The aim of this investigation is to find out how the concentration of the solution a plant is in affects the rate of osmosis in the plant.

Prediction

I predict that the potato tubes in the solution of distilled water will increase the most in weight and length and that the potato tubes in the 3M solution will decrease the most in weight and length.  I predict this because osmosis always occurs from an area of low sugar concentration to an area of high sugar concentration.  

Osmosis= diffusion from:  low sugar concentration         high sugar concentration

The potato tubes will contain a certain amount of sugar, but the distilled water will contain no sugar, therefore osmosis should occur from the distilled water into the potato tubes, meaning water will go into the potato tubes and they will consequently become turgid and increase in length and weight.  

However, the 3M concentration of sugar solution will contain more sugar than the potato tubes, and therefore osmosis will occur from the potato tubes to the plant, meaning water will go out of the potato tubes and they will become flaccid and decrease in weight and length.

The 1M and 2M solution do not have such extreme differences in concentration to the potato tubes, and therefore not so much osmosis will occur because the tubes will not be particularly turgid or flaccid.  It is harder to predict as I am not sure of the exact concentration of sugar in the potato tubes, but I would predict the potato tubes in the 1M solution would gain around 0.1 cm of weight and the potato tubes in the 2M solution would lose around 0.1 cm of weight on average.

Method

Preliminary Work

It is really important to have a reliable, well-planned method in order to produce accurate results and come to an accurate conclusion about how concentration affects the rate of osmosis.  Therefore, I carried out a preliminary experiment (included on next page).  I found that my results were accurate and the osmosis theory was proved because in a more concentrated solution the lengths and weights decreased, they changed very slightly in a dilute solution and increased in distilled water.  However, it was difficult to be quantitative when analysing my results because I did not record the specific concentration in molars of each solution, and I also weighed each group of potato tubes together rather than separately, so I didn’t get a good idea of individual weight changes.  I also feel I could have used another concentration, as the wider range of results you get, the more accurately you can conclude.  In this experiment I have applied all these points to my method so it will be a better experiment.

Apparatus

• Non-washed potato
• Cork borer&needle
• Scalpel
• Accurate cm ruler
• Scales
• 4 beakers
• 75ml distilled water
• 75ml 1M sugar and water solution
• 75ml 2M sugar and water solution
• 75ml 3M sugar and water solution
• Timer

Step-by-Step

1. Cut the potato in half to make extracting the tubes easier using the scalpel.

2. Push the cork borer through the curved top of the potato down to and out of the flat side so one tube is inside the cork borer and push it out with the needle.  Do this 12 times, making sure to keep the holes made in the potato separate so no tube has a piece cut out of it.

3. Using the ruler to measure, cut each potato tube to 4cm in length using the scalpel.  If a potato tube is shorter than 4cm, cut each potato tube to it’s length so we can easily see changes in length and make the original weights as close as possible.

4. Take 4 beakers and fill the 1st with 75ml distilled water, the 2nd with 75ml 1M  solution, the 3rd with 75ML of 2M solution and the 4th with 75ml 3M solution.  Label each beaker with the solution they contain.

5. Place 3 potato tubes by each solution.  Weigh the each group of potato tubes one at a time, one potato tube at a time and record their weights in a table, as shown below.  Try to keep tube 1 on the left of each group, tube 2 in the middle and tube 3 on the right so each tube’s weight change can be recorded after being placed in their solution.

6. Set the stopwatch to 20 minutes.

7. Place each group of chips in their solution as quickly yet carefully as possible, so they will have as close as possible the same time in their solutions yet not get mixed up or have the solution split.  Start the stopwatch immediately.

8. When the stopwatch goes off in 20 minutes, carefully remove the potato tubes from the beakers, keeping them in the left, middle, right order and in groups.  Group by group, potato tube by potato tube, measure the new lengths carefully, using the ruler, and to 0.1cm (e.g. 4.6cm) and record them in a table like the one below.

9. In the same way as before, weigh each potato tube, using the scales, to 0.01g (e.g. 1.88g) and record the results in the table.

I feel that this method is accurate and suitable for the experiment.  By using 3 potato tubes for each concentration of sugar I have made my results more reliable because I will be able to spot any anomalous results better, whereas if I only had one result I would have nothing else to judge on.  This also makes my experiment more accurate because I can work out an average result.  To make it more accurate, I could repeat the experiment, but I don’t have the time.  Using potato tubes is also very useful because they are solid and visible cells that won’t dissolve or fall apart in water.

I also feel that the method is very safe, though wearing safety glasses and hand protection will be a safety precaution when using the scalpel as it is very sharp and could easily jerk when cutting the potato.

Variables

Variables are things that can be changed in an experiment to affect the results, and it is important that only the chosen variable (or factor) is varied because otherwise  it will be impossible to tell which factor is affecting the rate of osmosis, and by how much.  As my chosen variable is the sugar concentration of the water around the potato tubes, the things that must be kept the same to make it a fair test are;

• The size of the potato tubes, because there are more cells, more cell membrane and larger cells for osmosis to occur in, so if I used different sized potato tubes, the rate of osmosis would be changed and the results wouldn’t be in proportion to one another.  Therefore each potato tube will be 4cm long.  Unfortunately it is impossible to make each the same weight, but having them the same length will make this as close as possible.
• The time each potato tube is kept in their solution, because the more time there is for the water to pass through the cell membrane of the potato tube, the more this will happen, so if one group was kept in for longer the lengths and weights of the potato tubes would have more chance to change.  Therefore the potato tubes will all be in their solutions for 20 minutes – although this will not be exact, osmosis is a fairly slow process so the time taken to put them in and take them out will not really affect the results.
• The temperature at which the experiments will occur, because on a hotter day, the rate of transpiration (water loss) from a plant is higher, so more water is taken into the plant by osmosis, so if one group was put into hotter conditions the rate of osmosis would be higher and make the results inaccurate.  Therefore, the whole experiment will occur at the same time in the same room temperature in the same amount of sunlight with the same cold water in each solution.
• The amount of solution each group of potato tubes is in, because if there was 75ml of solution for one group and 20ml for another, the 75ml group would have more chance to take water in (i.e. perform osmosis), so the results would be inaccurate.  Therefore there will be 75ml of each solution in its beaker.
• The ruler/scales used to measure the lengths/weigh the potato tubes, because different rulers/scales have different degrees of accuracy – for example some rulers may not use millimetres, which are essential to get accurate lengths and some scales may be broken, meaning the changes in the lengths and weights would be impossible to work out accurately.  
• The units measured/weighed in, because although it would be possible to work out the equivalent in the correct unit, that would be a waste of time and could lead to error.  Therefore, the lengths will be measured in cm and to 0.1cm and the weights in g and to 0.01g, so there is the same degree of accuracy.
• The potato used, because, depending on the conditions it has been kept in, different potatoes could contain different concentrations of solution, and this would change the rate, or even direction of osmosis, because if there was a 1M concentration of sugar in one potato and 2M in the other, there would be more likely to be osmosis out of the 1M and into the 2M.  The results would be completely inaccurate, because you can’t change the concentration of both the potato tubes and the solutions!
• Keeping the potato tubes unwashed, especially after measuring and weighing, because washing them is enabling water to diffuse through the cell membranes into the cells, in other words osmosis to occur, before the experiment takes place, making it impossible to see how the concentration of the solutions really affects the rate of osmosis.

Keeping these variables the same will help me have a fair test, using the method I have planned to give me accurate results.

Results

Shown on the next page are line graphs of the results.  I have used the average results to make the graphs easier to understand.

Conclusion

From looking at my results graphs, I can see a clear pattern in that the length and weight of the potato tubes in the distilled water increased the most in 20 minutes, because both lines slope upwards and the potato tubes in the 3M solution decreased the most in both, in length by 0.6cm and in weight by 0.07g, with a downward sloping line.  The potato tubes in the 1M solution stayed the same length on average and only increased a little in weight, showing that the concentration of sugar in the potato tubes was around 1M.   The potato tubes in the 2M and 3M solutions decreased in length and weight, a clear pattern showing that if the concentration is lower in the solution than in the potato tubes, the length and weight of the potato tubes will increase, and if the concentration is higher in the solution than in the potato tubes, the length and weight of the potato tubes will decrease, which is what the graphs show.  My prediction of these changes was correct based on the osmosis theory, although the concentration of sugar in the potato tubes being lower than I predicted meant that the 1M solution potato tubes gained less weight and length than I predicted, and the 2M solution potato tubes lost more.

I can conclude that the pattern of these changes was caused by the concentration of the solutions affecting the rate and direction of osmosis, as I predicted.  This is because osmosis is the movement of water from a solution with a low sugar concentration to a solution with a high sugar concentration. If water diffuses into the potato tube it is because the potato tube has a higher sugar concentration than the solution it is in, so becomes turgid and gains length and weight, which is what happened to the potato tubes in distilled water and a couple in the 1M solution (the concentrations were not exactly the same here).  On the other hand, if water diffuses out of the potato tube it is because the solution it is in has a higher sugar concentration than the potato tube, so the potato tube becomes flaccid and loses weight and length, which is what happened to the potato tubes in the 2M and 3M solutions and one in the 1M solution.  

My results show that the bigger the difference between the concentrations of the two solutions osmosis is occurring between, the more osmosis will occur because there is a bigger demand in the potato tube to gain or lose water and it pulls in or pushes out more water through it’s cell membrane.  This is why the 3M solution had the biggest changes in length and weight- the concentration of sugar in the potato tube was around 1M, so only 1M different to the 2M solution and the distilled water but 2M different to the 3M solution, although the distilled water showed slightly larger changes than the 2M  solution (0.3cm to 0.2cm and 0.05g to 0.04g).

I can conclude that my prediction was correct:

“I predict that the potato tubes in the solution of distilled water will increase the most in weight and length and that the potato tubes in the 3M solution will decrease the most in weight and length.  I predict this because osmosis always occurs from an area of low sugar concentration to an area of high sugar concentration” because as I have shown above, my results show this.

Evaluation

In this investigation I planned my method thoroughly and followed it thoroughly to try and get the most accurate results possible.  The experiment went to plan and I didn’t have any big problems, but I feel it could have been improved.  

Firstly, it was very hard to extract the chips from the potato fairly because it was hard not to cut into a hole that had already been cut, so I had to use a new potato and start again as a variable was to use the same potato for all the tubes.  I also didn’t specify that the potato had to be peeled in my method, as the peel would have changed the rate of osmosis completely, and although I did, someone following my method may not have realised that and would have had an inaccurate experiment.  It was also very hard to measure the exact lengths of the potato tubes due to their rounded shapes, and there was nothing to hold them in place along the ruler, though the weighing was fine as the scales were precise and accurate.  I feel it was a successful plan to keep the potato tubes in groups and label them tubes 1-3, going from left to right, ad putting them into the beakers in that order, as this worked really well and enabled me to compare the correct before and after weights.  The main strength in my method was that I thought about the variables I had to keep the same carefully as changing a variable other than the concentration of the solutions would make the whole experiment inaccurate, and the experiment was also very safe.  

Despite the problems I had with being as accurate as I would have liked, my results supported my prediction really well because the scientific osmosis theory behind it was correct.  I found it hard to predict quantitatively because in my preliminary experiment I didn’t record the actual concentrations of each solution.  However, my results are accurate in relation to:

• how the change in lengths and weights vary in different concentrations – for example they gain length and weight in distilled water and lose it in the 3M solution;
• how each group’s before and after lengths and weights change in the same way – for example they all decrease by around 0.2cm and 0.04g in the 2M solution and;
• how each individual potato tube’s before and after lengths and weights change – for example tube 1 in the 1M solution increases in length so increases in weight, whilst tube 3 decreases in length so decreases in weight.

which I feel proves that osmosis worked how it should have and the results back this up even if the lengths aren’t exact.

Based on the fact that hardly any osmosis occurred in the 1M solution, I concluded that 1M was around the concentration of sugar in the water solution in the potato tubes, and so the 0M distilled water should have increased in length and weight by the same amount as the 2M solution decreased, as the difference in concentration between them and the potato was 1M.  This occurred with the weights as there was a 0.01g difference in change (0.05g increase in distilled water to 0.04g decrease in 2M solution), which is tiny and occurred because, as shown on the graph, although the average length of the potato tubes in the 1M solution didn’t change, the weight increased a little, so the potato concentration was a little higher than 1M.  This shows that measuring the weight is more precise and accurate, as the length results show that the potato tubes in distilled water increased by 0.1cm more than in the 2M solution (0.3cm to 0.2cm), which is a large amount caused by the need for a more precise ruler.

Of course, there is not going to be the same amount of osmosis in the same concentration all the time, as my experiment has shown, because the plant isn’t a machine that I could set up to work in a certain way, though I tried to by making the starting lengths, weights and potato concentrations the same, you can’t actually control the plant whilst osmosis is occurring.  Apart from using a more precise ruler to measure the lengths more accurately, a larger potato to get as many good potato tubes as possible and possibly a square shaped borer to extract the potato tubes so they were easier to measure, I would have definitely repeated the experiment to see if the anomalous results I got occurred again, as I feel I could have been more quantitative if I had more experience of using my method, though this experiment did prove my prediction and fulfil my aim.  

If I had a chance to do further work on this experiment to look more in depth at osmosis in plants, I would look at the rate as which osmosis occurred by measuring the lengths and weights of the potato tubes at different times throughout the experiment to see if the rate of osmosis was quicker at first when the difference in concentrations between the potato tubes and solutions was greater and then slowed as the transfer of water balanced the concentrations out.  I would also repeat the experiment with different potatoes just to see how concentration of sugar varies in different types of potatoes.

 I would also investigate the other factors that affected the rate of osmosis;

• Time, by putting tubes of the same length and the same weight from the same potato into beakers with the same amount of the same concentration of sugar and water solution at the same temperature.  I would leave them in the solution for different lengths of time and then measure the changes in lengths and weights, but not for too long as when the plant has the right amount of water in relation to the amount available osmosis will stop occurring.
• Size, by putting tubes from the same potato of around 5 different lengths (2cm, 4cm, 6cm, 8cm, 10cm) and therefore 5 different weights into beakers with the same amount of the same concentration of water and sugar solution for the same amount of time.  I would then measure how much the lengths and weights had changed to see how size affects the rate and direction of osmosis.
• Temperature, I am not sure exactly what plant I would do this with as the reason osmosis occurs more in higher temperatures is due to a higher rate of transpiration from the leaves which results in more water needing to be drawn up the plant by osmosis in the cells, and potato tubes have no leaves.  However, I would put (plant) of the same length and weight if possible into beakers with the same concentration of the same amount of sugar and water solution for the same amount of time in 5 different water baths (10°c, 20°c, 30°c, 40°c, 50°c).  I am also not sure how I would measure the changes in the cells, but I would like to do this experiment as it would be interesting to see how the whole plant works together to alter the rate of osmosis.

I would like to do the experiments comparing different types of plants, for example potato and carrot, that grow in the same way, to see if one carries out osmosis quicker, or which loses most water in high temperatures, or which contains the most sugar.  Temperature would be especially interesting with tropical or desert plants compared to British plants, as tropical or desert plants are in higher temperatures where more water could be lost from leaves, but often are adapted for this by having high water stores, whereas British plants are used to cooler conditions and don’t have these features.

I think this experiment has a lot of scope, but the one I carried out went well and proved that osmosis always occurs from a solution of high water/low sugar concentration to a solution of low water/high sugar concentration.