Bio5LAManual12f

5 as each bag is finished trim the excess string from

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Unformatted text preview: then label each with either H2O or 0.5 M sucrose. The groups at each table will share these beakers. 2. Add 200 ml of the correct solution to the beakers. Biology 05LA – Fall Quarter 2012 Lab 3 – page 4 3. Each lab group should obtain 4 strips of dialysis tubing and soak them in distilled water until they are soft and pliable. 4. With reference to Table 2, and using the method described above, create dialysis bags with the specified contents. 5. As each bag is finished, trim the excess string from each end of the bag (don’t cut off the label). 6. After weighing and recording that value (in grams), place them into the specified bathing solution. 7. Incubate the bags for about an hour (no longer). At the end of that time, quickly blot the excess solution from the surface of each bag, weigh it, and record the weight. Bag # Bag Content Bathing Solutions 1 10.0 ml H2O water 2 10.0 ml H2O 0.5 M sucrose 3 4 10.0 ml 0.5 M sucrose 5.0 ml 0.5 M sucrose + 5.0 ml H2O water 0.5 M sucrose Table 2 Analysis. For each trial, give the tonicity (either hypo-, iso-, or hypertonic) of the bag content relative to the bathing solution. After considering your results, what generalizations can you make about how differences in tonicity influence the direction of osmotic water movement? Osmosis In Living Plant And Animal Cells. The osmotic relationships between plant and animal cells with their environment are closely regulated in both cell types, but are not identical. Much of this difference is related to some fundamental differences between these cells. Plant cells differ from animal cells in many ways, but only two are important to this discussion. First, plant cells possess a fairly rigid cell wall. Second, healthy plant cells are most often hypertonic in comparison to their immediate exterior. This circumstance has a significant osmotic consequence. Given the hypertonic condition of cell’s interior, water should move into the cells by osmosis. If the cells were not bounded by the cell wall, water would keep moving into the cell until it burst. This does not happen. Instead, the cell wall resists the predicted osmotic swelling and pressure builds up within the cell. Plants use this “turgor” pressure to provide support for the plant or to drive long distance translocation of solutes. A characteristic feature of most animal cells is that they usually lack any sort of a structure comparable to the plant cell wall. Thus, the cytoplasm of the animal cell must be maintained in osmotic balance with the cell’s exterior. That is to say that the osmolarity of the inside and the outside of the cell should the same (isotonic). Maintaining this osmotic relationship is vital to the health of the cells because an animal cell exposed to a hypotonic environment would rapidly swell and eventually burst. Conversely, an animal cell exposed to a hypertonic environment would shrink because of excessive water loss. The following exercises will demonstrate the behavior of plant and animal cells exposed to different osmotic environments. In the first part of this exercise, we will manipulate the external solute concentration around the leaf cells of Elodea, and observe the consequences of this treatment with the light microscope. In the second part we will make similar observations on red blood cells (RBCs) exposed to hypo-. iso-, and hypertonic solutions. We will conclude by exposing rbcs to a range of glucose concentrations extending from hypo- to hypertonic in an attempt to estimate the osmolarity of the cells. Biology 05LA – Fall Quarter 2012 Lab 3 – page 5 Osmosis in the Leaf cells of Elodea. Elodea is a common aquatic plant used in many fresh water aquaria. Its leaves are thin and comprised of fairly large cells. These features allow the microscopist to make useful observations of cell structure in intact leaves. Procedure: 1. With the use of a forceps, remove a healthy looking, green leaf from the tip of an Elodea twig. Make a wet mount of the leaf using water from the tank where it was collected. (Given this is fresh water, there are very few solutes dissolved in it. We will consider the osmolarity of this solution to be hypotonic relative to the leaf cells). 2. Examine the leaf first with low power (40x total magnification). Then select a suitable cell and focus on it with higher power (100x total magnification – no more!). Triangular cells that project from the edge of the leaf are most easily brought into clear focus, but if you focus carefully, cells the middle of the leaf can be studied. 3. Observe the following features in the healthy cell (it is important that you know what these cells look like in the healthy condition before applying the test solution): The cellulosic cell wall. The cell cytoplasm forms a layer just inside the cell wall. A large vacuole occupies the central core of the cell. This vacuole may be identified if you focus through the depth of the cell. It appears as a clear region in which no chloroplasts (globular green structures) come into clear focus. The chloroplasts are located in the thin layer of cy...
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