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Diffusion and Osmosis


Enrique Hueyopa


What is the effect of temperature on the rate diffusion of Iodine through a semipermeable membrane?


        Biology/Life Sciences
        Cell Biology
            1.The fundamental life processes of plants and animals depend on a variety of chemi­cal reactions that occur in                     specialized areas of the organism’s cells. As a basis for understanding this concept:
                a.Students know cells are enclosed within semipermeable membranes that regulate
                their interaction with their surroundings.

Investigation and Experimentation
    1.Scientific progress is made by asking meaningful questions and conducting careful
    investigations. As a basis for understanding this concept and addressing the content
    in the other four strands, students should develop their own questions and perform
    investigations. Students will:
        a.Select and use appropriate tools and technology (such as computer-linked
        probes, spreadsheets, and graphing calculators) to perform tests, collect data,
        analyze relationships, and display data.
        b. Identify and communicate sources of unavoidable experimental error.
        c. Identify possible reasons for inconsistent results, such as sources of error or
        uncontrolled conditions.
        d. Formulate explanations by using logic and evidence.

Experimental Design

Independent variable

The temperature variation of the iodine solution.

Dependent variables

The rate of diffusion of Iodine.


Room temperature

Heated solution


semipermeable membrane

amount of iodine, water, starch used

containers, apparatus, probes



large plastic containers (able to hold more than 1 liter)

fruits and vegetable grocery plastic bags

eye dropper

graduated cylinder

distilled water


light sensors, temperature sensors, usb link adapters

Datastudio software


Semipermeable membrane preparation

  •  cut a grocery plastic bag to 25 cm
  • cut two strings (one at 15cm and the other at 50 cm)
  • fold the bottom (the side of the bag that is sealed) portion of the bag into thirds and tie with the short string (15cm)
  • prepare a mixture of 500 ml distilled water and 20 g of starch
  • pour the solution into the plastic bag and tie the open end with the 50 cm string 
iodine solution preparation
  • pour 1000 ml of distilled water into the container
  • pour 2 ml of iodine into the container
Experiment run
  • place container in a microwave for two minutes
  • place bag in the container with the long string hanging out of the container
  • place probe into the solution and start the Data studio program and run

Sample data and graphs

Photographs and Movies


Diffusion - the process by which molecules spread from areas of high concentratiion, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM

Concentration gradient - a difference between concentrations in a space.


Watch this animation of water molecules moving across a selectively permeable membrane. Water molecules are the small blue shapes, and the solute is the green.

The solute is more concentrated on the right side to start with, which causes molecules to move across the membrane toward the left until equilibrium is reached.

Start Animation

Selectively Permeable - membranes that allow some things through, the cell membrane is selectively permeable, water and oxygen move freely across the cell's membrane, by diffusion

Osmosis - the diffusion of water (across a membrane)

Water will move in the direction where there is a high concentration of solute (and hence a lower concentration of water.

A simple rule to remember is:

Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty.

Type of Solutions

If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side.

"ISO" means the same

The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell.

The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting

In animal cells, the cell may be in danger of bursting, organelles called CONTRACTILE VACUOLES will pump water out of the cell to prevent this.

The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction.

In plant cells, the central vacuole loses water and the cells shrink, causing wilting.

In animal cells, the cells also shrink.

In both cases, the cell may die.

This is why it is dangerous to drink sea water - its a myth that drinking sea water will cause you to go insane, but people marooned at sea will speed up dehydration (and death) by drinking sea water.

This is also why "salting fields" was a common tactic during war, it would kill the crops in the field, thus causing food shortages.

Diffusion and Osmosis are both types of PASSIVE TRANSPORT - that is, no energy is required for the molecules to move into or out of the cell.

Sometimes, large molecules cannot cross the plasma membrane, and are "helped" across by carrier proteins - this process is called facilitated diffusion.


Osmosis and Diffusion in Dialysis Tubing 

 It is very difficult to measure or see osmosis actually occurring in cells because of the small
size of most cells. However, if an artificial membrane that acts in some ways like a real plasma membrane
could be found, than a study of osmosis using a model cell would be possible. Dialysis tubing is a
manmade semi-permeable membrane that is used to treat people who have kidney failure. Dialysis is a
process where substances in solution are separated by their difference in molecular weight (size). The
driving force behind dialysis is the concentration difference between two solutions on opposite sides of
the membrane.

Materials: (per lab team)
• 250 ml beaker
• 1- 10 cm length of Dialysis tubing
(soaked in warm water)
• 2 pieces of Cotton string 10 cm long
• 15 ml Starch solution with transfer
• 5 ml Supersaturated glucose solution
with transfer pipette 
• 10 ml Iodine solution
• Tap water
• Glucose test strip
1. Gather one piece of dialysis tubing and two pieces of string.
2. Under gently running water rub the tubing between your fingers until it opens up to form a tube. !!!BE
3. Once the tubing has opened twist one end shut and tie a knot at least 2 cm from the edge.
4. Fold the tail upwards and tie a second knot. 
5. Fill the tube with water and check for any leaks.
6. If no leaks are found empty the tube and fill it with 5 cm of starch solution.
7. Add one pipette full of glucose solution to the tube.
8. Twist the top end of the tube shut just above the surface of the starch-glucose solution and tie a knot.
9. Fold the tail over and tie a second knot.
10. Rinse the tubing off under gently running water.
11. Cut off any remain string
12. Fill your 250 ml beaker to the 150 ml line with tap water.
13. Place the tube in the beaker and add water if necessary to cover the tube.
14. Add several drops of iodine so that the color of the water in the beaker turns orange.
15. Use masking tape to label your beaker with your group name and period #.
16. Clean up your lab area and return materials to the cart.