Osmosis is defined as 'the movement of water molecules from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane' (Collins, 1999).

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Prediction

Osmosis is defined as 'the movement of water molecules from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane' (Collins, 1999).

If you get information from books put the copied text in brackets and at the end put the authors surname and date the book was published in brackets. Then put the full details of the book at the end of the course work in the 'References'

In a high concentration of water the amount of solute (e.g. sugar) is low. This could be called a weak or dilute solution.

In a low concentration of water the amount of solute (e.g. sugar) is high. This could be called a strong or concentrated solution.

When two such solutions are divided by a semi-permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium).

This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. If done with potato cells the cells would increase in length volume and mass because of the extra water.

If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.

The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.

However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.

Conclusion

The prediction made is supported by the evidence obtained from this investigation. It shows that the potato cells increase mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. At concentrations above 0.6 M, there appears to be no further water loss, suggesting that the cell is fully plasmolysed.

From the graph an estimate to the concentration of the potato cell can be made as 0.132 M. As this is the point where the potato is not increasing or decreasing in mass. However, it is important to realize that this is only an estimate as the potato cells will not be uniform in their concentration.

Evaluation

The accuracy of the investigation was adequate, however the concentrations were measured using a measuring cylinder and could be made more accurate using titration. Also when the potato was dried to remove surface liquid it was not necessarily done the same on each potato, a more accurate and uniform way of drying would improve the accuracy further.

Further work could be carried out to include concentrations that increased in 0.1 M rather than 0.2. This would increase the accuracy and improve the graph. Other investigations could include using different varieties of potato or different plant tissues e.g. carrot.

Fair test:

In this experiment there should only be two changing variables: 1: Different molarities of sugar solution. 2: Changing weight of the potato samples. To keep these two variables the only variables in the experiment I must:

: Keep the potato samples the same length (2cm). This is because if one potato sample is 1cm long and one is 3cm long then the 3cm long sample will have a larger surface area and will osmosise much more quickly.

2: Use the same potato. This is because many factors due to the potato may affect the experiment. For example the age, species e.g. King Edward, size and any imperfections in the potato can be kept to a minimum.

3: Stop the evaporation of any of the molar solution. This is because if the sugar solution evaporates past the level of the of the potato then the potato sample will have less surface area in the solution so may osmosise slower. To stop any solution evaporating a foil lid can be placed on top of the test tube.

4: Accurate amount of sugar solution: More Bathing solution may affect the rate of solution. To make the amount of solution placed in the test tube as accurate as possible a syringe will be used to measure out the exact amount needed.

5: Contamination: As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop this the beaker and syringe must be washed every time they are used.

6: Average: To make the experiment as accurate as possible an average will be taken out of 5 results taken. Also any clearly anomalous results will be ignored.

7:Temperature: The temperature may affect the reliability of the experiment for example at extreme temperatures the cells of the potato may die and at less extreme temperatures the experiment may be speeded up. To keep this from happening all the test tubes will be kept in the same place

Prediction:

I predict that at around the 0.3 molarity solution there will be not much change. This is because on a pilot test done before hand there was not much change at around this point. I also predict that if a sample floats in the solution then it will gain weight and osmosise and if it sinks then the potato will ex-osmosise.

Revised Procedure.

. Set up Test tubes, five for each molarity level, making sure they are labelled.

2. Prepare the potato samples, cut out tubes using the same cork borer and then cut them to 2cm long using a scalpel. Making sure you clean your equipment to prevent contamination.

3. Immediately weigh each sample, and then place in the bathing solution.

4. Leave for 24 hours.

5. Remove samples; wipe off excess water on dry tissue paper.

6. Immediately re-weigh the samples and record the results.

Number and Range:

There will be 7 different molarity levels 0, 0.5, 0.1.15, 0.2, 0.25, and 0.3. The results will also be taken five times per molarity level and an average taken this will help reduce anomalous results.

Appropriate Equipment.

. 5 test tubes for each molarity level (35).

2. Bungs for test tubes (35).

3. Labels for test tubes (35).

4. Test tube rack for each molarity level (7).

5. Bungs for test tubes (35).

6. Scalpels for cutting potato samples.

7. Cork borer.

7. Cutting board.

8. Weighing machine.

9. Beakers for measuring solution.

0. Tissue for drying potato samples.

Diagram:

Procedure:

Use a syringe to measure out 50 mls of a molarity level of a sugar solution and fill up five test tubes with 50mls of the solution. This will give a range of results and if the average is taken then. Cut out five samples of potato for each molarity level all from the same potato as if the potatoes are different then their cellular make up will be to and therefore they would have different osmotic properties. Make sure that they are two cm in length. Then place them in the test tube and cover with foil so that the solution does not evaporate over the 24 hours you have to leave it. <INSERT OSMOSIS DEFINITION HERE>During this 24 hours if the potato gains wait it is osmosising and the water molecules in the solution will be able to pass through the semi-permeable membrane of the potato sample. However the sugar molecules will not be able to pass through and therefore if no sugar molecules pass through the potato will become equally balanced with the water outside and as it needs to get water into its cells to do this then it will gain wait. On the other hand if there is more of a concentration of water in the potato then the water will ex-osmosise into the sugar solution therefore losing weight. Therefore if the sugar concentration in the solution is high the water inside the potato will pass through the potatoes semi-permeable membrane and into the sugar solution.

Prediction:

The weight of the potato should not change at some point just below or above 0.25m. This was worked out using preliminary tests.

Preliminary work:

Prior to this experiment I did an experiment to help with a prediction and also to help indicate a number and range for the main experiment. The experiment was done quickly using the molarity levels 1, 0.5, 0.25 and 0. There was no average taken but it helped identify a closer range to work with. The potato sample did not gain or lose much weight at around the 0.25 mark. This meant that we could now produce a more accurate experiment at around the 0.25 molarity mark and instead of having several molarity all the way up to 1% I could be more accurate and pin point at around what position exactly I would do the experiment. This also helped establish that if the potato sunk then it would probably lose weight and if it floated then it would gain weight and therefore showed me whether it would osmosise or ex-osmosise.

Section O:

Safety: To make the experiment safe the equipment that was sharp i.e. Scalpels and core borers were used carefully and a cutting tile or board.

Table to show the average loss/gain in % of a sample of potato tissue submersed in Sugar solution for a period of 28 hours (to 2dp).

Molarity Sample Weight

Original: New: Loss/Gain (g): Loss/Gain (%): Average Loss/Gain (%):

0.0m 1 1.17 1.34 + 0.17 + 14.53 +12.66

2 1.05 1.18 + 0.13 + 12.38

3 1.11 1.25 + 0.14 + 12.61

4 1.07 1.20 + 0.13 + 12.15

5 1.03 1.15 + 0.12 + 11.65

0.05m 1 1.06 1.17 + 0.11 + 10.38 +9.70

2 1.25 1.36 + 0.11 + 8.80

3 1.13 1.24 + 0.11 + 9.73

4 1.01 1.10 + 0.09 + 8.91

5 1.22 1.35 + 0.13 + 10.66

0.10m 1 1.13 1.20 + 0.07 + 6.19 +6.97

2 1.13 1.21 + 0.08 + 7.08

3 1.11 1.20 + 0.09 + 8.11

4 1.11 1.19 + 0.08 + 7.21

5 1.12 1.19 + 0.07 + 6.25

0.15m 1 1.15 1.20 + 0.05 + 4.35 +4.40

2 1.14 1.20 + 0.06 + 5.26

3 1.01 1.05 + 0.04 + 3.96

4 1.13 1.18 + 0.05 + 4.42

5 1.00 1.04 + 0.04 + 4.00

0.20m 1 1.14 1.18 + 0.04 + 3.51 +3.26

2 1.01 1.04 + 0.03 + 2.97

3 1.03 1.06 + 0.03 + 2.91

4 1.11 1.15 + 0.04 + 3.60

5 1.20 1.24 + 0.04 + 3.33

0.25m 1 1.03 1.05 + 0.02 + 1.94 +0.40

2 1.01 1.00 - 0.01 - 0.99

3 1.01 1.00 - 0.01 - 0.99

4 1.00 1.00 +/- 0.00 +/- 0.00

5 1.02 1.04 + 0.02 + 1.96

0.30m A 1.03 0.99 - 0.04 - 3.88 -5.94

B 1.02 0.96 - 0.06 - 5.88

C 1.01 0.93 - 0.08 - 7.92

D 1.00 0.93 - 0.07 - 7.00

E 1.00 0.95 - 0.05 - 5.00

The range was close to being inaccurate but showed that the potato samples solute content was between 0.25 and 0.3.

Flow diagram of how we obtained the results:

Fill a test tube with the sugar solution

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Cut out potato samples with an apple corer

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Cut each sample to 2cm in length

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Place a sample inside the test tube

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Place a foil lid over the neck of the test tube

ê

Leave for 28 hours

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Remove potato sample from the test tube and wipe off excess water

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Re-weigh and record the results

Scatter graph to test the accuracy of the results:

Section A:

Table of results showing molarity level and % loss/gain in the potato samples weight.

Molarity Sample Weight

Original: New: Loss/Gain (g): Loss/Gain (%): Average Loss/Gain (%):

0.0m 1 1.17 1.34 + 0.17 + 14.53 +12.66

2 1.05 1.18 + 0.13 + 12.38

3 1.11 1.25 + 0.14 + 12.61

4 1.07 1.20 + 0.13 + 12.15

5 1.03 1.15 + 0.12 + 11.65

0.05m 1 1.06 1.17 + 0.11 + 10.38 +9.70

2 1.25 1.36 + 0.11 + 8.80

3 1.13 1.24 + 0.11 + 9.73

4 1.01 1.10 + 0.09 + 8.91

5 1.22 1.35 + 0.13 + 10.66

0.10m 1 1.13 1.20 + 0.07 + 6.19 +6.97

2 1.13 1.21 + 0.08 + 7.08

3 1.11 1.20 + 0.09 + 8.11

4 1.11 1.19 + 0.08 + 7.21

5 1.12 1.19 + 0.07 + 6.25

0.15m 1 1.15 1.20 + 0.05 + 4.35 +4.40

2 1.14 1.20 + 0.06 + 5.26

3 1.01 1.05 + 0.04 + 3.96

4 1.13 1.18 + 0.05 + 4.42

5 1.00 1.04 + 0.04 + 4.00

0.20m 1 1.14 1.18 + 0.04 + 3.51 +3.26

2 1.01 1.04 + 0.03 + 2.97

3 1.03 1.06 + 0.03 + 2.91

4 1.11 1.15 + 0.04 + 3.60

5 1.20 1.24 + 0.04 + 3.33

0.25m 1 1.03 1.05 + 0.02 + 1.94 +0.40

2 1.01 1.00 - 0.01 - 0.99

3 1.01 1.00 - 0.01 - 0.99

4 1.00 1.00 +/- 0.00 +/- 0.00

5 1.02 1.04 + 0.02 + 1.96

0.30m A 1.03 0.99 - 0.04 - 3.88 -5.94

B 1.02 0.96 - 0.06 - 5.88

C 1.01 0.93 - 0.08 - 7.92

D 1.00 0.93 - 0.07 - 7.00

E 1.00 0.95 - 0.05 - 5.00

The weight increased in each potato sample until just above 0.25 and below 0.30. This shows the solute content of the potato is equal with the concentration of the bathing solution. How ever the last results at 0.3 have a difference of 4%. This may produce anomalous results.

To find the exact solute content of a potato a graph will have to be plotted showing molarity against % weight change. The point at which the potato does not gain or lose any weight should be just above 0.25m. This is because there was little weight gain at 0.25 and a loss at 0.30m.

Graph of weight (%) change vs. Molarity of bathing solution (m):

Conclusion:

The graph above shows that at 0.23 there is 0% loss or gain in the weight of the potato. This shows at this point the solute content of the potato is exactly equal to the concentration of the sucrose bathing solution. Therefore no water moves between the potato and the bathing solution. This means that the weight of the potato sample does not change. So the concentration of the solute content of potatoes must be 0.23%.

Prediction:

In Section P. I predicted that the potatoes would have a solute content at around 0.25%. The real solute content of the potato was 0.23%. This proves my prediction was accurate.

Section E:

Accuracy and general comments:

The evidence obtained was accurate. Most anomalous results were cancelled out by the averages taken. The scatter graph in section A shows that the results taken were accurate and the results were all quite close to the line. This means that the results were accurate enough to make a reliable conclusion. However there were some anomalous results at some point as the table suggests that the solute content of the potato would be between 0.25 and 0.30, however the best fit line on the graph shows that this is not true and that the actual solute content of a potato is around 0.23%.

The accuracy of the experiment was accurate to suit our purpose to make it more accurate several steps could have been taken.

: Human error: Human error could have been reduced by taken more accurate measurements to a uniform result. This could be attempted by using a tool which could be set to a set length to cut the potato.

2: Instead of the range of the potato being 0.0 to 0.30 the range could be decreased to 0.20 to 0.30. This would produce more accurate results. Also increasing the number of results so instead of taking results from 7 different molarity levels 10 molarity levels could be taken. This would make the lines of best fit on the graphs more accurate, therefore the end results would be more accurate.

3: If the potatoes did not rest against the sides then they would all have the same amount of surface area. This is the same for the potato samples that float therefore exposing themselves to air and the samples that sink stopping osmosis occurring on the areas that are touching the bottom of the test tube.

Reliability: The results were reliable to take a reasonably accurate result. However the steps above (1,2 and 3) would produce an even more accurate result.

Further work: To extend this experiment it could be repeated exactly as before. However this time results at the molarity levels 0.20, 0.21, 0.22,0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30. This would produce much more accurate results.

Other variables in the experiment could be changed for example instead of changing the weight of the potato the species of the potato could be changed. For example New potatoes, King Edwards etc could be used.

Also the shape and size could be changed. However this would not affect the results much. This is because the variable would only change the rate of osmosis because of a different weight and size.

Temperature could also be changed for example the samples could be placed in different water baths and brought up to different temperatures to see if temperature played its part in the osmosis of potatoes. 5 sets of 5 potatoes could be placed in water baths at 10oC, 20oC, 30 oC, 50 oC and 60 oC. Then leave them for 24 hours making sure all the variables in the first experiment still apply however just using one molar solution. Then after 24 hours re-weigh the samples and record the result. I would expect that at high temperatures the potato samples would osmosise the most. This is because at high temperatures the solutions water molecules would move faster and therefore equal the concentration faster. A preliminary experiment could be set up beforehand to find out how long the experiment should be kept going because if the concentration of the potatoes equalises then the weight of the potatoes will be almost exactly the same.

Introduction

Knowing that osmosis (a diffusion of water) will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of them, and thus change their volume, we want to test the hypothesis that:

If the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less than that level it will expand.

We have studied turgidity and plasmolysis in a textbook (Key Science-Biology, pages 143-144) and in a preliminary experiment, where we first added 2% sucrose solution to rhubarb epidermal cells, and saw them become plasmolysed, and then added water, and saw them become turgid. However, we did not use different solution concentrations, and did not measure the amount of contraction or expansion that took place. From our results in the main experiment, we should be able to work out not only the amount of contraction or expansion caused by each strength of solution, but also the concentration of the sap inside the cells.

Apparatus

For the experiment we will require:

Either cylinders of potato with a diameter of 6.5mm and a height of 5mm, or a potato, a borer with a diameter of 6.5mm and a scalpel. (To allow us to make our own).

Solutions of varying strengths (of sucrose and NaCl), or a solution of a known strength and distilled water. (To allow us to make our own).

Pins (To ensure that cylinders remain separate while in the solutions.)

Test-tubes

Callipers (To measure cylinder height and diameter.)

Diagram

One of the test-tubes during the experiment.

Three potato discs on a pin, not touching.

Method

We take a cylinder of potato, with a diameter of 6.5mm, from the potato, and cut it into separate cylinders each with a height of 5mm. We then thread at least three of the cylinders, to make the experiment fair (in case one of the cylinders is abnormal or damaged), on to a pin, keeping them apart from each other. We then make up solutions of either sucrose or sodium chloride, either by % strength or by molarity, and place 4 millilitres of each strength into a separate test-tube. We used a range of % sucrose solutions, going from distilled water (0%) to 2% (which we knew from earlier experiments would plasmolyse the cells), and a range of sodium chloride solutions from distilled water (0) to 0.4 molar (which would again be enough to plasmolyse the cells). We then place each of the sets of three cylinders on a pin into each of the different solutions, making sure that the cylinders are covered by the solution, and leave all of the test-tubes close to each other for 24 hours.

We assume that this means that the pressure and temperature in each case is the same, as these are factors which could affect osmosis, and we know that the volume, size and surface area of each cylinder is the same, and as they are all from the same potato, the only variable that we are altering is the concentration of the solution. Although ideally the experiment would be repeated several times, we were not able to do this as we did not have sufficient time.

After 24 hours we remove the cylinders from solution and, with callipers, which are more accurate than a ruler and would cover the likely range of sizes (from 4mm to 7mm), measure the new diameter and height of the cylinders. The results, in table and graph form are recorded below in the Results section.

Results

Concentration Cylinder Diameter/mm Cylinder Height/mm Volume/mm3 (2dp) Ave. Cylinder Volume/mm3

Pre-immersion 6.5 6.5 6.5 5 5 5 165.92 165.92 165.92 165.92

Sodium Chloride solution

0.0 Molar 6.8 6.6 6 5.5 6.4 5.2 199.74 218.96 147.03 188.58

0.1 Molar 6 6.5 6.8 4.4 4.9 4.9 124.41 162.6 177.95 154.99

0.2 Molar 5.6 5.9 5.7 5 4.5 4.5 123.15 123.03 114.83 120.34

0.3 Molar 6 6.1 5.9 4.9 4.9 4.5 138.54 143.2 123.03 134.92

0.4 Molar 5.9 6 5 5.6 5.4 5 153.1 152.68 98.17 134.65

Sucrose Solution

0% 6.8 7 6.8 5.7 5.5 5.3 207.01 211.66 192.48 203.72

0.25% 5.5 6 5 5 5.5 5 118.79 155.51 98.17 124.16

0.50% 5 5.2 5 5.5 6.6 5 107.99 140.17 98.17 115.44

% 5.5 5 4.9 5.9 5.1 5 140.17 100.14 94.29 111.53

2% 4.4 4.6 4.4 4.8 5.2 4.4 72.99 86.42 66.9 75.44

Concentration of Solution Average % Change in Volume From Original

NaCl solution

0.0 Molar 13.66

0.1 Molar -6.59

0.2 Molar -27.47

0.3 Molar -18.68

0.4 Molar -18.84

Concentration of Solution Average % Change in Volume from Original

Sucrose Solution

0% 22.78

0.25% -25.17

0.50% -30.42

% -32.78

2% -54.53

Analysis

The results show that, in accordance with our hypothesis, the cylinders will expand when external solute concentration is low (high water concentration), and contract in strong solutions (low water concentration). This is due to osmosis, where water passes from weak solutions to strong solutions across a semi-permeable membrane, such as a cell membrane. The graphs of % change against solution strength show that the results tend to form a curve, crossing the x axis (where there is no change in volume), at approximately 0.07 molar concentration for the sodium chloride solution, and at approximately 0.2 % for the sucrose solution. This concentration is the osmolar concentration (the total solute concentration) of the sap inside the cell. The volume change forms a curve when plotted against solute concentration because the cells, which have cellulose cell walls in addition to a cell membrane, will not expand or contract indefinitely, and will be held in shape within certain limits. However, the relatively low number of solutions tested (5) means that there is a range of possible values for the osmolar concentration of sap in the cell, and means that we cannot accurately predict values for volume change at different concentrations. To conclude, therefore, the results support our hypothesis, and we were also able to discover the approximate concentration of the sap in the cell.
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Evaluation

Although the results of the sodium chloride and sucrose experiments support the hypothesis, there are several anomalous results and a large deviation for each result. These could be improved by altering the experiment, for example by keeping the test-tubes in a water bath at a set temperature, by keeping them at a constant pressure, and by measuring the sizes of potato cylinders before and after with a more accurate method, e.g. accurate weight measurement or volumetric displacement. The test might also be more accurate if the potato cylinders were left in the solutions for a longer ...

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