This experience aims to demonstrate dialysis through a simulation of the dissemination of different substances through a semi-permeable membrane, represented by the dialysis bag. Experience illustrates the key concepts of cell biology and chemistry, such as the permeability of membranes, diffusion, and specific chemical reactions to test the presence of certain molecules in a solution.
Educational Goals
Preparation of the solution and heating: The beginning of the experiment is to prepare an aqueous solution and to heat a test tube containing glucose to simulate the preparation of the “virtual cell” and the surrounding solution. Preparation of reagents for tests: preparation of buckets with specific reagents for glucose, starch, and salt prepares the ground to test the presence of these substances after dialysis.
Preparation of the dialysis bag: The experience simulates the cell membrane using a dialysis bag, in which starch, salt, and glucose solutions are placed. The bag is then immersed in distilled water to simulate the extracellular environment. Diffusion and dialysis: The implementation makes it possible to observe the process of diffusion of molecules through the semi-permeable membrane of the dialysis bag, imitating the functioning of a living cell in its environment.
Chemical tests: After a period of dialysis, chemical tests are carried out to identify the substances that have disseminated through the bag. These tests include the use of Lugol to detect starch, Fehling A and B for glucose, and silver nitrate for salt.
Observation of changes: Experience makes it possible to observe the changes in the chemical composition of the surrounding water and inside the dialysis bag, as well as any change in volume in the bag, illustrating the principles of osmosis and diffusion.
Objectives of experience:
- Understand dialysis: demonstrate how substances diffuse through a semi-permeable membrane according to their gradients of concentration.
- Illustrate the principles of diffusion and osmosis: observe directly how molecules move from a high concentration area to a low concentration area, and how this affects volume in the dialysis bag.
- Application of chemical tests: use specific chemical reactions to test the presence of glucose, starch, and salt, stressing the importance of chemical indicators in detection of substances.
This experience offers a practical understanding of fundamental biological and chemical processes, using laboratory techniques to explore key concepts in biology and chemistry.
Protocol
Control solutions
Fill 100 mL of tap water into a 250 mL beaker.
2. Place the 250 mL beaker on the heating plate.
3. Add a certain number of different solutions as follows:
Put ten drops of starch solution in the cup labeled (starch).
Put 10 mL of glucose solution into the test tube labeled (glucose).
Put ten drops of saline solution into the cup labeled (salt).
4. Add a certain number of the different solutions as follows:
Add 10 mL of Fehling A to the test tube labeled (glucose)
Add 10 mL of Fehling B to the test tube labeled (glucose)
Add ten drops of Lugol into the cup labeled (starch).
Add ten drops of silver nitrate into the cup labeled (salt).
5. Mix the labeled cells (starch) and (salt) using a clean glass rod.
6. Place the labeled test tube (glucose) in the 250 mL beaker prepared in step 1 using universal tongs.
Set the hot plate to 75°C and wait for 30 seconds.
Preparation of the osmosis bag
8. Turn off the hot plate.
9. Put 300 mL of warm water into the 600 mL beaker.
10. Soak the dialysis bag in warm water to make it more flexible.
11. Attach the cap at the bottom of the dialysis bag.
In the graduated cylinder, successively pour 3 mL of starch solution, 3 mL of saline solution, and 3 mL of glucose solution.
13. Pour the contents of the graduated cylinder into the dialysis bag.
14. Hold the open end of the dialysis bag tightly and rinse the bag in the 600 mL beaker.
15. Place the empty 250 mL beaker next to the universal stand.
16. Attach the “virtual cell” to the universal support using the universal clamp and place it all vertically in the 250 mL beaker.
Fill the 250 mL beaker with distilled water so that the contents of the bag are immersed in the water. The water should not touch the end where the opening of the bag is located.
18. Take a few drops of water from the beaker and apply as follows:
Put 10 mL of the water solution from the beaker into the labeled test tube (Glucose Test).
Put ten drops of the beaker’s water solution into the cup labeled (Starch Test).
Put ten drops of the beaker’s water solution into the cup labeled (Salt Test)
Wait 24 hours. (Use the clock button to advance the time)
Use of indicators
20. Take a certain amount of water from the beaker and put it as follows:
Put 10 mL of the water solution from the beaker into the test tube labeled (Glucose Test).
Place ten drops of the beaker’s water solution into the labeled cup (Starch Test).
Put ten drops of the solution from the beaker’s water into the cup labeled (Salt Test).
Fill 100 mL of tap water into a 250 mL beaker.
22. Place the 250 mL beaker on the hot plate and heat at maximum intensity until boiling.
23. Observe the volume of water contained in the dialysis bag and note if there are any changes.
24. Put 10 mL of Fehling A and 10 mL of Fehling B into the two labeled test tubes (Glucose Test).
25. Put ten drops of Lugol in the two labeled cups (Starch Test).
Put ten drops of silver nitrate into the two cups labeled (Salt Test).
Place the labeled test tube (Test Glucose) initially in the 250 mL beaker prepared in step 21 and wait a few moments. (Fehling’s reaction).
28. Repeat the previous step with the test tube labeled (Glucose Test) after 24 hours (Fehling’s reaction).
Anticipated Outcomes
The iodine (I-) contained in Lugol’s solution reacts with starch to form an iodine-starch complex. When iodine is added to starch, it fits inside the helical structure of the starch molecules, resulting in a color change to blue-black. This reaction is often used as a qualitative test to indicate the presence of starch.
The Fehling A solution is essentially a CuSO4 solution with a molarity of 0.05M, with its’s characteristic sky-blue color due to Cu2+ ions. The Fehling B solution contains Rochelle salt (potassium sodium tartrate) and NaOH 0.0625M. Fehling’s B solution is typically a clear, colorless liquid.
When Fehling’s A solution is mixed with Fehling’s B solution without heating, the two solutions react to form a deep blue complex. This complex is a result of the copper (II) ions from Fehling’s A reacting with the tartrate ions from Fehling’s B in an alkaline environment, forming a copper (II)-tartrate complex. The mixture will be a deep blue color. Heat is necessary to drive the reduction of copper (II) to copper(I), resulting in a deeper blue color.
When you introduce reducing sugars to the heated Fehling’s solution (a mixture of Fehling’s A and B), a chemical reaction occurs where the reducing sugars donate electrons to the copper (II) ions, reducing them to copper(I) ions. The solution’s color changes from deep blue to light blue, followed by the appearance of a red precipitate, is indicative of the presence of reducing sugars.
This test is specific for reducing sugars, which are sugars that have free aldehyde or ketone groups capable of acting as reducing agents. Common reducing sugars include glucose, fructose, lactose, and maltose. Non-reducing sugars, like sucrose, do not react in this test unless they are hydrolyzed to their reducing sugar components. Silver nitrate reacts with chloride (Cl-) to produce a white precipitate of AgCl (s).
Water has moved from outside the cell to inside. This is observed by the water level in the beaker, which has slightly decreased, and by the bag, which has slightly enlarged. Glucose and starch have moved from inside the bag to the external medium. They are detected in the water surrounding the bag using Lugol’s iodine and Fehling’s solution tests. The concentration of substances inside and outside the membrane, as well as the size of the particles relative to the size of the membrane’s pores.
This could be explained by 3 principles:
- Water movement: This describes a process like osmosis, where water moves across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. In this case, the water inside the beaker (outside the cell or bag) moves into the bag (representing the cell), causing the water level in the beaker to decrease and the bag to expand as it fills with water.
- Movement of glucose and starch: This indicates that glucose and starch, initially inside the bag, have moved to the outside environment (the beaker water). This movement could be due to dialysis, a process where smaller molecules and ions can move through a semi-permeable membrane, while larger molecules cannot. The presence of glucose and starch in the external solution is confirmed using specific tests: Lugol’s iodine test for starch, which turns blue-black in the presence of starch, and Fehling’s solution test for reducing sugars like glucose, which results in a color change when glucose reduces the copper (II) ions in Fehling’s solution to copper(I) oxide.
Concentration and particle size: This statement refers to the factors influencing the movement of substances across a membrane. The concentration gradient (the difference in substance concentration inside and outside the membrane) and the relative size of the particles compared to the membrane’s pore size determine which substances can pass through the membrane. Larger particles or molecules that exceed the pore size of the membrane cannot pass through, while smaller ones can.
Summary of Assignment by Grade Range
Grades 3-5 (Ages 8-10)
- Focus: Basic introduction to osmosis and diffusion, simple preparation steps, and basic observations.
- Activities: Preparing simple solutions, basic use of a dialysis bag, and introductory chemical tests.
Grades 6-8 (Ages 11-13)
- Focus: Intermediate understanding of osmosis and diffusion, detailed preparation steps, and intermediate observations.
- Activities: Preparing and heating solutions, using a dialysis bag for diffusion experiments, and applying chemical tests.
Grades 9-12 (Ages 14-18)
- Focus: Advanced understanding of osmosis and diffusion, detailed preparation and observation, and comprehensive chemical testing.
- Activities: Preparing detailed solutions, performing complex diffusion experiments with a dialysis bag, and conducting in-depth chemical tests and analyses.
Laboratory essentials
Instruments
Beaker (250 ml & 600 ml)
Erlenmeyers (50 mL)
Bucket plate
Droppers
Pipette
Graduated cylinder (10 ml & 100 ml)
Hot plate
Lab stand & Clamps
Osmosis bag
Test tubes
Glass rod
Products
Fehling A solution
Fehling B solution
Glucose solution
Lugol 2% solution
Silver nitrate solution
Sodium chloride in solution
Starch solution