I predict that as the concentration of the sugar solutions increases, the weigh of the potato would decrease.
Water diffuse from a region of higher water potential to a region of lower potential by osmosis.
In a hypertonic solution the potato’s weigh would decrease, because in a hypertonic solution there is a lower concentration of water molecules compared to the cells in the potato cord, so there is a net movement of water molecules out of the potato cells.
In a hypotonic solution the potato’s weigh would increase, because in a hypotonic solution there is a higher concentration of water molecules compared to the cells in the potato cord, so there is a net movement of water molecules in the potato cells. This causes the cells to swell and become turgid.
Diagram showing the set-up of apparatus:
Method:
The different concentrations of sugar solutions were already measured out.
50cm3 of each different concentration of solutions were measured out using a measuring cylinder and poured into a polystyrene cup.
12 potato cords were distracted from a potato. The potato cords were sliced into identical shapes, the 12 potato cords were separated into 6 pairs- 2 potato cords were weighed and their weighs were recorded.
The pairs of potato cords were placed into 6 different polystyrene cups (2 to each cup). The cups were labelled according to the concentrations that would be sampled.
The stopwatch was started when the potato cords were placed into the cups.
The pairs of potato cords were reweighed after every 30 minutes (30mins, 60mins and 90mins). Their weighs were recorded and the change was worked out and represented as percentages.
d = s x t
New Mass – Original Mass X 100 = % change
Original Mass
Results:
The results obtained were plotted onto a graph (y- the % changes, x- Strength of sugar solutions mol dm-3).
Analysing and Conclusion:
The graph shows 3 different lines showing the changes in weigh in different concentrations of sugar solutions over different periods of time.
The lines that represent the changed in weigh after 30 minutes, 60 minutes and 90 minutes all shows the correlation that as the concentration increases, the weigh of the potato cords would decrease.
In most of the concentrations of the sugar solution (0.4, 0.5, 0.6, 0.8 and 1.0), the potato cords have significantly decreased in weight. This is because if the solution’s concentration had a lower concentration of water molecules compared to the cells in the potato (hypertonic), then the direction of the net movement of water molecules would diffuse out of the potato cells and into the solution. The potato cord would be flaccid and decrease in weight.
In the water, the weight of the potato cords increased, this is because the solution’s concentration was hypotonic meaning that the solution had a higher concentration of water molecules compared to the cells in the potato, then the direction of the net movement of water molecules would diffuse out of the potato cells and into the solution. The potato cord would become turgid and increase in weight.
This is because osmosis is a net diffusion of water from an area of high water potential to an area of lower water potential.
There are two anomalous results that don’t fit in with the pattern (line representing 30 mins), in which the potato’s weight increased rather then decreasing, this can be explained in the evaluation. The anomalous result is circled in green.
I have controlled the pattern and trend by using graphs, as you can see from my graph that as the concentration is increased, the slope becomes steeper, indicating a faster rate of diffusion of water. But as time increases, the lines becomes shallower then the previous indicating a slower rate of diffusion of water. This is because as time increases, the concentration of water molecules in the potato and the solution are about the same (becomes isotonic). This is when there is a constant volume of water diffusing into and out of the potato at the same rate, and hence the potato’s weight becomes constant.
I conclude that my results justify and prove my hypothesis, as it shows that as the concentration of sugar solution increases, the weight of the potato decreases. It also shows that in hypertonic solutions, water molecules diffuses out of the potato cells and into the solution, making the potato cord flaccid and decrease in weight. It also shows that in hypotonic solutions, water molecules diffuses in the potato cells from the solution therefore the weight of the potato cords would increase and become turgid.
Evaluation:
My method tested my hypothesis. The evidence I have obtained and reliable as it shows that osmosis is effected by different concentrations of Glucose solutions. My graph and results show that in hypertonic solutions, the weigh of the potato cords decreases as water diffuse from an area of high concentration to an area of low water concentration. It also shows that in hypotonic solutions, the weigh of the potato cords increases as water diffuse into the potato cells. These results must therefore be valid.
There were some anomalous results, as shown in my graph. The anomalous results could be explained by:
- There were changes to the temperature, heat affects the rate in which osmosis occur.
- Even though the cords weighed the same, the surface areas were not the same. The variation of surface area causes different rates in which osmosis occurs.
- Different Potatoes were used, some of the potato might not have been as fresh as the others. Different variety of potato have different concentration of water molecules in them, and also the cell walls can be different. This makes the experiment unreliable.
- When the potato cords were taken out to be reweighed, they were placed on tissues, the tissues absorb the soak up some of the water. This makes the weight of the potato cords to decrease from its actual weight.
- There are errors in the accuracy of equipment.
- Different parts of the potato were used, organic matter is different, different parts do different things. (Eg in humans, the liver tissue is different to the skin tissues, the same applies to all organic matter).
- Pure water was not used, instead tap water was used, this mean there were ions and solutes in the water which affects osmosis. Solute molecules make weak chemical bonds with water molecules, this constrict the movement of water molecules, making them move more slowly then those in pure water.
To improve and check the results, I would repeat the experiment several times to ensure consistency and to ensure that my results are similar and valid and that there were no major differences. Other improvements could be:
- Using 5 tubes per cup (the more the better)
- Use the same potato
- Use pure water
- Research on different varieties of potatoes and see if osmosis are the same in all.
- Repeat the experiment on the same variety of potato
- Measure the temperature and keep it at a constant
