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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
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
4 10.0 ml 0.5 M
5.0 ml 0.5 M
5.0 ml H2O water
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
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.
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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- Fall '12