Diffusion and Osmosis Lab
PART 1: SURFACE-AREA-TO-VOLUME RATIO AND CELL SIZE
Background
Cell size and shape are important factors in determining the rate of diffusion. Think about cells with specialized functions, such as the epithelial cells that line the small intestine or plant root hairs. What is the shape of these cells? What is the size of these cells? How do such cells obtain nutrients?
Pre Lab Questions
1. Use the centimeter side of a ruler to measure each of these four cubes (to the nearest tenth). After taking accurate measurements, calculate the surface area, volume, and surface-area-to-volume ratio for each of the four cubes.
a. What trend do you notice as the cell size increases?
______
Cube A
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Cube B
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Cube C
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Cube D
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Problem
· Are cells more efficient at moving materials across their surface when the surface area to volume ratio is higher or lower?
Hypothesis
· Create your own hypothesis. What size/shape cell do you think is most efficient at transport across the cell membrane? (Bigger? Smaller? Other?)
Materials
· Agarose, Water, 1% Phenolphthalein, 0.1 M NaOH, 0.1 M HCl or Vinegar
Demonstration Task Procedure
· Place phenolphthalein in two test tubes. Add 0.1 M HCl to one test tube. Add 0.1 M NaOH to the other test tube. Swirl to mix the solutions and observe color change.
Data
· In this section of your lab notebook, write what you observed during the demonstration task.
Analysis/Conclusions
· In a couple of sentences, summarize the effect that phenolphthalein has when added to an acid and added to a base. (Make sure you identify which solution was the acid, and which was the base).
Independent Task Procedure
· To create agar cubes: Measure 20 g of agarose, and add to a beaker containing 1 L of water. Place on hot plate and heat until agarose is fully dissolved and solution is clear. Remove beaker from hot plate. Add 10 mL of 1% phenolphthalein. Add a few drops of 0.1 M NaOH, until solution turns bright pink. Pour the agarose mixture in shallow pans and refrigerate 12 hours. (This step has already been done for you).
· Using a dull knife, cut three blocks of agar of different size. These three blocks will be three different sized models of cells.
· Measure the surface area and volume for each of these “cells” and determine the reduced surface-area-to-volume ratio for each of the three cells. (Create a data table)
· Using the materials listed, design an experiment to test the predictions you just made regarding the relationship of surface area and volume to the efficiency of diffusion. Once you have finished planning your experiment, have your teacher check your design. When you have an approved design, write your procedure in your lab notebook, run your experiment.
· After observing diffusion for equal amounts of time in each cell, immediately cut the blocks in half and measure the depth to which the solution has penetrated each cell. This is the diffusion depth. Record in millimeters. From that, you can calculate the rate of diffusion (mm/min) by dividing the distance traveled, by the time it took to for that diffusion to occur.
· Finally, calculate the percent penetrance. Do this by dividing the penetrated volume by the total volume.
Data
· This section should include a data table that shows the surface area, volume, and the reduced surface-area-to-volume ratio for each of your three cubes. This data table should also include the diffusion depth, diffusion rate, and percent penetrance.
· You should also have written observations describing what you observed when you conducted your experiment.
· Finally, you may want to include a sketch of your cubes at the end of your experiment to help show your final results.
Analysis/Conclusions
· In this section, analyze your results. How does the surface-area-to-volume ratio affect the efficiency of diffusion in the cell? Do your experimental results support your predictions? Draw conclusions from your data table.
Diffusion and Osmosis Lab
PROCEDURE 2: MODELING DIFFUSION AND OSMOSIS
Background
In this experiment you will create models of living cells using dialysis tubing. Like cell membranes, dialysis tubing is made from a material that is selectively permeable to water and some solutes. You will fill your model cells with different solutions and determine the rate of diffusion.
Pre Lab Questions
*Rewrite and answer these questions in your lab notebook.
1. What is kinetic energy and how does it differ from potential energy?
2. What environmental factors affect kinetic energy and diffusion?
3. How do these factors alter diffusion rates?
4. How are concentration gradients important in diffusion and osmosis?
5. What is the explanation for the fact that most cells are small and have cell membranes with many convolutions (folds)?
Materials
Distilled or tap water, 1 M sucrose, 1 M NaCl, 1 M glucose, 5% albumin (egg-white protein), 5 pieces of 20 cm- long dialysis tubing, cups, balances
Problem
· How can you use weights of the filled cell models to determine the rate and direction of diffusion? What would be an appropriate control for the procedure you just described?
· Will protein diffuse? Will it affect the rate of diffusion of other molecules?
Hypothesis
After choosing the solutions you will test for your experiment, use your knowledge about solute gradients to predict whether the water will diffusion into or out of the cell.
Procedure
1. Choose up to four pairs of different solutions. One solution from each pair will be in the model cell of dialysis tubing, and the other will be outside the cell in the cup. Your fifth model cell will have water inside and outside; this is your control. Knot the dialysis tubing in one end, fill with 10mL of solution, and knot to close the tube. Make sure to leave enough space for water to diffuse into the tube. Also, keep the dialysis tubing moist!
2. Before starting, use your knowledge about solute gradients to predict whether the water will diffuse into or out of the cell. (This is the hypothesis section!!) Make sure you label the cups to indicate what solution is inside the cell and inside the cup.
3. Weigh each cell, record the initial weight, and then place it into a cup filled with the second solution for that pair. Weigh the cell after 30 minutes. Create a data table and record the final weight.
Data/Observations
1. Construct a data table for your lab. The data table should represent each of your five solutions. It should include original weight, final weight, and percent change in weight.
2. Calculate the percent change in weight using the following formula: (final – initial) / initial x 100. Show your calculations in your lab notebook and record your results in your data table.
3. Diagram the flow of water based upon the contents of your model cell and the surrounding solution.
Analysis/Conclusions
1. In your analysis, consider the following:
a. What factors determine the rate and direction of osmosis?
b. Compare the solutions you used. Which were more/less hypertonic? What is your evidence?
c. Which pair(s) that you tested did not have a change in weight? How can you explain this?
Post Lab Questions
1. How is the dialysis tubing functionally different from a cell membrane?
2. When will the net osmosis rate equal zero in your cell model?
3. How could you test for the diffusion of glucose? (Hint: think back to McMush lab)
4. You are in the hospital and need intravenous (IV) fluids. You read the label on the IV bag, which lists all of the solutes in the water.
a. Why is it important for an IV solution to have salts in it?
b. What would happen if you were given pure water in an IV?
c. How would you determine the best concentration of solutes to give a patient in need of fluids before you introduced the fluids into the patient’s body?
PART 3: OBSERVING OSMOSIS IN LIVING CELLS
The interactions between selectively permeable membranes, water, and solutes are important in cellular and organismal functions. For example, water and nutrients move from plant roots to the leaves and shoots because of differences in water potentials. Based upon what you know and what you have learned about osmosis, diffusion, and water potential in the course of your investigations, think about these questions.
What would happen if you applied saltwater to the roots of a plant? Why?
What are two different ways a plant could control turgor pressure, a name for internal water potential within its cells? Is this a sufficient definition for turgor pressure?
Will water move into or out of a plant cell if the cell has a higher water potential than its surrounding environment?
Procedure