Osmosis.


Introduction:

Osmosis is a net diffusion of water between two solutions that differ in solute concentration and that are separated by a selectively permeable membrane.

Hypertonic solution – higher solute concentration outside the cell.

Hypotonic solution – lower solute concentration outside the cell.

Isotonic solution have the same solute concentration outside and inside the cell.


Hypothesis:

Potato cells in 0% solution of salt will swell up. Potato will gain weight, because net movement of water will be into the cells. Water moves down the concentration gradient.

Potato cells in 2% and 3% solutions will shrink. Water moves out of the cells to regions of greater solute concentration.

Potato cells in 1% solution will change little in cell size or shape. The difference between solution concentration and potato concentration is small. So there is no big movement of water.


Materials: Knife, potato, 4 salt solutions ( 0%, 1%, 2%, 3%), paper, razor blade.


Method:

Cut 12 plugs from similar potatoes. Remove the ends of each plug and skin.

Make up four solutions of salt ( 0%, 1%, 2%, 3%). Weight the plug sets. Place plug sets into each solution and continue to measure or weight them at specified time intervals. Report mean values only fo measurements or weights. Determine the percent change in lenght or weight for the potato in each solution. Calculate the exact percent of salt in the potato cell from the graphed data.


Salt

Solutions

Initial weight (g)

Weight of plug sets after some time:

Mean percentage change in weight

12 min.

22 min.

40 min.

52 min.

0%

5.61

5.87

6.01

6.13

6.25

11.4%

1%

8.84

8.85

8.87

8.90

8.86

0.22%

2%

10.02

9.61

6.51

9.20

8.99

-10.3%

3%

6.64

6.01

5.77

5.48

5.31

-20%


We can’t compare the changes in weight using grams, because each plug set had different initial weight before placing them into salt solutions.


Conclusion and evaluation:

From the graphed data the percent of salt in the potato cell is 1.1%. The original potato cell concentration is 0.9%. The error could be, because the water outside potato has influence on weight and not accurate measurement of mass by not exact scales, which showed only one decimal place.

If the concentration of water is not equal across a potato cell membrane (concentration of water outside the sell is more or less than 0.9%), osmosis tends to occur. Water will move osmotically to regions of greater solute concentration (from hypotonic to hypertonic solutions). If water concentration outside and inside the cell is equal, water shows no net osmotic movement from one to the other – isotonic solutions.

The experiment can be improved: Firstly, by drying potato from water outside the potato. Secondly, by using scales, which have more accuracy.


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