Biology Coursework

My aim is to investigate osmosis in plant tissue. Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration through a selectively permeable membrane. In this experiment the plant cells will be potato chips and the solution will be sucrose, later sucrose diluted with water.

Osmosis is a passive process that will occur across a selectively permeable membrane whenever there is a difference between the water concentrations on the two sides of a selectively permeable membrane. Osmosis is really a special kind of diffusion, because water moves down the concentration gradient. Unlike diffusion, by definition Osmosis only refers to the movement of water across a selectively permeable membrane.

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I predict the higher the concentration above the isotonic point the shorter the chip, and the lower the concentration below the isotonic point of the solute the longer the chip. I also predict that double the length solute concentration, i.e. double the concentration of sucrose, will result in double the length change.

A high concentration of water is called a weak or dilute solution, and it is said to be hypotonic to the stronger solution. A low concentration of water is called a strong or concentrated solution, and it is said to be hypertonic to the weaker solution. When two such solutions are divided by a selectively permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides have reached equilibrium (become isotonic), are equally concentrated.

Water moves from the hypotonic solution into the hypertonic.

Solution A is hypertonic and solution B is hypotonic.

Water moves from A to B until the concentration is equal, i.e. isotonic.

Selectively permeable membrane (only allows water through)

One important example of Osmosis is water entering the roots of plants. If plants cells are placed in different strength solutions they will swell up (gain turgor) or shrink (become plasmolysed). The cell membrane in cells is selectively permeable and the vacuole contains a solution.

Hypotonic solution

In a hypotonic solution water enters by osmosis through the selectively permeable cell membrane, and the vacuole swells. When the vacuole swells the cytoplasm is pushed up against the cell wall. When no more water is taken in it is called fully turgid. In the case of potato cells, the chip would increase in length

Hypertonic solution

Cell loses water by osmosis through the selectively permeable cell membrane, so the vacuole shrinks. Eventually when it loses a large amount of water and the cytoplasm is no longer pushed up against the cell wall and in extreme cases the cell membrane breaks away from the cell wall, the cell is referred to as plasmolysed. The potato cells would decrease in length.

If the concentration of a solution into which a potato chip is placed is greater than a certain level the potato chip will contract (length will decrease), and if the concentration is less than that level it will expand (and lengthen). It is this change in length I will examine. There is also 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 of the potato chip, and no osmosis will occur. Thus, I will also be looking for the isotonic point and discover the concentration of the potato chip’s vacuole.

The more hypotonic a solution the more water enters the cell. The more hypertonic a cell the more water leaves a cell. The bigger the difference the in concentration between the hypotonic solution and the hypertonic solution, the faster the change in concentration.

Therefore I predict the higher the concentration above the isotonic point the shorter the chip, and the lower the concentration below the isotonic point of the solute the longer the chip. I also predict that double the length solute concentration, i.e. double the concentration of sucrose, will result in double the length change.

Variables to be kept constant

To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable is changed. I have chosen to vary the concentration of the sucrose solution. This will give me a varied set of results from which I hope to make a conclusion. If any of the non-variables below are not kept constant it would mean it would not be a fair test. In this experiment I will keep the:-

* Chip starting length

I will keep the chip starting length constant. All the chips will be 50mm long to the nearest 0.5mm (I do not feel able to cut the chip more accurately than 0.5mm). I will use and ruler marked in mm, not cm and use a sharp razor blade to cut straight and accurately.

* Duration of experiment

All the chips will spend 40 minutes in their solution; this is accurate to about five seconds it will most likely take me to take the chips out. I will use a stopwatch to measure the time, though a normal clock with a second hand would do.

* Initial water content of potato

To ensure the initial water potential of the potato is the same I will use potatoes that have all been treated the same way, contained in the same box, peeled the same way etc.

* Solution volume

Although I will be changing the concentration of solution there will always be 20.0 ml, I will use a glass measuring cylinder accurate to 0.1ml to ensure accuracy.

* Temperature

All the components (potato chips, sucrose and water) will be stored at room temperature to ensure the entire experiment is carried out at room temperature.

* Width and breadth of potato

Since I want the chips to all start the same length and measure the length change I need to keep the width and breadth the same. Thus the chips I will be using are cut to exactly the same width and breadth.

In this experiment I will use a:-

* Ceramic Tile

* Distilled Water

* Glass measuring cylinder

I will use a glass measuring cylinder accurate to 0.1ml to measure out the 50.0ml of sucrose (and water).

* Sucrose

* Potato chips

* Razor blade

* Stopwatch

* Test tube

I will use one test tube for each different concentration.

* Test Tube Rack

I need enough test tube racks to hold all the test tubes.

Preliminary Experiment

I needed a quick experiment to decide on appropriate proportions of sodium thiosulphate and water to use, check there is a noticeable change in length and check that prediction is not completely inaccurate. So I performed a preliminary experiment.

I used concentrations of 0.9 moles, 0.6 moles, 0.3 moles and 0.0 of Sucrose. To work out how much sucrose to use and how much water I need simply used 1.0 molar sucrose, and multiply the desired amount of moles of sucrose I want by 20, the rest of the 20ml should be water:

e.g. for 0.9 moles I multiply 0.9 by 20 which equals 18. So I measure 18.0ml sucrose in the measuring cylinder and pour it into the test tube. Then I pour another 2ml of water in and I have a concentration of 0.9 moles.

1. Assemble the apparatus.

2. Fill four different test tubes with 20ml of sucrose at four different concentrations (previously chosen). Mark which test tube is which.

3. Cut four potato chips to 50mm each using the razor blade and using the tile as a cutting board.

4. Put one chip in one of the four test tubes (with the four different concentrations), the one with the highest concentration.

5. Start the stop watch.

6. After 1 minute place another chip in the next test tube, the one with the next lowest concentration. Repeat this twice at one minute intervals.

7. After 40 minutes after start of experiment remove the chip from the test tube with the highest concentration, and measure with a ruler on the tile. Record the result.

8. After 41 minutes since experiment began remove the next highest concentration and measure with ruler on tile. Repeat this twice, each after a minute’s interval.

I obtained the following results:-

Concentration

(molars)

Initial Length

(mm)

Volume of Sucrose

(ml)

Change in Length

(mm)

% change

0.9

50

18.0

-4

8

0.6

50

12.0

-3

6

0.3

50

6.0

0

0

0.0

50

0.0

+2

6

It all seems to be reasonable and it places the isotonic point roughly around 0.3 moles.

Method

I will use three chips for each concentration and eight different concentrations. This is because this will ensure greater accuracy; more results ensure greater accuracy as long as they are not averaged. When you average results you hide anomalous results and the line of best fit is less likely to be accurate.

I will use concentrations of 1.0 moles, 0.8 moles, 0.6 moles, 0.4 moles, 0.3 moles 0.2 moles, 0.1 moles and 0.0 moles of sucrose. I decided to use these concentrations so I could gain a wide range and a focus around the isotonic point, which I already know from the preliminary to be around 0.3 moles.

To work out how much sucrose to use and how much water I simply use 1.0 molar sucrose, and multiply the desired amount of moles of sucrose I want by 20, the rest of the 20ml should be water:

e.g. for 0.9 moles multiply 0.8 x 20 = 16. So I measure 18.0ml sucrose in the measuring cylinder and pour it into the test tube. Then I pour another 4ml of water in and I have a concentration of 0.8 moles.

1 Assemble the apparatus. Make sure the test tubes and measuring cylinder is clean.

2 Measure room temperature.

3 Measure out the desired amount of sucrose accurate to 0.1ml and pour into test tube. Place test tube on test tube rack and fill test tube with desired amount of water. Repeat with all eight desired concentrations. Mark each concentration next to test tube.

4 Cut 24 potato chips to 50mm long, as accurately as possible so they should be to the nearest 0.5mm. Use tile as chopping board.

5 Place three chips in one of the eight test tubes, preferably the one with the highest concentration. Start the stop watch.

6 After 1 minute place another three chips in the next test tube, the one with the next lowest concentration. Repeat this after 2, 3, 4, 5, 6 and 7 minutes.

7 After 40 minutes after start of experiment remove the first three chips from the tube (the highest concentration) and measure with a ruler on the tile. Record the results.

8 After 41 minutes since experiment began remove the next highest concentration and measure with ruler on tile. Repeat this twice, each after a minute’s interval.

9 Clean apparatus, and tidy away.

Obtaining

The room temperature was 20�C and I obtained the following results:-

Concentration

(molars)

Initial Length

(mm)

Volume of Sucrose

(ml)

Final length

(mm)

Change in Length

(mm)

% change

1.0

50.0

20.0

44.0

44.5

44.0

-6.0

-5.5

-6.0

-12

-11

-12

0.8

50.0

16.0

46.0

47.0

47.0

-4.0

-3.0

-3.0

-8

-6

-6

0.6

50.0

12.0

47.5

47.5

47.5

-2.5

-2.5

-2.5

-5

-5

-5

0.4

50.0

8.0

48.5

47.5

48.5

-1.5

-2.5

-1.5

-3

-5

-3

0.3

50.0

6.0

49.5

50.0

49.0

-1.5

0.0

-1.0

-3

0

-1

0.2

50.0

4.0

50.5

51.0

50.0

0.5

1.0

0.0

-1

-2

0

0.1

50.0

2.0

51.0

51.5

52.0

+1.0

+1.5

+2.0

+2

+3

+4

0.0

50.0

0.0

53.0

52.5

53.0

+3.0

+2.5

+3.0

+6

+5

+6

To work out the percentage change column simply multiply the change in length by 100/50 or 2.

I did not plot the average of the results because that would hide anomalous results and affect the line of best fit.

Analysis

From the graph I can see that when the concentration of the solution was lower than 0.28 moles the chip’s vacuole gained water became more turgid and lengthened. There was no sign however of the chip becoming fully turgid. When the concentration of the solution was higher than 0.28 moles the chip’s vacuole lost water and would shrink. There was no sign however of the cell becoming plasmolysed.

The graph is a negative strong correlation but there was an anomalous result, circled in green on the graph; it is 1.5mm longer than predicted. This is not too serious but a larger difference than all the others are from the line of best fit. The isotonic point or molar concentration of the potato chip was 0.28 moles.

These results support my prediction ‘the higher the concentration above the isotonic point the shorter the chip, and the lower the concentration below the isotonic point of the solute the longer the chip.’ They also support my quantitative prediction that the length would double when the concentration is doubled. At 0.4 moles I obtained a change in length of -1.5mm, -2.5mm and -1.5mm, and at 0.8 moles I obtained -4.0mm, -3.0mm and -3.0mm. Their averages are a.-1.83mm and -3.3mm, the 0.8 moles is not quite double and the 0.4 moles not quite half, yet the percentage error is 6.07%, this is within reasonable experimental error.

Evaluation

I think the experiment worked because the results were consistent, there was only one anomalous result, and are also reproducible. The most likely reason for the one anomalous result was because the chip was on the border between measuring the result as 47.0 (I could only reasonably measure it to 0.5mm); this would suggest a measuring problem. It could also have contained slightly more water than the other chips this would make it more hypotonic and the solution more hypertonic, resulting in more water lost and length decreasing.

The results allowed me to make a firm conclusion (discussed in my ‘Analysis’) and so on therefore the experiment was a success. There is some room for improvement however:-

1. More chips

More chips mean more results and more results mean greater accuracy on the line of best fit, this is because anomalous results are more evident and many reasonable ones.

2. More narrow range of concentrations

As with more chips, a more focussed range of concentrations would expose anomalous results more effectively.

3. Use burette

This would ensure greater accuracy in the volume of solution.

4. Use water bath

Using a water bath would ensure the temperature would remain constant. It would also mean the experiment would be performed in the dark, removing light as a factor.

5. Weigh chips

It would be far more accurate to weigh it with scales accurate to 0.01g than by eye with a ruler. It would also allow for width and breadth shrinking or expanding.

Doing all of the above would greatly increase accuracy, leaving no major variables out. However due to time I am not able to perform the experiment in the above way. In the extension though I show how I would perform it.

Extension

In this experiment I would use a:-

* Ceramic Tile

* Distilled Water

* Burette

* Burette stand

* Sucrose

* Paper towels

I would use paper towels to wipe excess water off chip before weighing.

* Potato chips

* Razor blade

* Stopwatch

* Scales

I would use scales accurate to 0.01g.

* Test tube

I would use one test tube for each different concentration.

* Water Bath

* Water Bath Test Tube Rack

I would perform a preliminary to discover how accurately it is possible to cut the chip, to the nearest what of a gram? 0.05g?

I would use six chips for each concentration and eight different concentrations. This would ensure greater accuracy; more results at a more focussed concentration. More results expose anomalous results, and so the line of best fit would be more likely to be accurate.

I would use concentrations of 0.50 moles, 0.45 moles, 0.40 moles, 0.35 moles, 0.30 moles 0.25 moles, 0.20 moles and 0.15 moles of sucrose, and with a burette I would be able to measure out these more precise concentrations. Choosing these concentrations would mean you can focus around the isotonic point, which we already know from the previous experiment to be around 0.28 moles.

1 Assemble the apparatus, preheating the water bath to 21�C. Make sure the test tubes and measuring cylinder is clean.

2 Measure out the desired amount of sucrose accurate to 0.01ml and pour into test tube. Place test tube on test tube rack inside water bath and fill test tube with desired amount of water. Repeat with all eight desired concentrations. Mark each concentration next to tube.

3 Cut 48 potato chips to 50mm long, using the tile as chopping board. Then weigh each individual chip cutting each down till they are all exactly the same weight, to between the nearest 0.01g and 0.05g.

4 Place six chips in two of the sixteen test tubes, three chips in each test tube, preferably the one with the highest concentration. Start the stop watch.

5 After 1 minute place another six chips in the next two test tubes, the one with the next lowest concentration. Repeat this after 2, 3, 4, 5, 6 and 7 minutes.

6 After 40 minutes after start of experiment remove the first six chips from the two test tubes (the highest concentration) and weigh each individually to the previous scale used. Record the results.

7 After 41 minutes since experiment began remove the next highest concentration’s chips and weigh. Repeat this twice, each after a minute’s interval.

8 Clean apparatus, and tidy away.

The use of a water bath also allows use to use temperature as a variable, instead of concentration. Most reactions give double the rate of reaction when the temperature is raised by 10�C. I would suggest this is what would happen here, the reactions would speed up and the chip would lose and gain more water. It would allow us to find us the fully turgid point and when the chip becomes plasmolysed.

To carry out such an experiment follow the method for e mass except instead of changing the concentrations change the preheated temperature. It would mean that each different experiment (different temperature) would have to be carried out separately at a different time. At higher temperature the test tube would also need a bung.