Diffusion and osmosis are related concepts, which involve movement of materials from areas of high concentration to areas of low concentration. Diffusion involves movement of any chemical from one place to another; osmosis refers to movement of water across a membrane. Only water can undergo osmosis.
The rate of diffusion depends on the molecular weight of the chemical.
In this demonstration, the blue dye, methylene blue, has a higher molecular weight than the compound potassium permanganate. Smaller, lighter molecules diffuse faster than larger, heavier ones, and the potassium permanganate solution (on the right) has diffused further from the well than the methylene blue (on the left).
The rate of diffusion depends on the characteristics of the medium through which the substance diffuses.
Crystals of potassium permanganate were placed in distilled water (left) and on an agar gel (right). The agar gel is still about 98% water, but the gel forms a crisscrossing network of carbohydrate that the potassium permanganate ions must work through. Since the ions must move up, down, forward, and even backwards to diffuse, it takes much longer for them to move away from the source.
The rate of diffusion through a membrane depends on the characteristics of the membrane and of the materials.
In this demonstration, we put starch inside a dialysis bag, and an iodine solution outside it. The dialysis bag is made of a membrane that prevents molecules with molecular weights of more than about 10,000 from passing through. Starch, with a molecular weight in the 100,000's, can't pass through the membrane and remains inside the bag. Iodine, with a molecular weight of less than 100, can pass through the bag, and reacts with the starch inside to turn purple. Dialysis tubing is an example of a semipermeable membrane, because only small molecules can get through it. Other membranes are selectively permeable because they can restrict passage to specific molecules.
Osmosis is the diffusion of water across a membrane.
In this demonstration, we put dialysis bags of either water or a sucrose solution into beakers that contain either water or the sucrose solution:
If we consider distilled water to be 100% water, and the sucrose solution to be 90% water and 10% sucrose, then we can see that there is a concentration difference between water and sucrose in beakers 2 and 3, but that there is no concentration difference in beakers 1 and 4. We should see no net movement of water or sucrose in beakers 1 and 4, but we should see a net movement of water iout of the dialysis bag in beaker 2, and into the bag in beaker 3. Osmosis will occur in beakers 2 and 3.
We can also refer to the relative concentrations of water and sucrose in the solutions as hypotonic, isotonic, and hypertonic. In a hypotonic solution, the concentration of solute (sucrose, in this demonstration) is lower than in the solution you are comparing it to. So in beaker 2, the water in the dialysis bag is hypotonic to the sucrose solution in the beaker. In a hypertonic solution, the concentration of solute (sucrose, in this case) is higher than in the solution you are comparing it to. So in beaker 2, the sucrose solution in the beaker is hypertonic to the water in the dialysis bag. In beaker 4, the solution inside the bag has the same solute concentration as the solution in the beaker. The solutions are isotonic. Now, for practice, describe the relationships of the solutions in beakers 1 and 3. Check your answer.
The water in the dialysis bag and in the beaker 1 are isotonic with respect to each other. The sucrose solution in the bag in beaker 3 is hypertonic to the water in the beaker; the water in the beaker is hypotonic to the sucrose solution in the bag. Return to osmosis.