Diffusion and Osmosis

concept map diffusion and osmosis



Diffusion is the process by which particles move from a region of higher concentration to a region of lower concentration. This process will occur until there is an equal distribution of these particles, so that there is no longer regions of higher or lower concentration so that no movement is needed. Diffusion can occur is liquids and also in gases. It is easier to use some examples to explain:

Example 1) Smells propagating: when somebody uses a sparay deodorant in a room, for instance, the smell spreads to the whole room, within minutes. That's because the particles of deodorant move through the air (a gas) from regions of lower concentration (close to the deodorant container) to regions of lower concentration. A fart propagates the same way, as en example of diffusion of a gas (fart) into another gas (the air we breath).

2) Coloured pellets dissolving in water: if you dissolve purple potassium permanganate pellets in water, the purple colour will diffuse through the whole container.

IMPORTANT:It is no needed to stir the contents! The diffusion will occur spontaneously, thanks to the random movement of the particles. The higher the temperatute, the higher the velocities of the molecules and as a result the faster the diffusion occurs.

3) Animation of diffusion in gases. The particles of two different gases are represented by blue and red colours (gas molecules don't have colours, this is just to be able to tell one from the other). The two gases are initially separated, each in one compartment , but they can interact through a small hole in the wall separating them. As diffusion goes one, the gas mix together, as it can be observed in the figures 1 to 3 (below).This example would also work if there was only one gas and the other compartment was empty. In this case the gas would occupy both containers equally.

Figures 1, 2 and 3: Snapshots of the animation picturing gas diffusion.

You can try the animation here

The concept map on the top of this page illustrates the importance of diffusion in biologic systems, allowing for the entrance of oxygen in the cell and the elimination of the waste carbon dioxide. As the cell consumes oxygen, its concentration falls below of that on the blood and oxygen diffuses in. These gases can cross freely the cell membrane.

Glucose also enters cells by diffusion (although it is tecnically a bit different, it is a process called facilitated diffusion, because glucose is a polar molecule and cannot cross the cell membrane freely, it requires a transporter protein located at the membrane). As the glucose in the cell is consumed and the concentration falls, more can come in. It is interesting that if the glucose is not being consumed very much, the cells can store excess glucose in the form of glicogen, a polymer. Because the glucose has been transformed into the polymer, there is little glucose in the cell as a result and more can come in by diffusion.


Osmosis is the movement of solvent, through a semi-permeable membrane, towards regions of higher concentration of solute. The solvent is usualy water. A semi-permeable membrane let some particles through and stop others. It does so based on the size of the particles involved. Water molecules are smaller than solute molecules so that it can croos the membarne while the solute can't.

You could think of osmosis as a kind of diffusion, where water moves from a region where there is more water to a region where there is less (because there is more solute dissolved).

Example 1) Killing a slug with salt. The slug skin is a kind of semi-permeable membrane. When you deposit salt on top of the slug, the salt concentration outside will be higher than inside the slug, so that water will move to the outside and dehydrate the slug to death.

2)Potato slices. This is the classical example, often brought in chemistry and biology exams. A potato slice has an internal concentration of solute that is higher than that of tap water. If you place it in a glass with tap water, it will absorb water, by osmosis. On the other hand, if you place the potato slice in salty water (more concentrated than the potato), water will flow from the potato to the outside and the potato slice will shrink.

3)Red blood cells. The potato example above may look a bit unimportant, but it illustrates what happens with red blood cells (RBC's). That is, if the concentration of blood is reduced, the red blood cells will take water in until the point the break, causing serious problems to the organism. The process is analogous to the one described for the potatoes.

4)Fishes. A freshwater fish will loose its water if placed in the sea. A sea fish will get swollen if placed into salt water.

Isotonic: That means, of the same concentration. For instance, blood and red blood cells must be isotonic, so that RBC's neither shrink nor get swollen.

Isotonic drinks are those that have concentrations, of salts, minerals and sugars, similar to those of a human body. In general, they are just ordinary drinks with added sugar, salt and minerals. You can do it yourself by adding sugar and cokking salt to whatever fruit juice. Some drinks may have other minerals, in which case you can crush a mierals tablet and put it in the drink. That will save you money.

Saline solutions are used when water and other substances must be injected into the circulatory system. They are also called isotonic saline. Their salt concentration will ensure that osmotic problems will not occur. Even when forensic scientists need to collect a sample of blood in a crime scene, saline solution must be used because ordinary water would destroy the RBC's.

Osmosis is a fundamental principle for life. For instance, plants absorb water from the soil by osmosis through their root hair cells. 

So, although osmosis is a fundamental principle that operates to ensure the good functioning of living organisms, it may also cause death (as described in 3)).