To investigate the process of osmosis and discover the water potential of a potato or the concentration of sucrose in a potato cell.

Factors

* Concentration of sucrose solution - this is our variable.

* Type of potato

* Surface area of potato piece

* Amount of potato exposed to sucrose solution

* Length of time between taking the potato out of the solution and weighing.

* Temperature

All the factors apart from the variable should remain constant in order to provide a fair test and therefore accurate results.

Diagram

Apparatus

* 4 x Petri dish

* 4 x Sugar solution (ranging from 0M to 1M concentration)

* Potato

* Corer

* Scalpel

* Scales

* Stop-clock

Method

A cylinder of potato was made by pushing the borer through the potato. This cylinder was cut into discs with a scalpel, each disc 5-mm thick. This ensured that the potato discs were all of approximately the same surface area. Each petri dish was filled up with a different concentration of sugar solution. The molarity of each solution was labelled on the bottom of the petri dish.

3 discs of potato were then weighed and placed in one of the petri dishes as a stop-clock was started. Every 15 minutes the potato discs were removed from the solution and weighed. All excess solution was wiped off with a paper towel before weighing to prevent disproportionate readings being obtained. The potato discs were then placed back in the solution as quickly as possible and the stop-clock was restarted. The masses obtained were noted down. 4 of these readings were taken, so the potatoes were in the solution for an hour in total.

This process was repeated for each petri dish. To shorten the experiment the 4 concentrations were tested simultaneously. However the times the potatoes were put in was staggered to ensure that the readings didn't clash with each other and could be completed quickly.

Safety

Great care should be taken when cutting the potato cylinders with scalpels because they are extremely sharp.

Results

This is the table of raw data obtained during the experiment.

Mass (g)

Concentration (mol/dcm3)

0 mins

Av.

5 mins

Av.

30 mins

Av.

45 mins

Av.

0.0

0.78

0.79

0.75

0.77

0.77

0.77

0.74

0.76

0.83

0.86

0.81

0.83

0.83

0.87

0.83

0.84

0.2

0.87

0.88

0.93

0.89

0.88

0.88

0.94

0.90

0.77

0.76

0.81

0.78

0.92

0.91

.00

0.94

0.4

0.64

0.71

0.83

0.73

0.59

0.66

0.80

0.68

0.58

0.62

0.80

0.67

0.60

0.66

0.81

0.69

0.6

0.70

0.69

0.73

0.71

0.66

0.65

0.69

0.67

0.59

0.59

0.62

0.60

0.61

0.60

0.64

0.62

0.8

0.72

0.73

0.72

0.72

0.68

0.70

0.68

0.69

0.61

0.66

0.62

0.63

0.57

0.65

0.56

0.59

.0

0.64

0.67

0.73

0.68

0.61

0.66

0.71

0.66

0.52

0.58

0.58

0.56

0.49

0.53

0.56

0.53

These graphs represent the change in mass in the pieces of potato throughout the experiment. Each line represents one piece of potato.

This table shows the percentage gain or loss of mass of the potato in each solution.

Concentration of

Solution (mol/dcm3)

Increase of

Potato 1

Increase of

Potato 2

Increase of

Potato 3

Average Percentage

Increase

0

6.41%

0.13%

0.67%

9.07%

0.2

5.75%

3.41%

7.53%

5.56%

0.4

-6.25%

-7.04%

-2.41%

-5.23%

0.6

-12.86%

-13.04%

-12.33%

-12.74%

0.8

-20.83%

-10.96%

-22.22%

-18.00%

-23.44%

-20.90%

-23.29%

-22.54%

This is a graph of concentration against average percentage increase. It is similar to the one in my hypothesis.

Conclusion

The data collected during the experiment proved that my hypothesis was correct. The percentage change of mass of potato went down as the concentration of the sucrose solution increased. To calculate the concentration of sucrose in a potato cell we need to look at the last graph. The 'x-axis intercept' can be calculated using the formula of the trendline:

Y = -33.75x + 9.5592

If we substitute the value 0 into the equation in place of Y, the value of X is calculated at:

0.28 mol/dcm3

This is very close to the value predicted in the hypothesis; any difference could be put down to experimental error.

The graphs showing the change in mass of each piece of potato show that the weight of the potato goes down first, even when the mass of the potato eventually rises.

When an animal cell is placed in a dilute solution, water will keep entering the cell until the 2 concentrations (outside and inside) are equal. This often leads to the cell exploding under the pressure of the water entering, this pressure is known as 'turgor pressure'. However plant cells have a strong cell wall surrounding them to stop turgor pressure from breaking the cell. When the amount of turgor pressure allowed by the cell wall is reached, no more water can enter the cell. At this point the plant cell is known as 'turgid'; a potato cell with turgid cells would be heavier, slightly larger and would feel much firmer to the touch.

When a cell is placed in a concentrated solution, water will leave the cell, reducing the turgor pressure. When the turgor pressure is very low the cell is known as 'flaccid'. A plant that has lost much water will have flaccid cells, this can be seen when a plant wilts.

When a cell is placed in a solution that has the same 'osmotic strength' or concentration as the solution inside the cell, a state of 'incipient plasmolysis' occurs. When this happens osmosis occurs neither one way nor the other. This is what is happening at the 'x-intercept' on my graph, this point is known as the 'isotonic point'.

Evaluation

My results seem to be very accurate. Large quantities of results were obtained that have formed smooth curves on graphs with no obvious anomalous results.

The reason my graph is not a perfect straight line is that as the 2 concentrations get closer together, the concentration gradient gets smaller. The speed at which osmosis occurs is directly proportionate to the concentration gradient, so when the concentration gradient is reduced, the speed of osmosis is also reduced. The graph I believe would have been a straight line if I had measured the weight of the potatoes to 'constant mass'. This means I should have continued replacing the potatoes back into the solution until they become fully turgid and the mass no longer changed.

The method could have been improved by measuring to 'constant mass' and also by measuring the percentage change of mass in more concentrations, e.g.) 10 instead of 5. This would have let me plot more points on my graph and I would have been able to state my conclusions with more certainty.

There were many places where errors could have happened in this experiment. The potato discs could have been different shapes and therefore had different surface areas, the solutions may not have been completely accurate, not all of the excess solution may have been removed before each weighing and the time between weighings could have varied. It would have been very difficult to regulate these factors exactly, a machine could have been used to cut the potatoes to size but this would have been needlessly expensive. If more time was available, the weighing of each potato could have been done separately so there wasn't so much of a hurry every time the stop-clock said it was time to weigh.

Biology Osmosis Practical

Page 1 Andrew MacKay 11S