Scientific Knowledge:
Osmosis is the movement of water molecules from a high concentration of solute to a lower concentration of solute through a semi-permeable membrane. When a potato tuber is put in a test tube filled with a 1m sucrose solution, the water molecules move from inside the potato tuber to outside, into the solution, because inside the potato tuber is less concentrated than the solution surrounding it. The water molecules inside move through the potato’s ‘skin’ – the semi permeable membrane – into the solution, because water molecules move from the lower concentration to the higher concentration, through the semi permeable membrane, which allows only water to pass through it. However, water moves from both sides of the membrane at the same time, but more water leaves the more dilute solution than the concentrated solution. In the 0m sucrose solution the opposite happens. The concentration inside the potato tuber is higher than the solution surrounding it. So the water molecules from the solution surrounding it move through the semi-permeable membrane and into the potato. Because of this, when the potato is in the 1m solution, it ‘shrinks’ and becomes flaccid because of the loss of water inside it. When put in the 0m solution it swells and becomes turgid.
Diagram from the textbook.
Low concentration Semi-permeable membrane High concentration
The cells inside plants are different to humans. They are surrounded by a cell wall.
Diagram from biology website
When animal cells are submerged in water, they swell up and eventually will burst because animals don’t have a cell wall. When the plant cell is submerged in water, the cell swells up, but the cell wall prevents it from bursting. When plants cells are swelled to this level they are said to be turgid. The process is called turgor. When plant is then submerged in a more concentrated solution, and the water molecules pass out of the cell, through the semi-permeable membrane, the cell becomes plasmolysed, where the cell membrane tears away from the cell wall and shrivels. When this happens it usually kills the plant cell because the membrane is damaged. Inside the cell, when plasmolysed, the vacuole shrinks as does the cytoplasm, which causes the cell wall to ‘bend’ inwards, although it doesn’t shrivel because it is still quite strong. This causes the plant to wilt or droop.
Diagrams from the textbook
Apparatus:
- 12 boiling tubes
- 1 pot
- 2 beakers
- 2 Potatoes
- Potato ‘corer’
- Glass rod
- 2 beakers
- Razor blade
- Ruler
- Sucrose solution
- Pure water
- Measuring cylinder
Diagram:
Method:
I will be doing the experiment twice.
- I collected all my apparatus.
- I placed my boiling tubes in the rack and labelled them.
- I cored 12 tubers from the two potatoes and cut them each to 4cm long, and placed each one in a boiling tube.
- I measured out the different concentrations of solution for each boiling tube, at 25mls and then added each to their boiling tubes.
- I waited for 2 days and then measured each tubers length and recorded my results.
Risk Assessment:
On a scale of 1-5 with 1 being least dangerous and 5 being most dangerous, I would rate this experiment as danger 1. This is because razor blades were used, which could cause an injury if not used properly. Goggles were worn to protect the eyes from the sucrose solution. Long hair was tied back to prevent it getting in the way.
Results:
Results of start/finish lengths for potato tubers
Conclusion:
From my results I can see that as the concentration of the solution increased, the potato tubers decreased in size, but as the concentration of the solution decreased, the tubers increased in length. From my graph, I can see that at around 0.25M the concentrations of the solution and inside the potato would have been in equilibrium and there would have been no change in length, however water would still pass through the semi permeable membrane, but in equal quantities, hence no change in size. My prediction was correct, and my results verified it. It was at equilibrium between 0M and 1M (0.25M) and the tubers increased in length with less concentrated solution and increased in length with more concentrated solution. This was true because of osmosis. In a more concentrated solution, the water molecules with pass through the semi permeable membrane and into the solution surrounding the tuber, thus causing a decrease in size because of plasmolysis – the shrivelling of the plant cell due to water loss/osmosis. In a less concentrated solution, the water molecules pass through the semi permeable membrane and into the potato tuber, because inside the tuber is more concentrated than the solution surrounding it, thus causing an increase in size due to water gain/swelling, called turgor. At 0.25M the tuber will not have increased or decreased in length because neither inside the tuber or the solution surrounding it would have been more concentrated – it was in equilibrium.
Evaluation:
At first I thought that the entire experiment was anomalous, but I later discovered I had mislabelled our boiling tubes, and the results were simply backwards. Our only anomalous result was for 0.4M, where the length decreased only by an average of 0.5mm, whereas the others increased steadily between 2-4mm each time. This may have been because a slight mistake was made with the ratios of sucrose to water, or made an error with the measurements, either before putting the tuber in the solution, or after the experiment.
There were some problems with the experiment, such as the temperature may not have been kept consistent throughout (night and day, temperatures differing each day) and that the concentrations were not exactly at 0m, 0.2, 0.4, 0.6, 0.8 and 1m. I also used two different potatoes for the experiment, and they would have had different concentration inside. I also measured the tubers with a ruler, which wasn’t very accurate.
To solve these problems, I could have kept the experiment in an environment with a consistent, controlled temperature, and measured the concentrations exact, with more accurate measuring equipment. I could also have used a single potato for the experiment, and perhaps weighed them before and after the experiment, instead of measuring them.
To investigate osmosis further, I could conduct experiments that involve different quantities of solution, larger potato tubers, different plants, and experiment with different temperatures, to see if it affects osmosis. I could also use more concentrations of sucrose, such as increasing in steps of 0.05 or 0.1m instead of 0.2m. This would give a more varied set of results. I could also repeat it several times and calculate the averages.
Laura Evans 105:LLB
