of the cell are stopped from escaping. However, some exchange between the cell and its environment is essential. Diffusion & Facilitated Diffusion If you open a bottle of perfume in a room, it is not long before molecules of its scent have spread to all parts of the room. This will happen, even in still air, by the process of diffusion. Diffusion is the movement of molecules from a region of their higher concentration to a region of their lower concentration. The molecules move down a concentration gradient. It happens because of the natural kinetic energy possessed by molecules or ions, which make them move about at random. As a result of diffusion, molecules tend to reach an equilibrium situation where they are equally spread within a given volume of...

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