Membrane transport how size of molecule affect their ability to diffuse a cross plasma membrane
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The plasma membrane is composed of a phospholipid bilayer with embedded proteins, which creates a semi-permeable barrier that regulates the movement of substances in and out of the cell. Show more…
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What is the effect of molecule size on a semipermeable membrane?
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What is the effect of molecule size on a molecule's ability to diffuse across a semipermeable membrane? If molecules are small enough, then....
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tells us how much matter is present. A demonstration of the effect of molecular weight on diffusion will be prepared at the front table. In the demonstration approximately equal-size crystals of potassium permanganate and methylene blue are placed on the surface on an agar plate. Both of these materials dissolve readily in water and begin diffusing as soon as they touch the plate. The agar plate consists of 99% water and 1% agar. The molecular weight of potassium permanganate is 158, while that of methylene blue is 374. Thus, a molecule of methylene blue has over twice the mass (is over twice as "heavy") as a molecule of potassium permanganate. Observe the agar plate about an hour after the crystals were added and measure the diameter of the colored areas to determine the distance each substance has diffused. Diffusion and Molecular size One quality of the plasma membrane is that it allows some molecule to pass through it easily, while others are not allowed through. Such a membrane is said to be semi-permeable, a characteristic which can be attributed, at least in part, to the presence of many very tiny openings. These openings permit smaller molecules to pass through but prevent the passage of larger ones. It can be chemically demonstrated that glucose molecules are relatively small, while starch molecules, which consist of hundreds of glucose molecules all linked together, are comparatively huge. Cellulose tubing is somewhat similar to the cell membrane in that it has minute pores or openings. Investigate the effects of molecular size on diffusion through a semi-permeable membrane by observing the demonstration performed by your instructor at the front table and as shown in Figure 6-1. A large test tube has been filled about two-thirds full with a solution of iodine and water. A cellulose sac has been filled with a mixture of water, starch, and glucose. The cellulose sac will be inserted into a large test tube and secured with a cork. Figure 6-1.
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