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NS 1011: Introduction to Biology: Cells and Organisms

Diffusion and Osmosis Lab

INTRODUCTION:


In order for cells to interact with their environment, chemicals, including water, must be able to move across the cell membrane and across the cell. Movement within the cell occurs by a process known as diffusion. Molecules move across the cell membrane by a related process known as osmosis. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. This happens because of random molecular motion. Molecules move around randomly until there is an even mixture throughout the container in which they are enclosed. The overall effect is that molecules move "down" a concentration gradient from a region of high concentration to a region of low concentration.

Osmosis is the movement of molecules down a concentration gradient and at the same time across a membrane. Cell membranes do not allow all molecules to cross them. They are said to be "selectively" or "differentially" permeable. Only certain molecules can cross the membrane into or out of a cell. For example, water can cross the membrane while sodium and chlorine ions (dissolved salt) cannot. If there is a concentration gradient across the membrane (if there is more salt and less water on one side than on the other), water will move across the membrane down the concentration gradient while the salt cannot. If there is more salt and less water inside a cell than outside, water will flow into the cell from the surrounding environment. This process is called osmosis.

When the environment outside a cell has a lower concentration of dissolved molecules than inside the cell, the solution is said to be hypotonic, and water will move from the solution into the cell. If the surrounding solution has a higher concentration of dissolved molecules than the cell, the solution is hypertonic. In that case, water will move from the cell out into the surrounding solution. An isotonic solution is one in which the concentration of dissolved molecules is the same inside and outside the cell, and there is no net movement of water across the membrane. When cells are placed in a hypertonic solution, water flows out of them and they shrink or shrivel up. When cells are placed in a hypotonic solution, water flows into them. If the cell does not have a cell wall or some other means of protecting the membrane, it will burst in a hypotonic solution.

In this lab you will observe diffusion and osmosis.  Then you will set up an experiment to test osmosis under different conditions.
 

Part 1: Demonstration of Diffusion

  1. Obtain two Petri dishes, one empty and one with agar.
  2. Place water in the empty one so that it covers the bottom of the dish.
  3. Add a drop of ink to the center of each dish.
  4. Measure the size of each drop across the diameter, and record the time.
  5. Observe what happens.
  6. When the ink nears the edge of the dish of water, measure and record the diameter of the spot, and record the time.  How far has the ink diffused in the agar in that same time?
  7. Allow the ink in the agar to continue to diffuse until the end of the period.  Measure the diameter of the ink spot again at the end of the period, and record the total length of time it has been defusing.
  8. Calculate the rate of diffusion in mm/min.: convert any measurements taken in centimeters to millimeters by multiplying by 10.  Divide diameter measurements by two to get the radius.  Subtract the first radius measurement from the second radius measurement to get the total distance moved.  Divide the distance moved by the number of minutes to get the rate.

Part 2: Observation of Osmosis

  1. Obtain and label 2 test tubes.
  2. In one tube place 10% salt solution.
  3. In the other tube place distilled water.
  4. Place a slice of potato in each tube.
  5. Allow the potato to stay in the tubes for 30 min.
  6. Remove the potato slices and observe and record any differences between them.
  7. What happened?

Part 3: Osmosis Experiment

Dialysis tubing acts like an artificial cell membrane.  Some molecules can pass freely through the tubing, others cannot.  Water can pass freely, sugar cannot.  You will plan and execute an experiment to test conditions which affect the rate of osmosis across the dialysis tubing.

Materials:

Procedure:

  1. Obtain a section of dialysis tubing which has been soaking in water.  Tie off one end with string
  2. Using a funnel, pour some syrup into the tubing until it is about half to three-quarters full.
  3. Tie off the top end of the tubing.
  4. Place the tube with the syrup in a beaker with about 500 mL of water (the exact amount of water isn't important).
  5. Remove the tubing, blot it on paper toweling, and measure the mass.
  6. Replace the tube into the water. Allow it to remain in the water for 5 minutes.
  7. Remove the tube, blot, and measure the mass. 
  8. Repeat steps 6 and 7 five times, recording the time and mass each time.  This is your control data.
  9. Discuss with your lab group conditions which may affect the rate of osmosis.
  10. Choose one set of conditions to test.
  11. Make a hypothesis about your conditions.
  12. Design an experiment to test your hypothesis.
  13. Gather the equipment needed and perform your experiment.
  14. Collect your data.

Calculations:

  1. Graph your data
  2. Calculate the slope of each of your graphs:
    a.  Choose 2 data points which fall on the line.
    b.  Subtract the first (earlier time) mass from the second mass
    c.  Subtract the first time from the second time.
    d.  Divide the difference in the mass by the difference in the time.  This is the rate of diffusion in grams per minute.
  3. Compare the rates of the initial experiment with the variable you tested.

Lab Report:

  1. Title: A few words which give a sense of the lab.
    Minus 1 point if no title

  2. Introduction defining diffusion and osmosis (do not copy my introduction!). 
    1 point

  3. Data and analysis, Part1: Report your observations of diffusion and calculations of the rate of diffusion in mm/min.
    1 point

  4. Data and analysis, Part 2: Report your observations of osmosis in potato strips.  Why did the observed changes occur?
    1 point

  5. Hypotheses, Part 3: Include your hypothesis for the initial observation of osmosis and for your osmosis experiment. 
    2 points

  6. Procedure, Part 3: Describe your experiment, including procedure, controls, and independent and dependent variables. 
    1 point

  7. Data, Part 3: Present your osmosis data in a data table.  Include the data for the control experiment. 
    1 point

  8. Graph, Part 3: Graph your osmosis data.  Both the control and your experiment may be on the same graph. 
    2 points

  9. Calculations, Part 3:  Calculate the rates of osmosis for both your control and your experiment (see directions above). 
    2 points

  10. Conclusions, Part 3: Explain your results. Why did the observed mass changes occur?  What affects the rate of osmosis?
    3 points

  11. Metacognitive Analysis:  What contributed most to your learning in this lab?  What was confusing or didn't work in this lab, and why?  What would you change? 
    1 point


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