Passive Transport through Membranes Lab Report

Passive Transport through Membranes Lab Report



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Lab 5: Passive Transport through Membranes – Online

20pts (including online pre-lab)


Learning Outcomes 

After completing the lab exercises you should be able to:

  1. Discuss the role of transport molecules found in plasma membrane.
  2. Explain the difference between passive and active transport.
  3. Describe the nature of molecules able to diffuse through cell membrane.
  4. Discuss how size of molecules contributes to their diffusion through plasma membrane.
  5. Define isotonic, hypotonic, and hypertonic solutions.
  6. Compare and contrast the behavior of animal and plant cells in solutions with different tonicities.


The plasma membrane separates the inside of the cell from its environment. This membrane is selective and semi-permeable, which means that the membrane allows some things to pass through and not others. The process we’re looking at today is passive and requires no cellular ATP. Passive movement across a membrane is called diffusion and is defined as movement of material from a region of high concentration to a region of lower concentration. Small non-polar molecules dissolve in the hydrophobic interior of the membrane and can pass through directly (this is simple diffusion). Almost all other molecules must pass using a membrane protein, a passive process called facilitated diffusion.  In facilitated diffusion the proteins act as transporters, helping to move molecules from one side of the membrane to the other. Cells can also transport things from areas of low concentration to areas of high concentration, a process that requires ATP energy and is known as active transport. Active transport proteins are called pumps. In this lab it’s all about passive transport; no active transport will be measured. Water will also diffuse across membranes from areas where it is more concentrated to areas where it is less concentrated (think of high concentration as more “free” water). The diffusion of water is a type of passive transport and is specifically called osmosis.

Before completing your pre-lab and lab, review vocabulary for tonicity (see Ch 7 in your text).

Lab Exercises

A. Diffusion through a semipermeable membrane

The presence or absence of glucose and starch is easily determined by two chemical tests. Starch can be detected with iodine solution (IKI); the iodine solution (IKI) turns dark blue in the presence of starch, but remains yellow in the absence of starch (see Fig. 5-1a). Glucose can be measured using the Benedict’s test.  Here the reagent is added to a solution containing a reducing sugar, such as glucose, the solution heated in a water bath, and the solution turns green, orange, or orange-red. If no reducing sugar is present, the solution remains blue (see Fig. 5-1b). Recall these tests from lab 2.

Diffusion through a dialysis bag 8pt

For this series we will imagine you’ve placed solutions of glucose and starch into a dialysis tubing bag and then put the bag into a beaker of IKI in water. The dialysis bag is made of cellulose and has small pores that allow movement of small molecules but not large molecules.  The images below show this arrangement.  Note the tubing has been tied off on both ends; movement of molecules across the tubing then must use the pores.  Also note that there are no proteins present.

How large are the molecules we’re using?  Small molecules include water, glucose, and IKI.  Large molecules include starch.


  • Based on the left image above (before any movement has happened), what molecule(s) besides water is/are in the beaker compartment (top arrow)?


  • What molecule(s) is/are in the bag compartment (lower arrow)?
  • Now allow the experiment to proceed to equilibrium where no net movement will occur. Based on the right image above (after all movement has happened), what molecule(s) besides water is/are in the beaker compartment (top arrow)?

What molecule(s) is/are in the bag compartment (lower arrow)?

If you test the bag and the beaker solution will the IKI show the presence of starch in either compartment?

What about the Benedict’s test: will it show the presence of glucose in either compartment?

Table 1: Prediction of results of Benedict’s test before and after. Indicate whether the test will be positive or negative (+ or -), and what the color of the solution will be.

Benedict’s Test (see Fig. 5-1 for reference)
Start of Experiment (Before Diffusion) End of Experiment (After Diffusion)
3Beaker solution Bag solution Beaker solution Bag solution


Table 2: Prediction of results of IKI test before and after.  Indicate whether the test will be positive or negative (+ or -), and what the color of the solution will be.

IKI Test (see Fig. 5-1 for reference)
Start of Experiment (Before Diffusion) End of Experiment (After Diffusion)
Beaker solution Bag solution Beaker solution Bag solution

Table 3: Actual results of dialysis experiment you would measure in the lab.

  Starting Contents Starting Color Color of solution after 1 hr of diffusion Benedict’s test
Dialysis bag  Glucose, starch Clear Blue Orange
Beaker IKI Light brown Light brown orange

You can watch a time-lapse of an actual starch/IKI diffusion experiment at


  1. Did your results support your hypotheses? If not, why not? Briefly discuss the results of the chemical tests (both Benedicts and IKI) and indicate which molecule(s) was/were permeable to the dialysis bag, and which was/were not.








  1. Hypothesize results if the solutions were reversed.

Imagine a different experiment in which you began with glucose and iodine/potassium iodide (IKI) inside the bag and starch in the beaker. In the table below predict the test outcomes on the contents of the bag and the beaker as done before (glucose, starch, and/or IKI) both before and after dialysis.



Table 4: Predicted results for Imaginary Dialysis Experiment.  Fill in the table as above with your predictions of test results and colors.

  Starting Contents Starting Color Color of solution after 1 hr of diffusion Benedict’s test
Dialysis bag        


  1. Recall movement of molecules across membranes- it is either passive (no ATP used) or active transport. Which type of membrane transport functions most like dialysis tubing? A), is it passive or active transport? Briefly explain your answer below. B) Yes, it’s passive transport, of course, but what do you think is the specific type of passive transport being displayed during dialysis? (To best answer this question, please review the two types of passive transport). Briefly explain your thinking below (note there is no absolute right or wrong answer to #B).



B. Osmosis in Red Blood Cells and plant cells (7 points)

Red Blood Cells: You will observe images of the effects of hypotonic solution (distilled water from your dropper bottle), isotonic solution (0. 9% NaCl), and hypertonic solution (saturated 25% NaCl) on erythrocytes (RBC).

  • Regarding cell size and shape, what do you expect to see in:
    1. an isotonic solution?




  1. distilled water?




  1. a hypertonic solution?


Below are three images of RBC under different tonicities.  Use these to answer question 2 below.


Isotonic                 hypotonic       hypertonic

What is actually observed in the following solutions, again with regard to cell size and shape?

  1. the isotonic solution:




  1. distilled water:




  1. the hypertonic solution:


In plant cells, there must be an overall flow of water into the cells, not only to provide them with the molecules they need for metabolism, but also for much of their structural integrity (plants can wilt). When water flows into plant cells, it creates an internal pressure called turgor pressure. The force of turgor pressure against plant cell walls is what enables a plant to stand upright.


Elodea is a common aquatic plant with a fairly typical plant cell structure. The cytoplasm and chloroplasts are normally pressed to the outside edges of the cell by the large central vacuole. The vacuole inside each cell expands or contracts as it responds to environmental conditions. If Elodea cells are placed in a hypertonic solution, water will tend to leave the vacuole and the cell will plasmolyze, or collapse. You should understand why water will leave the cell if they’re placed in a hypertonic solution- see the RBC experiment above for a hint. During plasmolysis, the cytoplasm contracts and the cell membrane separates from the cell wall.  This is reversible.


What do you think will happen to Elodea cells if you place them in a solution that is hypotonic relative to their cell contents?  What is this fresh-water organism already in, a hypotonic (i.e. DI-type water with essentially no solute) or isotonic solution?    What’s the difference do you think?


What is actually observed in the Elodea samples in the following solutions?  Consider distribution of chloroplasts in answering the question.

  1. the isotonic solution:





  1. hypertonic solution:




Explain why the red blood cells in your body are so vulnerable to a hypotonic solution, whereas plant cells do not have this problem.