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# Cell Homesostasis: A Steady State (page 2)

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Author: Janice VanCleave

### Try New Approaches

1. How long does it take to fill the egg to capacity? Repeat the experiment measuring the time from the moment the egg is placed into the water until liquid is seen at the base of the straw.
2. Does the surface area of the membrane affect the rate the water flows into the egg? Repeat the original experiment exposing more of the membrane by breaking away larger pieces of the shell. Again, measure the time needed for the egg to fill to capacity. Science Fair Hint: Prepare an egg with a straw for display. Show photographs to represent the various experiments and their results.
3. Once the egg is filled to capacity, at what rate does the water flow into the egg? Does the rate continue to remain the same? To find the answers to these questions, use an egg that is filled to capacity. Use a marking pen to mark the liquid level in the straw every hour for eight hours or until the straw is filled. Use the distances between the hourly markings to determine whether the flow rate is constant. Science Fair Hint: Include diagram5 showing the height of the liquid in the straw in the project report as well as part of the project display.

1. Do cells dehydrate (lose water) when placed into hypertonic solutions (solutions with low concentrations of water)? Mix 1 tablespoon (15 ml) of table salt (sodium chloride) with 1 cup (250 ml) of water. Pour the salt solution into a bowl. Cut four slices from a potato, each about 1/4 inch (6 mm) thick. Place the potato slices into the salty water. After 15 minutes, cut another slice from the potato and test the firmness of this potato slice by trying to bend the slice back and forth with your fingers. Test the firmness of the potato slices in the salt solution. A lack of firmness indicates that water has moved out of the cells. Thus, the cells have lost turgor pressure (pressure inside cells due to the presence of water). Try testing different objects and solutions. Display diagrams to represent your results.
2. What happens to plants that live in fresh water? What stops the water from flowing into their cells? Prepare a wet-mount slide of one leaf of elodea. See Appendix 1 for instructions on preparing slides. Use a microscope to examine the slide. With low-and high-power magnification, observe the structure of each cell and notice the cell wall. Remove the water from the cells by adding drops of a table salt solution [made with 1 tablespoon (15 ml) of sodium chloride in 1/2 cup (125 ml) of water at the edge of the coverslip] (see Figure 6.2). Observe the structure of the cell in the salt solution. Remove the salt solution by placing a paper towel at the edge of the coverslip. Add drops of distilled water at the edge of the coverslip while watching the cell through the microscope. You should observe that the cells shrink in the salt solution and expand in the water. They rarely expand enough to rupture, however, because of the strong pressure of the outer cellulose walls. When the pressure with in the cell equals the osmotic pressure, the water no longer enters or leaves the cell.

### Get the Facts

1. Fresh-water animals, such as fish and protozoans, do not have strong cellulose walls to protect them from water that continually enters due to osmotic pressure. How do the cells of these organisms protect themselves? Find out how energy is employed to pump the excess water back out into the environment. What is a contractile vacuole, what organisms have it, and how does it function?
2. Why does a fresh-water plant wilt when placed in salty water, while marine organisms survive daily in salty sea water? What does it mean when we say that sea water is isotonic to the cytoplasm of sea organisms but hypertonic to the cytoplasm of fresh-water organisms? What is plasmolysis?
3. The human kidney continuously filters blood, removes wastes, and maintains a proper salt-and-water balance. An artificial kidney machine is used to perform these same functions. How does this machine work? How does the machine use the process of osmosis to cleanse the blood?
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