Osmosis in living cells : celery

After 15 minutes, the celery was removed from the solutions, weighed and noted for texture.

Assuming that the osmotic potentials of the solutions in the beakers are as follows:

distilled water = 0 MPa

1% NaCl = -0.5 MPa

10% NaCl = -3.0 MPa

Since all the solutions are open to the air they are all subject to atmospheric pressure which is defined as zero. Recall that the water potential is the sum of the pressure potential and the osmotic potential. Knowing that the pressure potential is zero allows us to conclude that the water potential is equal to the osmotic potential.

Therefore the water potentials of the solutions are:

distilled water = 0 MPa

1% NaCl = -0.5 MPa

10% NaCl = -3.0 MPa

For each of the solutions was the water potential of the celery (prior to being placed in the solution) higher or lower than that of the solution?

Solution 1: distilled water

The water potential of the celery was lower than that of the distilled water. There was an increase in weight of the celery after soaking in the distilled water, so we know that water must have moved into the cells of the celery. Since water moves from an area of high water potential to an area of lower water potential, the celery had to have had a lower water potential than the solution.

Solution 2: 1% NaCl

The water potential of the celery was the same as that of the 1% NaCl solution. There was no change in weight of the celery after soaking in the solution, so we know that there was no net movement of water into or out of the cells of the celery. In order for no net movement of water to occur, the celery and the solution must have had the same water potentials.

Solution 3: 10% NaCl

The water potential of the celery was higher than that of the 10% NaCl solution. There was a decrease in weight of the celery after soaking in the 10% NaCl solution, so we know that water must have moved out of the cells of the celery. Since water moves from an area of high water potential to an area of lower water potential, the celery had to have had a higher water potential than the solution.

What is known about the osmotic potential and pressure potential of the celery tissue at equilibrium in each solution?

By definition, equilibrium is attained when there is no further net movement of water in any direction. Therefore, at equilibrium the water potential of the solution and the celery tissue must be equal.

In each case, recall that the pressure potential of the solution is equal to zero because the solutions are open to the air and subject only to atmospheric pressure.