Physics of Diffusion Across a Selectively Permeable Membrane

About this essay


Diffusion is movement of molecules from one area of concentration to another. This process is vital for the life functions of cells. Cells have selectively permeable membranes that allow only certain solutions to pass through them. Osmosis is a special kind of diffusion that allows water to go through semi-permeable membranes of high water potential to a region of lower water potential. Water potential measures the tendency of water to leave one place and into another. Diffusion and osmosis is responsible for the movement of molecules in and out of cells.

Diffusion and osmosis can be shown using dialysis tubing which is a membrane that is made of regenerated cellulose fibers into a flat tube. When solutions that contain dissolved substances, are separated by a membrane like the dialysis tubing, some may readily pass through it and others may not due to the size of the molecules.


Diffusion and osmosis will occur once dynamic equilibrium is reached. If a substance contains molecules small enough to pass through a selectively permeable membrane then they will readily pass through it without the solution or substance, which is inside the membrane, getting out.

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  • String
  • Starch Solution
  • I2KI Solution
  • Plastic cup
  • Disposable transfer pipettes
  • Strips of dialysis tubing soaked in water
  • Timer
  • Scale


First you will have your dialysis tubing soaked in water ready for the experiment. Once the dialysis tubing in being soaked, wash your hands to get rid of any oils or dirt that may be on them.

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Remove the tubing from the water and tie off one end. Open the other end using two fingers in a rubbing motion so that the tubing will separate and open. Use a disposable pipette to fill the tubing with starch solution. Once the tubing is filled leave a bit of space so that expansion may occur and tie off the end and place the tubing on the scale to see how much it weighs. Fill the cup with distilled water and add I2KI solution to the water. Place the tubing inside the cup for 30 minutes and record any color change. Record the color change of the bag every 5 minutes until 30 minutes have passed. Take the tubing out of the water solution and place on the scale and record any weight change.


  • Solution source
  • Original Contents
  • Original color
  • OriginalWeight
  • min color 5
  • min color 15
  • min color 25
  • min color 30
  • Final weight

Final Contents

  • BagWater 1% starch
  • White/milky10g
  • White slightly blue
  • Milky inside darker blue
  • Darker blue some white visible
  • Darker blue no white visible11g
  • Water 1% starch + iodine Beaker
  • Water, I2KIAmber yellow
  • N/AAmber
  • YellowAmber
  • YellowAmber
  • YellowAmber
  • YellowN/A
  • Water and iodine


The results show that the water, glucose, and I2KI molecules were small enough to pass through the selectively permeable membrane which was the dialysis tubing. I would expect for the color change to occur in the cup instead of inside of the tubing if we had started with the starch in the cup and the I2KI inside the tubing, because the iodine would instead of going into the bag, would leave through the bag and into the starch inside the cup.

Discussion and conclusion

The data shows what molecules can and cannot diffuse across a selectively permeable membrane. The color change that occurred showed that the I2KI molecules were small enough to pass through the pores of the membrane or dialysis tubing. This data also shows that the water and glucose solution moved out of the dialysis bag because water molecules are small enough to pass through the membrane. No mistakes were made in this experiment but many could have occurred. If the tubing was not tied correctly a leak could have occurred and the experiment would have failed and been inaccurate.


  1. “Diffusion and Osmosis.” Diffusion. N.p., n.d. Web. 19 Mar. 2014 “Diffusion.” And Osmosis. N.p., n.d. Web. 20 Mar. 2014.
  2. “Osmosis.” Osmosis. N.p., n.d. Web. 19 Mar. 2014.
Cite this page

Physics of Diffusion Across a Selectively Permeable Membrane. (2016, Apr 13). Retrieved from

Physics of Diffusion Across a Selectively Permeable Membrane

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