To determine how the concentration of salt water affects osmosis in potatoes

Investigation – To determine how the concentration of salt water affects osmosis in potatoes

Planning

Aim – To determine the effect the concentration of water has on osmosis in potatoes

Prediction – I think that when the concentration of salt water is higher then there will be a greater loss of weight in the potatoes. This is because the water molecules inside the potato move from an area of high water concentration to low water concentration. When the concentration of salt is higher it will therefore have a lower water concentration meaning that there will be a steeper concentration gradient. The salt molecules will not diffuse into the potato because they cannot pass through the partially permeable membrane.

I think that in all the salt solutions above 1% the potato will become plasmolysed. Whilst this doesn’t usually happen the potato will be surrounded by solutions of a very high concentration. It will become plasmolysed because a lot of water will have left the cell to the point when the cell wall can no longer shrink and the cell membrane will tear away from the cell wall. This will kill the potato cell. A diagram of a flaccid cell is shown to the right.

In 0% solution (water), the cell will become turgid and therefore gain weight. This is because when plant cells are put in pure water the plant cell will take in water and swell. The cell wall will prevent it bursting, even when the cytoplasm presses out. A turgid plant cell is shown above.

In 1% salt solution there will be a small loss in weight however plasmolysis will not occur, because not enough water will diffuse from the cell, therefore the cell membrane will not shrink enough to tear away from the cell wall.

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Method – First the potatoes will be chopped into 1cm thick discs, 3 cm in diameter and weighed. They will then be placed in salt water for 10 minutes and osmosis will occur. We found in our preliminary work that this was the optimum time to leave them in for, because generally osmosis occurred much less after ten minutes. The results of the preliminary work are shown below. After the 10 minutes is up we will record the change in weight. The different concentrations of salt solution we will use are 1%, 2%, 3%, 4% and 5%. 60mls of salt solution will be used. A measuring cylinder will be used to ensure accuracy. The experiment will be repeated until we have obtained two sets of results close to each other in value. Lab coats and goggles will be worn at all times to ensure safety.

The independent variable we are investigating will be the concentration of the salt water. The factors that will stay the same include: time, temperature, concentration of water and pH.

Results

* Believed to be an anomalous result and therefore excluded from calculations

Analysis

Whilst no formula can be determined to summarize the trend, there is a clear trend. The trend is shown clearly by the graph overleaf. When the salt water is more concentrated, above 2% more weight is lost from the potato because the cells become plasmolysed and die. However in tap water there is a small gain in weight, because the cell becomes turgid. There is only a slight loss for 1% because the 1% salt solution isn’t concentrated enough for plasmolysis to occur. This supports my original prediction made that when the concentration is higher there will be a greater loss of weight.

My conclusion can be explained by the fact that the water molecules diffuse from an area of high concentration to low concentration. They are in a high concentration in the potato and therefore move into the salt water where they are in a low concentration. When the salt solution is more concentrated there is a lower concentration of water meaning that the concentration gradient is steeper.

Also my prediction that the potato placed in solutions of high concentrations would become plasmolysed was right. This is shown by the large loss of weight and the change in texture/appearance of the cells. They had visibly shrunk and felt extremely soft and spongy.

Therefore the results all support the original prediction, with the exception of the anomalies.

Evaluation

I think that whilst our results were generally accurate (to within the nearest minute), there were several anomalies which have been identified in the results section. I think that our third section of results was so far apart from the other section that it maybe should have been disregarded. This could have been caused by a number of factors. We noticed that the potato discs were unusually heavy in relation to the other discs, so maybe the potato contained an abnormally large amount of water causing osmosis to occur more slowly. There may be a more simple explanation for it however. Perhaps the potato may not have been dried off properly or it was down to sheer experimental error. This nevertheless is unlikely considering that all of the results (with the exceptions of 0% and 1%) were odd in the batch.

For this reason I’m not sure if the evidence supports a firm conclusion. There were some results that may have been anomalous in the other two sets of results yet I think that they were close enough to not require another set of results to be taken. I therefore think that the evidence is sufficient.

I think that the experiment could have mainly been improved by being more precise generally. Due to time constraints we seemed to choose reliability over accuracy and produced several anomalies in the process. If I could repeat the experiment I would do only 2 sets of results but be as precise as possible. If I then didn’t have 2 sets of results close together I would do repeats where necessary. I think that the way we did it meant that our conclusion is not as firm as it could have been. Also we could have dried the potato off more thoroughly because the excess of surface moisture on the potato made it too heavy and adversely affected the results, especially in the 3rd set.

I think that using sugar water instead of salt water could extend the enquiry. This could finally prove the conclusion by showing that it works for solutions apart from salt. Or use a different plant apart from potatoes such as apples because at the moment the conclusion only describes potatoes in salt water and is therefore not particularly “concrete”.