======Osmosis and Diffusion with Dialysis Tubing======


<WRAP right box>
**Materials: **{{$demo.materials_description}}\\
**Difficulty: **{{$demo.difficulty_description}}\\
**Safety: **{{$demo.safety_description}}\\
\\
**Categories:** {{$demo.categories}}
\\


**Alternative titles:** Model Cell Membrane with Starch and Glucose, Demonstrating Selective Permeability
</WRAP>

====Summary====                   
{{$demo.summary}}



====Procedure====  
  - Soak 15 cm lengths of dialysis tubing in water for at least 15 minutes.  
  - Prepare 2% starch solution (2 g in 100 mL boiling water) and 30% glucose solution (30 g in 100 mL water).  
  - Combine equal volumes of starch and glucose solutions to make a 1% starch and 15% glucose mixture.  
  - Measure and record initial glucose concentration of the mixture using glucose test strips (after diluting 1 mL sample into 9 mL water).  
  - Prepare iodine solution by adding 1 mL iodine/potassium iodide (I₂/KI) solution to 100 mL water in a beaker. Test for glucose; record results.  
  - Tie one end of dialysis tubing securely. Half-fill with the starch–glucose mixture using a pipette, then tie the other end to make a sealed pouch.  
  - Place the tubing pouch in the iodine solution and leave for ~15 minutes.  
  - Observe color changes: note whether the tubing solution or surrounding solution changes color.  
  - Measure glucose concentration in the iodine solution after 15 minutes with a test strip.  
  - Carefully empty the tubing into a beaker, dilute 1 mL into 9 mL water, and test for glucose again. Record results.  
  - Compare initial and final glucose levels inside and outside the “cell” pouch.



====Links==== 
Dialysis Tubing Experiment - Bright Biology: 
{{youtube>pduvq1qzfbk?}}\\

Osmotic Pressure Dialysis Tubing - North Carolina School of Science and Mathematics: 
{{youtube>Ji1wetFngLo?}}\\

📄 Osmosis and Diffusion - Southern Biological: [[https://www.southernbiological.com/osmosis-and-diffusion/?srsltid=AfmBOorvzo8tSvppswkB0ZCZqcCD31m4W6TPRhMDXG5X5SjZsbn1OROl]]\\


====Variations====
  * Use a pouch made of parchment paper instead of the dialysis tubing, and fill with water and food colouring.
  * Leave a pouch overnight to show longer-term diffusion of glucose toward equilibrium.  
  * Repeat with different solutes to test permeability (for example, sucrose vs glucose).



====Safety Precautions====
  * Wear gloves, goggles, and a lab coat; iodine stains skin and clothing.  
  * Handle glassware carefully to prevent spills and breakage.  
  * Avoid direct contact with iodine solution and wash hands thoroughly after the experiment.  
  * Dispose of chemical solutions as instructed by your teacher.




====Questions to Consider====
  * Why does the solution inside the tubing turn blue? (Iodine passes in and reacts with starch to form a starch–iodine complex.)  
  * Why doesn’t starch diffuse out of the tubing? (Starch molecules are too large to pass through the membrane pores.)  
  * Why is glucose detected outside the tubing after the experiment? (Glucose molecules are small enough to diffuse through the membrane.)  
  * What does this experiment demonstrate about cell membranes? (They are selectively permeable—some molecules pass freely, others cannot.)  
  * How does molecular size affect diffusion across membranes? (Smaller molecules diffuse more easily, while larger ones are restricted.)