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Transport across plasma membranes The cell surface membrane is approximately 7.5mm thick and is a bi-molecular phospholipid bilayer with inwardly directed hydrophobic (substances which repel water molecules) tails. It is also a fluid structure. A partially permeable membrane is one which allows some substances through but not others. There are a number of different ways in which substances are transported across plasma membranes. The first being diffusion, which occurs across the cell surface membrane. This is a passive process (requires no energy) by which substances move from a region of high concentration to a region of low concentration, of the same substance. The rate of diffusion depends on a number of factors: * The concentration gradient * The distance between the areas * The size of the molecules that are diffusing Particles of gas or solute can also diffuse through a membrane, as long as the membrane has pores that are larger than the particles....

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