Osmosis
Objectives:
Students should learn to:
1. Distinguish diffusion and osmosis. 2. Explain the effects of a semipermeable membrane like the cell membrane on movement by
semipermeable pores of a dialysis tubing.
Diffusion is the movement of substances from a high concentration to a low concentration. It comes about because the molecules of the substance move anyway randomly in all directions all the time. For example, perfume molecules in air move randomly in all directions all the time. Therefore, if more perfume molecules are present in one area, more will move randomly to the other area with less perfume molecules than the other way around. Osmosis is the movement of substances by diffusion when it occurs through a semipermeable barrier. A barrier is semipermeable, when it lets some substances pass but others not. A semipermeable barrier might let water pass but not the other substances that are dissolved in water. The substances that are dissolved in water are called the solutes, whereas water is then called the solvent. In osmosis when only water can pass the semipermeable barrier, just like in general diffusion, more molecules of water will pass through the barrier to the side where its concentration is lower than the other way around. The concentration of water is lower, where it contains more solutes. Note that movement of water by osmosis is a spontaneous movement that requires no energy. It happens “on its own.” The rate at which this movement occurs depends on many factors, including the relative concentration of water on each side, and temperature, among others. In biology, we are generally concerned with osmotic movement of water in and out of cells across cell membranes. Cell membranes are the semipermeable barriers around our cells that allow water to pass unhindered but that prevent many of the solutes dissolved in water from passing. Cell membranes though can be permeable to some important other molecules than water.
In lab, we will investigate how a semipermeable membrane functions by using a dialysis tubing as our example. A dialysis tubing has tiny holes that let small molecules pass but not large ones. We will investigate whether starch, protein, sulfate ions and chloride ions pass the holes in the dialysis tubing. Do you know the relative sizes of these four compounds? Can you formulate a hypothesis as to which of them you expect to pass the dialysis tubing and which not? Write down your hypothesis: (1 pt)
BIOL%114%
The procedure is as follows:
1. Add approximately 100mL starch/sodium sulfate (Na2SO4) solution to a beaker.
2. Take a piece of dialysis tubing from its water bath and tie one end tightly with string so that it does not leak.
3. Using a graduated cylinder and a funnel, add 10mL protein/sodium chloride (NaCl) solution to the dialysis tubing bag, leaving a small amount of air space at the top of the bag. Tie the other end as before, ensuring it does not leak.
4. Place the filled dialysis tubing bag in the beaker solution. Make sure it is fully submerged.
5. Wait for 20 min, swirling the beaker occasionally.
6. Remove the dialysis tubing bag, blot it with a paper towel to remove excess solution, and then empty its contents into a clean beaker.
7. Distribute the dialysis solution about evenly into 4 test tubes.
8. Perform the 4 tests described in the table below with the 4 test tubes. Use 2-3 drops of each reagent. Enter the results in the data table below. (3 pts) Attention: Solutions tested with silver nitrate and Biuret must be discarded in the hazardous waste bottle for Silver & Biuret!
9. Repeat steps 7-8 with the beaker solution (the one the bag was submerged in for 20 min.)
10. Clean test tubes, beakers and graduated cylinder with soap water, then rinse with DI water.
Test Positive result Negative result IKI test for starch dark purple/blue/black color no color change or amber color
Barium chloride (BaCl2) test for sulfate ions cloudy white precipitate no color change or precipitate
Silver nitrate (AgNO3) test for chloride ions cloudy white precipitate no color change or precipitate
Biuret test for protein light purple color no color change or light blue color
Substances in BAG
Before Dialysis After Dialysis Starch Sulfate ions Chloride ions Protein
Substances in BEAKER
Before Dialysis After Dialysis Starch Sulfate ions Chloride ions Protein
Study questions (1 pt each)
1. Which substances was the dialysis tubing permeable to?
2. Which substances was the dialysis tubing impermeable to?
3. Do your results support your hypothesis? Or do you have to reject your hypothesis?
Applications in everyday life:
When we use perfume, we rely on diffusion to move the perfume molecules through the air to the noses of other people so they can smell them. Similarly, when we put a tea bag into a cup of hot water, we rely on diffusion to move the tea molecules from the bag, the area of high concentration, to the surrounding hot water, the area of low concentration.
Reverse osmosis can be used to produce pure, drinkable water from contaminated water or from salt water. Normally, when pure water and salt water are separated by a semipermeable membrane that only allows water molecules to pass, osmosis will happen. Water molecules will move from the pure water to the salt water, because where there is salt, the water concentration is lower. For reverse osmosis to occur, pressure must be applied that will force the water molecules through the semipermeable membrane in reverse of the flow that would otherwise occur.
