Passive Transport is Diffusion of a Substance Across a Membrane with No Energy Investment (Campbell Biology)
Molecules have a type of energy called thermal energy, due to their constant motion. One result of this motion is diffusion, the movement of particles of any substance so that they spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional. To understand this process, let’s imagine a synthetic membrane separating pure water from a solution of a dye in water. Study the photos carefully to appreciate how diffusion would result in both solutions having equal concentrations of the dye molecules. Once that point is reached, there will be a dynamic equilibrium, with roughly as many dye molecules crossing the membrane each second in one direction as in the other.
We can now state a simple rule of diffusion: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decreases (in this case, decreases). No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substances.
Much of the traffic across cell membranes occurs by diffusion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffuse across the membrane down its concentration gradient (assuming that the membrane is permeable to that substance). One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the oxygen as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.
The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Remember, however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, the presence of aquaporin proteins allows water to diffuse very rapidly across the membranes of certain cells compared with diffusion in the absence of aquaporins. As we’ll see next, the movement of water across the plasma membrane has important consequences for cells.
Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition. https://www.pearson.com/us/higher-education/series/Campbell-Biology-Series/2244849.html
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