Click on the name of the activity listed below to go to a specific key. Otherwise, you may scroll through all of the keys. They are presented in the order given below. You will be given the opportunity to return back to the top of this page at the end of each answer key.

• Exploratory Activity: Osmosis in Elodea Cells
• Exploratory Activity: Dynamic Equilibrium
• Exploratory Activity: Factors Affecting Diffusion
• Exploratory Activity: Osmosis
• Experiment: Osmosis and Red Blood Cells
• Quiz: Diffusion, Osmosis, and Cell Membranes
  

Exploratory Activity: Osmosis in Elodea Cells

Observations: The student will receive one point for preparing any sketches and two points for completing all five sketches. Labels on the sketches will earn another point, and one point will be awarded for sketches that are neatly and carefully drawn. One point will also be given for written observations of the cells under each set of conditions.

Answers to Conclusion Questions:

1. The shape of the typical Elodea cell is rectangular or boxlike.
2. The small green objects are the chloroplasts, which are responsible for photosynthesis (food production) in the plants.
3. The cells shrink although the shape of the cell wall stays the same. Thus, an open space appears between the cell membrane and the cell wall.
4. The cells swell until the cell membrane is pressed tightly against the cell wall.
5. The cell membranes would pull away from the cell wall as the cell lost water. This happens because water flows out of the cell faster than it flows back in.
   

Conclusions: Three points will be awarded for answering all five questions. Another point may be earned for answers that indicate accurate observation. Yet another point may be earned for a reasonable explanation of the changes based on the observations (question 5). A point will be deducted if answers are not in complete sentences.

Grading: Point totals will be recorded as part of the overall grade for the term.

Exploratory Activity: Dynamic Equilibrium

Observations: Two points will be awarded for a complete data table that includes a title, column headings, and a complete set of data. Two points will be awarded for a complete graph that includes a title, labeled axes that are correctly incremented, and a smooth curve drawn through the data points. One point will be awarded for neatness and precision.

Answers to Conclusion Questions:

1. In Trial 1, the water levels stayed the same. In Trial 2, the water level in the container that was initially empty rose. The water level in the container the was initially full dropped. When the point was reached where the transfer cups were both full for each transfer, the water level in both containers stayed the same, although it was lower in the container that was initially empty.
2. In Trial 1, the graph for both containers was a horizontal line. In Trial 2, the curve for the full container showed a decrease and then leveled off. The graph for the empty container rose, then leveled off at a position lower than the graph for the full container.
3. The amount of water transferred from the initially-empty container increases until the cup is full for each transfer.
4. In Trial 1, they are the same. In Trial 2, the water level in the full container drops and the water level in the other container rises, but only until the water transferred from the initially-empty container fills the transfer cup. Thus the water level in the initially-empty container is lower than in the other container when equilibrium is reached.
5. It would stay the same because the amount of water being transferred in each direction is the same, so the water level does not change.
   

Conclusions: Three points will be awarded for answering all five questions. Another point may be earned for answers that indicate accurate observation. Yet another point may be earned for reasonable explanations of the changes based on the observations (questions 4 and 5). A point will be deducted if answers are not in complete sentences.

Grading: Point totals will be recorded as part of the overall grade for the term.

Exploratory Activity: Diffusion

Observations: The student will receive a maximum of one point for the data table, one point for each graph, and one point for clearly reporting the distance traveled by the molecules in Part 2. One point will be awarded for neatness and precision in reporting the results.

Answers to Conclusion Questions:

1. It should travel farthest at the highest temperature.
2. As the temperature of the system increases, the rate of diffusion increases.
3. The molecules have more energy, so they move faster.
4. NH₃ should have traveled farther.
5. NH₃ diffused the fastest since it traveled farther in the same amount of time.
6. HCl has the greatest mass since, at the same temperature, it diffuses more slowly. Since the mass and the velocity of the molecules are inversely proportional, the mass must be greater.
   

Conclusions: Three points will be awarded for answering all five questions. Another point may be earned for answers that indicate accurate observation. Yet another point may be earned for reasonable explanations of the changes based on the observations (questions 3 and 6). A point will be deducted if answers are not in complete sentences.

Grading: Point totals will be recorded as part of the overall grade for the term.

Exploratory Activity: Osmosis

Observations: A maximum of one point will be awarded for each data table. One point will be awarded for the graph. One point will be awarded for neatness and precision.

1. Answers to Conclusion Questions:
   The liquid level should rise in the side with the starch solution.
2. The color should change from clear to blue-black in the side with the starch solution since the iodine can pass through the membrane but the starch cannot.
3. The solution with the highest starch percentage.
4. The solution with the highest starch percentage.
5. Yes, because the more solute molecules that are present, the greater the hindrance of the solute molecules leaving the starch solution.
6. The 8% solution because at the cell membrane, more water is leaving the cell than enters due to more hindrance by the starch molecules in the more concentrated solution.
   

Conclusions: Three points will be awarded for answering all six questions. Another point may be earned for answers that indicate accurate observation. Two more points may be earned for reasonable explanations of the changes based on the observations (questions 2,5,and 6). A point will be deducted if answers are not in complete sentences.

Grading: Point totals will be recorded as part of the overall grade for the term.

Experiment: Osmosis and Red Blood Cells

This will be a major component of the evaluation for this lesson. The student is expected to prepare a substantial report including all of the major elements of a research paper. Two points will be awarded for a well-supported hypothesis. Five points will be awarded for a clear description of the experimental method. Five points will be awarded for clearly presented observations and results. Ten points will be awarded for conclusions that are well-supported by the evidence and include the use of concepts regarding osmosis that have been explored previously in this lesson sequence. Three points will be awarded for clarity.

Quiz: Diffusion, Osmosis and Cell Membranes

I.

Diffusion

The movement of molecules from a region of high concentration to a region of low concentration that is due to the random movement of the molecules.

Dynamic equilibrium

A state that occurs when the rate of two opposing processes is the same

Semipermeable membrane

A membrane that allows some molecules to pass through but not others, usually as a result of the size of the molecules or their charge.

Passive transport

The movement of materials into or out of a cell that does not require the use of energy by the cell.

II.

a. The level of the water and iodine solution will fall as more water moves into the starch solution than leaves it.

b. The level of the starch solution will rise since more water molecules will enter the solution than will leave it.

c. The water will move in both directions across the membrane.

d. The color of the water and iodine solution will not change since starch cannot pass through the membrane to enter the solution.

e. The color of the starch solution will turn from clear to a blue-black as iodine molecules cross the membrane, enter the solution and complex with the starch molecules.

f. Five (5) grams of starch will be present in 100 grams of a 5% starch solution.

III. If a person were given an intravenous injection of pure water, the concentration of the water in the serum of the blood would be higher than in the red blood cells. More water would flow into the cell than out of the cell, causing it to swell. Since the cell membrane is not permeable to most of the other molecules in the cell, the cell would be unable to prevent the swelling and might burst. Should this happen on a large scale it would be dangerous for the person.

IV. The support mechanism for a plant consists of the cell walls. In plants (like most houseplants) with green stems, the cell walls of the stems are relatively thin and flexible. When the cell is full of water, it presses against the cell wall and makes it more rigid. If the plant lacks sufficient water, the cells shrink, and the pressure against the cells walls is reduced. This allows the cell walls to bend, so that the stem is unable to support the leave, and the plant wilts.

V. When the cells are placed in tap water, both types retain their original shape. When the cells are placed in a salt or starch solution, both types will shrink, although the plant cell walls will not change shape. When the cells are placed distilled water, both types will swell. The plant cell will expand to the limits presented by the cell wall. The blood cell, having no cell wall, will swell until it bursts.

Evaluation of the Quiz
Each of the definitions in Part I will carry a maximum value of one point. In order to earn full credit, the student must give a thorough definition in complete sentences. The responses to the questions in Part II will each be worth two points: one point for the correct response and one for the correct explanation. The questions in Parts III and IV will each be worth three points, and will be graded for evidence of an understanding of the concepts and on the logic of the argument.

Point totals will be recorded as part of the overall grade for the term. The term grade will be determined relative to a percentage scale, with 90-100% of the points corresponding to a "A", etc.

Teacher Materials:

Teaching Tips

Evaluation Keys

Student Materials:

	Background Reading
Osmosis in Elodea	Dynamic Equilibrium	Factors Affecting Diffusion
Osmosis	Osmosis and Blood Cells	Quiz

Science Education Connection Department of Biochemistry The University of Arizona Wednesday, February 12, 1997 johnmcc@dakotacom.net http://biology.arizona.edu/sciconn/lessons/mccandless/ All contents copyright © 1997; 1998. All rights reserved.