Diffusion is simply the net movement of atoms or molecules from a region of higher concentration to a region of lower concentration. The force behind the movement is heat or kinetic energy (also called Brownian motion). Diffusion occurs when you spill water on the carpet floor and it spreads out, or when you open a bottle of perfume and it leaves the bottle and spreads throughout the air in the room. Osmosis is a similar phenomenon that moves water from a region of high water concentration to a region of low water concentration. Imagine that a cell from your body is placed in a solution of water. If the concentration of the water inside the cell is the same as the concentration of water in the solution, then we describe the water solution as being “isotonic” or having the same concentration as the water inside the cell.
In this case, net movement of water will be zero and the cell will not swell or shrink. In other words, the same amount of water will move in the cells as will move out. On the other hand, if the cell is placed in a solution of water that has a higher concentration (of water) compared to the concentration of water inside the cell, then osmosis will cause more water to move into the cell than will move out and this will cause the cell to swell. In this case, we say that the solution of water is “hypotonic”. Now imagine that the cell is placed in a solution of water that has a lower concentration (of water) compared to the concentration of water inside the cell. In this case osmosis will cause more water to move out of the cell than will move in the cell and the cell will shrink. In this case, we say that the solution is “hypertonic”. Learn more about osmosis and diffusion from Chapter 3 of your textbook.
We can place celery in three different water solutions to observe the effects of osmosis on the celery stalks. Salt will serve as our solute to mix with water to make the water either hypotonic, hypertonic or isotonic. The hypotonic solution will contain less salt so that the amount of water compared to salt will be greater than the amount of water inside the cells (compared to the salt and other solutes inside the cell.) The hypertonic solution will contain more salt so that the amount of water compared to the salt will be less than the amount of water inside the cells (compared to the salt and other solutes inside the cell.) The isotonic solution will contain an amount of salt that will be similar to the amount of salt and other solutes inside the cells of the celery.
measuring cup and spoon
two stalks of celery (they should be fresh and firm, not bend like rubber) table salt
three containers with lids (or something to cover the containers) to hold the solutions and the celery stalks filtered or soft water, distilled water works best, tap water will work if it the mineral content is not too high (tap water with high mineral content is called “hard” water)
1. cut four pieces of celery that are each two inches long (be sure they are all the same length)
2. make four vertical slices or cuts into the celery but make the cuts only three fourths of the length or 1.5 inches; space the vertical cuts as evenly apart as possible
3. label the three containers A, B and C
4. add one cup of water to each container
5. add NO salt to container “A”
6. add ½ teaspoon of salt to container “B” and stir well
7. add 2 teaspoons of salt to container “C” and stir well
8. place one of your 2 inch cut pieces of celery into each of the three containers and keep the fourth piece as a control for comparison
9. cover the three containers with the lid or cover that you are using, you could use a cling plastic wrap as well
10. wrap the fourth control celery piece tightly in plastic wrap
11. leave the celery pieces in the three containers and the fourth wrapped (control) piece for 6 hours or overnight at room temperature
12. remove the pieces from the containers and bend them while observing how firm or flexible they are
13. record your observations of each of the four pieces of celery below
Flexibility compared to the control stalk (less flexible, similar, more flexible) More water moved into or out of the cells?
Indicate which solution was isotonic, hypotonic, and hypertonic
When the celery stalk was placed in solution A. The water moved through and in the cells of the celery which is an example of osmosis. Water moves through the cells. The stalk was firm and less flexible.
More water moved into the celery’s cells. The celery stalk was nice and firm after the allotted time.
More water and less to no solute, is an example of hypotonic solution. Which means there is “less” osmotic pressure.
In solution B the celery became a little more flimsy. Solution has a little salt.
Slightly less water moved into the celery’s cells. The reason the water didn’t move as freely through the cells is due to the small amount of solute.
This is an example of hypertonic, or greater osmotic pressure. This solution has a higher concentration of solute than the hypotonic solution.
In solution C the salt/ solute is very present. The celery became very flimsy after 4 hours. Even less water moving into cells.
The celery became “very” flexible and very flimsy. The water was not moving through the cells even at a higher rate. This solution is a full example of osmosis. In osmosis the solute is of greater concentration of the solute. The diffusion of pure solvent across a membrane in response to a concentration gradient,