Transcription
[Music] Osmosis is a special type of diffusion that applies to water and other solvents. If you take a liter of pure water and compare it to a liter of seawater, which is very salty, there is less water present in the seawater liter because some of that liter is occupied by salt. The saltwater has a lower water potential.
If there is a partially permeable membrane, like a cell membrane, separating two different water samples, there will be a net movement of water from the place with a higher water potential to the place with a lower water potential. This is the reason plant roots take in water. The rainwater that enters the soil has a higher water potential than inside of the roots, so water moves into the roots by osmosis.
Where we can demonstrate this in a lab setting using potatoes. First, you need a cork board to get your potato strips. Push it through the potato, then use the narrower piece to push the strip out. For this experiment, we need 18 strips. Once you've got as much as you need, tidy them up with a knife and remove the skins. All of the strips need to be the same length to begin with. Here, we're cutting them all down to 6 centimeters.
Once they're ready, get 6 beakers and put three tubes in each. This will give you 3 repeats for each solution. The next thing to do is make your solutions. Put a sheet of paper on the balance, hit tear to get the starting point of 0, and add salt until it reads one gram. [Music] Once that's in the beaker, you need 100 milliliters of water or another measuring cylinder. Keeping your eyes on the level with 100ml until it reaches the line, and then add that to the salt. Here, we've just made a salt solution of 1 gram per 100 milliliters.
In this experiment, we need 6 solutions: 0 grams per hundred ml, which is just pure water, and 1 to 5 grams per hundred ml in one gram increments. Once the solutions are ready, pour them in to immerse the potato strips. Start the timer. Leave them where they are for 20 minutes. [Music] [Music]
After 20 minutes, you have to measure all of the strips and measure them as accurately as possible. To the nearest millimeter is good for this experiment. As you're working through the potato strips, you'll probably notice that the ones immersed in the more concentrated solutions are more soft, while the ones immersed in lower concentrations are more turgid. Be sure to make a quick note of each value you measure as you're going along.
To process the data, you need a table of results with all concentrations listed, as well as the starting lengths of the strips. These all start at 6.0 centimeters. In the next column, put in the values you measured, and then calculate the percentage change for each individual strip. Notice the negative sign on some values; this tells us whether the strips increased or decreased in length.
Next, you can take an average of changes in length. These are the values that we need to put on the graph. The graph shows the average change in length against the salt concentration, and the results from this experiment are a nice downward curve. The lower salt concentrations result in an increase in length of potato strip, meaning water has entered the potato strips by osmosis. This means there is a higher water potential inside the solution compared with the potato. We have a hypotonic solution.
At higher salt concentrations, however, we have a decrease in potato strip length. This tells us the water potential is higher inside of the potatoes, so water has left the potato strips by osmosis. We can say that the solutions are hypertonic to the potatoes. At the point the graph crosses 0, which is about 1.4 grams per 100 milliliters of salt, the solution is isotonic, meaning the water potential inside the potato is equal to the solution. So, at that point, there is no net movement of water. [Music]