electrical charge. Small, uncharged molecules often pass through the membrane easily, while most large orcharged molecules are prevented from passing. Molecules which cannot diffuse across the membrane maybe able to cross through other regulated gateways located within the membrane. Dialysis Dialysis is the separation of molecules through diffusion. In dialysis, adifferentially permeable membrane is used to separate the components ofa mixed solution containing more than one type of molecule. Thismembrane allows the free passage of water, but limits the movement ofmolecules by their size. In one of the following experiments, you willdialyze a solution of glucose and starch to observe membranepermeability. The concentration of a solution may also impact the membranepermeability. A solution or its components may not be able to pass througha membrane if the concentration is too high. For this reason, serialdilutions may be performed to create solutions of lower concentrations.The original sample is a known volume and concentration. A small volumeof this sample is transferred sequentially among new tubes, called dilutionblanks. The sample is mixed with water (or another diluent) throughout theprocess to create more dilute solutions (Figure 2). ? Did You Know... Hemodialysis is a method ofremoving toxic substances fromthe blood when the kidneys areunable to do so. It is frequentlyused for patients with kidneyfailure, but may also be used toquickly remove drugs or poisonsin dangerous situations.
Diffusion and Osmosis 103 Figure 2: Serial dilution schematic. The dilution blanks all have 9 mL of diluent in them such that when 1 mL is added, it represents 1 / 10th the concentration of the previous tube. The dilution of the sample is written below each tube. Osmosis Osmosis is similar to diffusion, but refers directly to the movement of water molecules across a selectively permeable membrane, rather than any molecule. This often occurs across the plasma membrane of a cell. Osmosis is directed from an area of high water concentration to an area of low water concentration. Ultimately, membrane selectivity and the movement of water in and out of the cell regulate the concentration of intracellular material. As solute concentration increases, solvent concentration decreases. In order to achieve equilibrium, the solvent from a low solute concentration solution will move into an area of high solute concentration to create an equal ratio of solvent to solute in both areas. Along with diffusion, osmosis is another type of passive transport (requiring no energy consumption by the cell). Solution Tonicity Tonicity is a relative term that describes the solute difference between solutions and determines the net direction of movement of water molecules (osmosis). There are three types of tonicity: Hypertonic : A solution with a higher solute concentration than the solute concentration on the opposite side of the permeable membrane.
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