An investigation into the effect of increasing the concentration of salt solutions on potato cells.

Emma Highton

Science Investigation: Biology

Aim: An investigation into the effect of increasing the concentration of salt solutions on potato cells.

Prediction: Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

This diagram illustrates a concentrated sugar solution by a membrane. The membrane has pores in it, which are very small. (An example of a membrane like this is visking tubing.)

There are more water molecules on the left-hand side of the membrane than the right. Water molecules are very small. Each is made of two hydrogen atoms and one oxygen atom, where as sugar molecules are many times larger than this. In visking tubing the holes are big enough to let water molecules through, but not sugar molecules, as they are too big. This is a partially permeable membrane therefore because it will let only some molecules through.

There is a higher concentration of sugar molecules on the right hand side of the membrane in the illustration, than the left hand side. If the membrane were not there, the sugar molecules would diffuse from the concentrated solution into the dilute one until they were evenly spread out. However, they cannot do this because the pores in the membrane are too small for them to get through.

There is also a concentration gradient for the water molecules. On the left hand side there is a high concentration of water molecules. On the right hand side the concentration of water molecules is lower because the sugar molecules take up a lot of space. The water molecules therefore diffuse from left-hand side to the right-hand side. The water molecules are small enough to fit through the pores in the membrane.

The result of this is that water has diffused from the dilute solution, through the partially permeable membrane, into the concentrated solution, meaning that the concentrated solution will become more dilute, because of the extra water molecules transferring into it.

This process is called Osmosis. It is the diffusion of water molecules from a place where they are in a higher concentration (such as dilute sugar solution), to a place where the water molecules are in low concentration (e.g. concentrated sugar solution) through a partially permeable membrane. Cell membranes are like visking tubing. They will let some substance pass through them, but not others, working as a partially permeable membrane.

This diagram illustrates a plant cell in pure water. Plant cells do not burst in pure water.

Plant cells are surrounded by a cell wall. This is fully permeable, which means it will let any molecules go through it, so osmosis will not occur across it.

A plant cell however also has a cell membrane. This is partially permeable. A plant cell in pure water will take in water by osmosis through its partially permeable cell membrane, to dilute the concentrated cell solution (cytoplasm). As the water goes in, the cytoplasm and vacuole will swell.

However the plant cell has a very strong cell wall surrounding it. This is much stronger than the cell membrane and prevents the cell from bursting. The cytoplasm presses out against the cell wall, but the cell wall resists and presses back on the contents.

The plant cell in this state is like a blown up tyre, tight and firm. It is said to be turgid. The turgidity of its cells helps a plant that has no wood in it to stay upright, and keeps the leaves firm. Plant cells are usually turgid.

This diagram illustrates a plant cell in a concentrated solution. Plant cells plasmolyse in concentrated solutions.

It will lose water by osmosis because the solution surrounding it is more concentrated by the cytoplasm, then the water molecules will diffuse out of the cell. The cytoplasm and vacuole will shrink. As the cytoplasm shrinks it stops pushing outwards on the cell wall. Like a tyre when some of the air has leaked out, the cell becomes floppy, making it flaccid. When this happens the plant becomes flaccid, then the plant loses its firmness and begins to wilt.

If the solution is very concentrated, then a lot of water will diffuse out of the cell. The cytoplasm and vacuole will go on shrinking. The cell wall, though, is too stiff to be able to shrink much. As the cytoplasm shrinks further and further into the centre of the cell, the cell wall gets left behind. The cell membrane, surrounding the cytoplasm, tears away from the cell wall.

A cell like this is said to be plasmolysed. This does not normally happen because plant cells are not surrounded by very strong solutions. It usually kills a plant cell because the cell membrane is damaged as it tears away from the cell wall.

Osmosis therefore makes plant cells swell up when surrounded by weak solution making them turgid. If the plant cells are surrounded by very strong solution, stronger than the cell solution, then plant cells shrink, becoming flaccid, then the cell membrane pulls away from the cell wall, and the cell is plasmolysed.

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This is a preview of the whole essay

Previously, we conducted an experiment with sugar solution and visking tubing.

Here the visking tubing was filled with concentrated sugar solution and was placed in a beaker filled with pure water. The visking tubing acted as a partially permeable membrane, therefore this meant that the sugar molecules were too big to fit through the small pores but the water molecules weren’t. There was a higher concentration of water molecules on the outside of the tubing than on the inside, therefore mass of sugar solution rose, but became more diluted, because as in osmosis the water molecules moved through the partially permeable membrane from an area of high water concentration to an areas of low water concentration, in the idea that the water molecules were trying to balance out on either side of the membrane. The movement of molecules stopped when both sides were equal in water concentration. This meant that the solution inside the tubing had gained mass (water mass) showing that osmosis had taken place. The experiment I am going to conduct is with potato cells in salt solution. The potato cell is made up of plant cells, and like the visking tubing, its cell membrane acts as a partially permeable membrane. Therefore only water molecules, not salt molecules can fit through. If the two sides are un-balanced in water concentration then osmosis will take place over the membrane, moving the water from the area of high water concentration to the area of low water concentration.

Therefore, having looked at the previous experiment on visking tubing and sugar solution and what it teaches me about water movement, and my knowledge of osmosis, I predict that as the percentage of salt concentration increases, the mass of the potato will decrease through loosing water in osmosis. The potato cell will loose water mass. This is because osmosis occurs whenever there are two different concentrations of solution on either side of a partially permeable membrane. A potato cell, made up of plant cells, has a weak cell solution. The salt solution is going to increasingly get stronger than the cell solution, and salt molecules are larger than water molecules and since the potato is in the salt solution, and the potato has a partially permeable membrane, salt cannot move into the cell because the molecules are too big. Therefore because greater water concentration will be inside the potato cell, the water molecules will move out of the cell to dilute the salt solution, and balance the water concentration on either side of the membrane. Therefore the cell will shrink, meaning its mass will decrease. We will be able to tell whether this has happened by weighing the potato’s mass before and after the experiment, to notice a decrease in mass, representing that the water has diffused out of the cell to dilute the increasing salt solution, and that osmosis has occurred.

To predict the shape of the graph there are a variety of factors to explore. At first I will conduct a control experiment with the potato in pure water. Here the cell solution has less water concentration than the pure water, meaning that through osmosis the water molecules will move into the cell to dilute the cell solution and make the two sides balanced. When then salt solution replaces the water, the mass of the cell will decrease, as the greater water concentration will be inside instead of out meaning a reverse result of the control. Also as the salt solution becomes more diluted, eventually the concentration in and around the cell will balance out, meaning the graph will eventually even out as no more water can diffuse in and out.

Variables:

Factors I Will Measure Factors I Will Change Factors I Will Keep The

Same

Incubation Period

(Minutes)

Mass of potato(g)

Volume of salt

Solution(cm )

Volume of water for

control(cm )

Concentration of salt

solution(0%, 5%…)

Plan: This is what I intend to do.

In a group of three, I will collect all of my apparatus and set them up in a clear space. Using an apple corer, I will cut six cylinders of potato, making sure all the cylinders come from the same potato to make it fair. Using a ruler and knife I will cut the cylinders to be 3cm long. After, I will collect 12 beakers, labelling two with each salt percentage 0% 5% 10% 15% 20% 25%. For example, two pots will say 0% salt, one of these pots will have a cylinder of potato in, and later on, the other will have solution in it. For the time being, I will put one of the six potato cylinders in each beaker. Then I will take the labelled pots over to the scales, and weigh and record each potato cell, corresponding with its percentage on the label, making sure I put back the potato in the same pot, and I record the weight next to the correct %. After doing this, using a measuring cylinder of 0 – 100 cm I will measure out carefully 20 cm of each solution, pouring it into the corresponding pot, the pot without the potato cell in it. One person will start the stopwatch as the other people in the group drop the correct potato into the matching beaker of solution all at the same time. This will now be left for ten minutes in the incubation period. After 10 minutes, each of the group members will remove the potato from the beakers, placing them down in front of the beaker they came from so that they do not get mixed up. Using the scales, they will each be re-weighed and this result will be recorded next to its original weight. Finally I will rinse the beakers and throw away the potato cells. Repeat this procedure again to collect enough results for an average.

To make a fair test, the potato cylinders have to all come from the same potato, as the water concentration may differ in another potato. When each solution has been measured in the measuring cylinder, after it has been poured out, I will wash the measuring cylinder to make sure that solutions do not mix, which could effect the results, to make a fair test.

Apparatus:

Stopwatch (Mins)

12 small beakers

12 potato samples from the same potato

Salt solutions (0 – 25%)

Distilled water

Knife

Ruler (0-15cm) this is the smallest ruler available and we only need 3cm.

Measuring cylinder (0-100cm )

Weighing scales (grams)

Borer

I have chosen to use a stopwatch, as it is more accurate than looking at a clock, and the 15cm ruler is the smallest ruler available, since we only need 3cm in length.

Diagram of Experiment:

Safety: The use of the knife could be dangerous as there is a sharp blade. To be safer, I will keep the blade tucked away when it is not in use. Also I will keep my hair tied back as to not get in the solution, and keep my tie tucked in and sleeves back as to not splash the salt solution into eyes.

Table Of Results:

Experiment 1:

Salt Solution Original Mass of Final Mass of Change in Mass % Change

% potato (g) potato (g) (g) + or - In mass

0 1.06 1.07 +0.01 +0.94

5 1.20 1.15 - 0.05 - 4.17

10 1.11 1.00 - 0.11 - 9.91

15 1.25 1.13 - 0.12 - 9.60

20 1.18 1.05 - 0.13 - 11.02

25 1.30 1.15 - 0.15 - 11.54

Experiment 2:

Salt Solution Original Mass of Final Mass of Change in Mass % Change

% potato (g) potato (g) (g) + or - In mass 0 1.22 1.25 +0.03 +2.46

5 1.11 1.07 - 0.04 - 3.60

10 1.08 0.98 - 0.10 - 9.26

15 1.20 1.03 - 0.17 - 14.17

20 1.21 1.10 - 0.11 - 9.10

25 1.78 1.59 - 0.19 - 10.67

Average Results:

Salt Solution Change in Mass % Change

% (g) + or - In mass

0 +0.02 +1.70

5 - 0.05 - 3.88

10 - 0.11 - 9.59

15 - 0.15 - 11.89

20 - 0.12 - 10.06

25 - 0.17 - 11.11

Graph: This is on the next page.

Analysis:

From looking at the results, it can be seen that as the percentage of salt solution is increased, the mass of the potato decreases. For example in pure water the mass of the potato changed by +0.02 grams but in 5% salt solution the mass decreased to by –0.05, showing a loss of mass in the potato. The % change was notably greater between 0% salt solution, 5% and 10%, but after 10% salt solution the percentage change becomes less dramatic and shows a steady rate of balancing out. For example between 0% and 10% the percentage change went from +0.02 to –0.11, whereas between 10% salt solution and 25% the percentage change went from –0.11 to –0.17, a smaller difference. It can also be sent that between 10% salt solution and 25% salt solution percentage change rose then began decreasing again, making this an odd result. The graph seems to be evening out in the end.

Conclusion:

When the water was in the 0% solution the mass of the potato from both experiments before incubating the potato in salt solution was 1.06g or 1.22g. After the incubation period both of the potato samples went up to weigh 1.07g and 1.25g therefore shows that there has been an increase in mass.

When the potato samples were in 5% salt solution the mass of the potato decreased. The potato samples from each experiment weighed 1.20g and 1.11g. After incubating I knew that they had lost mass because there masses were now 1.15g and 1.07g. The increase in to 10% salt solution showed that the increase in salt solution was making the mass of the potato go down. I know that there was a change in mass for the next solution tested (15%) because the masses of the potatoes went from weighing 1.11g and 1.08g to 1.00g and 0.98. This means that it was a negative change and a decrease in mass.

There were changes in mass for every potato in each salt concentration. In distilled water the change in mass was positive, an increase in the potatoes mass. However once the salt concentration was being increased the change in mass of the potato was negative, a decrease in the weight of the potato.

The positive or negative change depended on where the greater water concentration was. This now uses the theory of Osmosis.

Osmosis is the movement of water molecules from an area of high water concentration to an area of low water concentration through a partially permeable membrane. It occurs whenever there are two different concentrations of solution on either side of a partially permeable membrane. The potato cell is a plant cell with a cell membrane acting as a partially permeable membrane. A potato cell has a large water concentration, yet since it is a plant cell, it has a cell solution (cytoplasm). The salt solution increasingly got stronger than the cell solution, and salt molecules are larger than water molecules. Since the potato is in the salt solution, and the potato has a partially permeable membrane, salt could not move into the cell because the molecules are too big. Therefore because greater water concentration was inside the potato cell, the water molecules moved out of the cell to dilute the salt solution, and balance the water concentration on either side of the membrane. Therefore the cell shrunk, meaning it’s mass decreased.

When the potato cell was in distilled water the greater water concentration was on the outside of the potato. Instead of the potato loosing mass it gained mass because through osmosis the water moved into the cell to dilute the weak cell solution.

There was a greater decrease in mass at one trial of salt concentration. This was in the experiment with 15% salt solution. The mass of the potato decreased dramatically and the higher concentrations after it did not loose as much weight. This was an odd result and is circled on the graph. This result could be because there was a different concentration in the potato cell than the others meaning that it could dilute the salt solution more. Or another reason for why the cell had a great decrease in mass could be because the salt concentration was not what we thought meaning it needed to be diluted more.

If we had continued with the experiment the graph would have evened out even more until eventually osmosis could not occur due to the two concentrations either side of the partially permeable membrane being balanced.

Evaluation:

In order to create a procedure that was fair, when we were measuring out the amounts of each solution, it was always the same person because different people have different reactions and this would make it fair. When I was pouring out eh different solutions I was always eye level with the measuring cylinder to make the amount even more accurate.

Another thing that we did to make sure it was a as possible was that when we were weighing the potato cells we used the same scales each time because another set of scales would not be exactly the same.

There was a greater decrease in mass at one trial of salt concentration. This was in the experiment with 15% salt solution. The mass of the potato decreased dramatically and the higher concentrations after it did not loose as much weight. This was an odd result and is circled on the graph. This result could be because there was a different concentration in the potato cell than the others meaning that it could dilute the salt solution more. Another reason could be because the concentration of the solutions could have been mixed up, causing a greater need for the solution to be diluted meaning the more water lost from the potato.

An improvement for my experiment could be that when the measuring cylinder was washed out we should have made sure that the water was shaken thoroughly out of it to prevent further dilution of a salt concentration.

When the potatoes were being dropped into the beaker for the incubation period we had to drop them all in at once. This meant that more that one person was needed to drop them in meaning that each potato cell would not have been placed in at exactly the same time. This makes the experiment unfair. To improve on this if I were to do the experiment again, I would experiment with each concentration of salt solution and its potato cell one at a time. This would make it more accurate as it means that each incubation period could be timed accurately instead of trying to get all six samples in at once, although this is very time consuming.