Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
DIFFUSION AND OSMOSIS
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Molecules in air and molecules in liquids are in motion, and they tend to become evenly mixed. When a person wears a fragrance, it is this natural mixing process that allows others to smell the fragrance. When you put food coloring in a glass of water, it gradually moves out from the center and colors all of the water. The term that describes such unlimited mixing is diffusion.
A demonstration of diffusion has been set up on the front desk. Pigment crystals have been put at the bottom of a cylinder of water. Observe the amount of change in 5 minutes. Is diffusion in water a rapid or a slow process? Is diffusion an effective mechanism of molecular transport over both short and long distances? If you heated the mixture, would diffusion speed up?
Cells exist in a watery environment and themselves contain water. Paramecium lives in ponds and is bathed with water; your own cells and the cells of plants are surrounded by water called tissue fluid. The plasma membrane that divides the cell from its surroundings is semipermeable, that is, small molecules such as water can pass through, and large molecules cannot. Since the tendency toward complete mixing is a natural phenomenon, cells may have a problem retaining enough water or getting rid of excess water.
If number of dissolved particles inside a cell is higher than that in an equal volume of surrounding water, the cell contents are said to be hypertonic to (more concentrated than) the environment. If the cell contents are more dilute than the environment, the cell is hypotonic to the environment. When both are balanced, the concentrations of the cell and its surroundings are isotonic.
When the cell contents and environment are of different concentrations, water will pass through the plasma membrane, and the two regions will become more similar in concentration. The process of water moving through a semipermeable membrane is called osmosis. An organisms whose contents were hypertonic to the environment would gain water; one that was hypotonic to the environment would lose water. Organisms are adapted to deal with their own specific kind of environment. This is why a fresh water fish cannot live long in salt water; they dehydrate.
A demonstration has been set up on the front table. A concentrated sugar solution that has been colored with dye fills the wide end of an inverted thistle tube. The opening of the wide end has been covered with a semipermeable membrane and immersed in water. As water crosses the membrane and dilutes the molasses, the volume of fluid inside the thistle tube increases, and a column of fluid ascends the thin part of the tube. You can measure the rate of its ascent. Would you expect the rate to be correlated with the concentration of molasses?
In this exercise you are provided with an artificial, semipermeable membrane called dialysis tubing and with a variety of concentrations of sugar solutions. Dialysis tubing is a model for the way that cell membranes function. You will tie off bags of the tubing and fill them with sugar solutions or distilled water. Then you will immerse these bags in an environment that is the same or different in concentration. It is possible to observe the result of osmosis. Bags that contain fluids hypertonic to the environment should gain water and weight. Bags that contain fluids hypotonic to the environment should lose water and weight. There should be no change in the weight of bags whose contents are isotonic to the environment.
Procedure: 15 cm. (approx. 7 inch) pieces of dialysis tubing have been cut and soaked in water to make them soft.
Design an experiment in which you test the effect of surface area on rate of osmosis.
The normal functioning of cells can be disrupted if they are in an environment different in concentration from that which is normal. Prepare a slide and examine the aquatic flowering plant Elodea or the green alga Spirogyra under the microscope; you will need to use high power to see the cells and the chloroplasts. In pond water the cells of each plant fill the box that is defined by the cell walls, and the plasma membrane is pressed tightly against the cell wall. Now remove the cover slip and blot the water. Replace it with a concentrated salt or sugar solution and put the cover slip back. After a few minutes you should see a change in the cells; they have been plasmolyzed. Do they still fill the boxes made by the cell walls? What has happened to the cells; and why has it happened? Can you reverse this plasmolysis? Try out your idea.
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