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| Osmosis and Diffusion | ||||||||||||
| Do the following lab activity entitled “Osmosis and Diffusion.” The instructions for this lab are below, and this lab must be done in the Biology Learning Center. There is a worksheet for this activity in your worksheet packet. When you have completed the activity, take the results to the front desk in the Biology Learning Center for lab points. Equipment and Supplies:
This activity was created by Dr. Gita Bodner, a biology adjunct faculty member at the Downtown Campus. Thanks, Gita! Part A: Diffusion Your textbook defines diffusion as “the movement of molecules from an area of high concentration to an area of low concentration.” The pictures below show red dye diffusing through water. Notice how the dye starts out concentrated in one place (where we dropped it into the water). After a few minutes, it has started to spread out in the water. Soon it is all through the water, at the same concentration everywhere. This example shows how diffusion is a passive process—we did no work here, didn’t stir the water or move it around at all, and no ATP was used.
Next we’ll look at diffusion across a membrane. As you learned in Chapter 4, cell membranes are "selectively permeable" (also called “semi-permeable”), which means that some substances will cross the membrane and others will not. As long as the membrane is permeable to a substance, the substance will diffuse across the membrane, just like it did in the plain water above (though membranes can slow diffusion down). The pictures below show a mock cell with red dye in it.
When we put the cell in water, the dye diffuses across the membrane. The dye moves across the membrane until it’s just as concentrated outside the cell as it is inside the cell. This is called moving “down a concentration gradient,” and your text shows the process in detail in Figure 4.12. Again, the cell has not done any work to move this dye across the membrane. In other words, it is a passive process. Diffusion across a cell membrane therefore also is called _________ transport. [Answer question 1 on your worksheet] Now look at the set of pictures below with the purple dye. We left this purple dye cell in water just as long as the red dye cell above.
Why do you think the purple dye did not diffuse across the membrane? [Answer question 2 on your worksheet. There could be several reasons. Just name one.] Our cells use many molecules that can’t diffuse across simple lipid bilayer membranes, either because the molecule is too big to cross, or because the membrane polarity or cell charge keep the molecule from crossing by itself, or because the substance is already more concentrated on the other side of the membrane. In other cases, cells need certain molecules faster than normal diffusion can supply them. For example, a cell may need to rid itself of excess sodium even though the seawater outside the cell is more concentrated than the inside of the cell. So, how do cells deal with this problem? On your worksheet, answer question 3. Name two ways (besides passive transport) that the cell can move the molecules it needs across its membrane and into the cell. [Hint: read the definitions box below, and chapter 4, pages 74-75 your textbook.] Definitions
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