NYS REGENTS LAB: Diffusion Through A Membrane

Summary

This lab procedure demonstrates diffusion and osmosis through membranes using dialysis tubing, chemical indicators, and wet mount slides. Students create a cell model by filling dialysis tubing with glucose and starch solutions, then submerge it in water with iodine to observe diffusion. Chemical tests for starch and glucose are conducted using iodine and Benedict's solution. For osmosis, students prepare wet mount slides with red onion skin and observe the effects of saltwater and freshwater solutions under a microscope. The process provides hands-on learning of key biological concepts related to diffusion, osmosis, and chemical testing.

Takeaways

• 😀 Dialysis tubing is used to simulate a cell by filling it with glucose and starch solutions to demonstrate diffusion through a membrane.
• 😀 The first step in setting up the dialysis tubing is to tie off one end tightly to prevent leakage of solutions.
• 😀 When filling the dialysis tubing, ensure to add about a quarter to halfway of glucose and starch solutions before tying off the other end.
• 😀 After preparing the cell, rinse it off and place it into a beaker of water with iodine solution to visualize starch diffusion.
• 😀 Iodine solution serves as the starch indicator, turning amber in the presence of starch and remaining yellowish in its absence.
• 😀 For chemical testing, use iodine to test for starch and Benedict's solution for glucose. Each solution should be added to separate test tubes.
• 😀 The Benedict's solution test for glucose requires heating in a hot water bath, where a positive result shows a color change to orange or red.
• 😀 For the wet mount slide, start by adding freshwater to a microscope slide and placing a red onion skin onto it for observation.
• 😀 When preparing the wet mount slide, add a few drops of saltwater next to the coverslip to observe osmosis in action as water moves across the membrane.
• 😀 The saltwater will cause water to leave the onion cells (plasmolysis), whereas freshwater will cause water to enter the cells (turgidity).
• 😀 The procedure highlights key biological concepts like diffusion, osmosis, and the use of chemical indicators for testing starch and glucose in solutions.

Q & A

What is the purpose of using dialysis tubing in this experiment?

Dialysis tubing simulates a semi-permeable membrane, allowing certain substances (like glucose and starch) to pass through while preventing others from leaking out. This demonstrates diffusion and osmosis processes in cells.

How do you ensure the dialysis tubing is properly sealed?

To seal the dialysis tubing, twist one end tightly, loop it, and pull it through to secure it. Repeat the same process for the other end, ensuring no leakage of the solution inside the tubing.

Why is it important not to overfill the dialysis tubing with solution?

Overfilling the dialysis tubing may lead to spillage or difficulty in sealing the ends properly. It's recommended to fill the tubing only about halfway to ensure proper closure and prevent leaks.

What is the role of iodine solution in this experiment?

Iodine solution acts as a starch indicator. It helps to detect the presence of starch by turning blue-black when starch is present and amber when it is not.

How do you test for the presence of glucose in this experiment?

To test for glucose, add Benedict's solution to the test tube containing the glucose solution. Heat the mixture in a hot water bath. A color change to orange or red indicates the presence of glucose.

What safety precautions should be followed when handling iodine and Benedict's solution?

Be cautious when using iodine, as it is a stain and can ruin clothing or skin. Handle Benedict's solution carefully, and avoid direct contact with skin. Always wear appropriate protective gear, such as gloves, when working with these chemicals.

Why is it necessary to label the test tubes during the chemical testing portion?

Labeling the test tubes ensures that you can correctly identify which solution (water, glucose, or starch) is being tested, preventing confusion and ensuring accurate results.

What observation would indicate a positive test for starch?

A positive test for starch will result in a color change from amber to black or blue after adding iodine solution to the test tube.

How does the wet mount slide setup help observe osmosis?

The wet mount slide setup allows the observation of osmosis by placing a piece of red onion skin in freshwater or saltwater and observing how water moves in and out of the onion cells under the microscope.

What should you do if the saltwater doesn't move across the coverslip during the wet mount slide procedure?

If the saltwater doesn't move, adjust the position of the paper towel to help draw the solution across the coverslip. Be careful not to dry out the specimen or move the cover slip too much.