Membrane Transport Activity: Simulation Guide

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The biological membrane is made up of mostly phospholipids, which enables the membrane to interact with the aqueous environment within and outside the cell. The same phospholipid gives the membrane a selectively permeable characteristic, hence limiting the number of substances that can enter and exit the cell. In this exercise, you will be simulating how membrane transport occurs using commercially accessible materials. Setting up the experiment should not take your more than 30 min. Please note the group discussion questions at the end of the protocol.

Materials Needed

    • Povidone iodine (e.g. Betadine)
    • Drinking water 
    • Cornstarch or laundry starch
    • 4x 5-inch long Sausage sacs or 4x ice candy bags/ziplock bags (DO NOT USE POLYPROPYLENE/PP BAGS)
    • Table salt 
    • Table sugar 
    • 4x clear small-sized cups 
    • String
    • Measuring cup or its alternatives 
    • Table spoon
    • Teaspoon
    • Funnel
    • Paper towels or tissue
    • Scissors
    • Marker 

Procedure

A. Preparation of Solutions  

  1. Best to use lukewarm water to dissolve as much of the solute as possible.
    • Solution 1 - 1/2 cup water + 1-2 tbsp table salt (dissolve)
    • Solution 2 - 1/2 cup water + 1-2 tbsp table sugar (dissolve)
    • Solution 3 - 1/2 cup water 
    • Solution 4 - 1/2 cup water + 1 tsp starch
  2. (To prepare the starch solution you have to first dissolve 1 tsp in a bit of cold water, then add it to boiling hot water. The resulting solution should appear translucent. Please let the solution cool before using it for the subsequent steps.)

B. Preparation of Sausage Sacs or ice candy bags 

  1. If using sausage sacs,  cut 4 sacs of about 5-inches in length. Tie one end securely with a string and make sure that nothing leaks. If using ice candy bags, you can skip this step. 
  2. In each sac/bag, put 3-5 tablespoons of the previously prepared solutions. A funnel might be helpful in the process. Make sure that you add equal amounts of the solutions in each of the bags.  
    • Casing 1 - Solution 1
    • Casing 2 - Solution 2
    • Casing 3 - Solution 3
    • Casing 4 - Solution 4
  3. Securely tie the other end of the sac/bag. Make sure that some air space is allowed inside the bags. 
  4. Pat dry the bags and leave them aside for the next step. Make sure you label them so that they do not get mixed up. 

C. Preparation of the Set-ups

  1. Fill about a third of each of the 4 cups with room temperature water (just enough to submerge the sacs). Make sure that there are equal amounts of water in all 4 cups and that the cups are not overfilled so nothing spills over when the sacs/bags are added. 
  2. To one of the cups with water, add 3-4 drops of povidone iodine or until an even dark yellow color (similar to honey) is observed. 
  3. Mark the water level in each of the cups. 
  4. Fully submerge the sacs/bags as follows: 
    • Cup Solution in Cup  Casing 
      1 water Casing 1 with salt
      2 water Casing 2 with sugar
      3 water Casing 3 with water only 
      4 water + povidone iodine  Casing 4 with starch
    • Make sure you label your set-ups properly! 
  5. Leave the set-up in a cool place for 2 hours. Make sure that your set-up is secured that it will not put bystanders at risk (there are no real risks except spillage). 📷 Take a selfie with all 4 set-ups and submit this. 

D. Analyzing the Set-Ups

  1. Take out each of the sacs/bags from the cups making sure that there is no water spilled in the process. DO NOT throw away the sacs/bags. 
  2. Mark the water level post-incubation. 📷 Take a picture of all 4 set-ups with the markings of before and after incubation. Do you notice any changes in the water level? What does this indicate? 
  3. For Cups 1 and 2, taste the solution in it (Hence, the need to use drinking water!) Do you notice anything different? What does this imply? (Note: If you used sausage sacs, skip this step.)
  4. Check Casing 4. Is there any change in the color of the solution inside the casing? What does this mean? 
  5. Check the remaining solution in Cup 4, is there anything different about the solution? What does this indicate?

Group Discussion Questions

  1. Discuss and compare your observations with the class about each set-up.
  2. Are there similarities or differences with your results?
  3. What is the most common result within the class?
  4. Is the common result consistent with the theoretical results of the set-up? If not, what could be the reason for the discrepancy?