Osmosis Coursework

Touba Jamil

Osmosis

Aim

The aim of my investigation is to find out how osmosis affects plant cells. I will be doing this investigation on potato chips in different concentrations of salt. From this investigation, I would like to know whether the concentration of the solution affects the rate of osmosis. I will use one molar salt solution in my experiment.

Definition

What is osmosis? Osmosis is a special a special type of diffusion of water particles. They move from a region of a higher concentration towards a region of lower concentration through a semi-permeable membrane. This membrane is porous and therefore allows small molecules such as water and glucose to pass through it but it disallows larger molecules such as sucrose and starch to pass through it. The small molecules move to the higher concentrated region until both sides of the membrane become equal.

The effect of osmosis on plant cells

When plant cells take up water, they begin to swell. The strong cell wall around the plant cell prevents it from bursting. When this happens, we refer to the cell as ‘turgid’.

A turgid cell is swollen and hard. Turgidity is very important to plant cells because this is what makes the green parts of the plant stand up into the sunlight.

When plant cells are placed in a concentrated solution, they lose water by another special type of diffusion called ‘plasmolysis’. Plasmolysis is the opposite of osmosis. Plasmolysed cells are known as ‘flaccid’. Flaccid and turgid cells are exact opposites.

Turgidity and Flaccidity

A cell, which has no water in it, is known as flaccid. Before it is placed in, a highly concentrated solution it looks shrunken likes this:

When a cell is in the same concentration as the solution outside it, it is in a state called equilibrium. The state is between turgidity and flaccidity and makes the cell look like this:

As water passes through it, it goes more and more swollen. Slowly it would look like this:

This cell has now become turgid. It has enough water in it to burst, but the strong cell wall surrounding it prevents it from doing so.

Plasmolysis

Plasmolysis is reverse osmosis. It happens when the cell membrane shrinks and pulls away from its cell wall. It occurs when the water inside the cell moves into the solution outside it. This transference takes place across the membrane moving from an area of high water concentration to a lower water concentration outside the cell.

Variables

Variables are things in the experiment that can change. Variables can affect the rate of osmosis.

The variables may include:

Temperature – The higher the temperature of the solution the faster the rate of osmosis. Heat causes the molecules to have more energy and move around faster than the molecules in a colder solution.

Surface Area – The surface area is the area of surface around the potato chip. The larger the surface area the faster the rate of osmosis. This is because there are more molecules that are exposed to diffuse in comparison to the molecules in a smaller surface area.

Concentration – The rate of osmosis will faster as the solution is more concentrated. This is because there are only a few particles to collide with the potato chip and fewer particles will have to diffuse into the potato to equalise the concentration of the solution and the potato inside it.

Time – Time is very important as the longer the chip is left in the solution the more osmosis that will take place. In other words the reaction will be more complete.

Prediction

I predict that when the potato chips are placed into their different beakers, having different concentrations of salt solution, the osmosis will take place at a different rate.

When the potato chip is inside a water concentration, which is lower than in the chip, water will enter through the semi-permeable membrane into the potato chip. This will cause the cells of the chips to turn turgid, making the potato chip gain weight and increase in length at a certain extent.

When the difference in the concentration is little, then there would not be much change in the ...

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Prediction

I predict that when the potato chips are placed into their different beakers, having different concentrations of salt solution, the osmosis will take place at a different rate.

When the difference in the concentration is little, then there would not be much change in the weight. The reason being that the concentration inside the potato cells is almost equal to the concentration outside. This will leave the potato cells in a state of equilibrium since there will be less particles of water diffusing into the potato.

When there is lower concentration inside the solution, the water from inside the cell will pass out from the cells into the solution. The cells inside the potato chip are now fully flaccid.

BEAKER A:

Beaker A contains only 100% distilled water. This means that the concentration of the water is much lower in the potato chip than outside it. Therefore, the water from the solution around the potato chip will enter its cells. These cells will swell up and become turgid. This means osmosis will occur in these cells, and as a result the potato will increase in length and mass (weight).

BEAKER B:

In Beaker B there is 75% distilled and 25% salt solution. There is still more water than salt so this beaker’s conditions are similar to Beaker A’s. I personally do not think that much change will take place in the length of the chip but it will increase in weight and length though less than beaker A.

BEAKER C:

Beaker C contains 50% distilled water and 50% salt solution. This means that the potato cells are in a state of equilibrium. I predict that the mass will decrease a little and therefore will become slightly flaccid. I think that the length will decrease to some extent. Although it seems as though the concentration of the water is the same, the amount in the solution is actually more than the concentration of water inside the chip. Therefore, there should be a little decrease in the mass and length of the potato chip because of water loss.

BEAKER D:

In Beaker D there is 25% distilled water and 75% solution meaning there is less water and a lot more salt solution. Therefore, I reckon that there will be a large decrease in the length and the mass. After some time it will become flaccid, causing the chip to decrease in both mass and length. The chip may also lose colour due to the water loss and because salt is not good for living cells.

BEAKER E:

The last beaker contains 100% salt solution, and no distilled water. The difference between the concentration gradients of the two substances is a lot. The concentration of the water in the potato chip is much higher. Therefore, most of the water will leave the chip and enter the solution surrounding it. This time the chip will lose almost all its colour because the salt affects the cell. My prediction is that the mass and length will decrease largely. The chip will be left very flaccid and small.

To sum up my prediction for this investigation, I think that the higher the concentration of the solution around it, the more turgid the potato cells will become.

Apparatus

5 X 200ml Beakers

Size 5 Borer

Electric Top-Pan Balance

Ruler

Scalpel

Measuring Cylinder

Tweezers

Distilled Water

1 Molar Salt Solution

Clock

Paper and Pens

Paper Towels

A borer is used to cut out a certain size of circular potato chips. My borer was size 5, and I used this size because I thought that it was the most suitable size, being neither too thick nor too thin.
A top-pan balance is used to weigh the potato chips.
A ruler is used to measure the length and width of the chips.
A scalpel is used to cut the potato chips so that they are all equal lengths.
A measuring cylinder is used to measure the solution and distilled water.
Tweezers are used to pick out the potato chips from the beakers.
A clock is used to make sure the timing is fair.
Paper and pens are used to record the results of the investigation.
Paper towels are used to dry the potato chips.

The rest of the apparatus are needed in the experiment.

Preliminary Work

Preliminary work is the work done before the actual investigation, to make it a fair test.

In my preliminary work, I did a practice investigation. The thing that I realised I needed to practice was putting the potato chips in the beakers at the exact same time. In order to achieve this I asked a couple of my friends to help me and after a few practices, we managed to put the potato chips in at the same time.

I also worked out that a suitable amount of time for osmosis to take place is 48 hours.

I will try my best after this work to obtain a fair test.

Planned Method

The first thing I would start with is making my results table. It will look something like this:

Next, I will make a one molar salt solution. After that I will add the different amounts of distilled water and salt solution in beakers. Then I will cut out five circular potato chips, and then cut them into the same lengths. After this I will measure and weigh the chips and record the results in my table.

When all of the results are recorded, I will ask two of my friends to help me put the potato chips into the beakers at the same time. Then I will leave the potato chips in the beakers for 48 hours.

After 48 hours, I will take the potato chips out of the beakers and place each of them on the paper towels. Then I will weigh and measure, and record the results in my table. Everything will be done at the same time because it has to be a fair test.

After this first investigation, I may repeat it to get results that are more accurate and will be able to come out with an accurate conclusion and theory.

Method

First, I made my results tale as shown in my plan. Then I took two healthy and hard potatoes. Using a size 5 borer, I cut out five potato chips. I placed all of them on a tile and cut them all to 5cm long. I used a scalpel to cut them and also made sure with a ruler that they were all the same lengths.

Next, I took five beakers and labelled them according to the amounts of water and solution that were going to go in them. Then taking a measuring

cylinder I measured out the different amounts of distilled water and salt solution and poured them into the five beakers.

I weighed all the potato chips on an electrical top-pan balance and recorded the weight in my table. As I had already planned, I used a friend to put the potato chips into the beakers at the same time. I recorded the time in the table. Then I observed the potato chips from time within 48 hours.

48 hours later, I took all the chips out with a pair of tweezers, and placed them onto paper towels.

While the potatoes were drying, I put a paper behind every chip and labelled it with the letter of its beaker. I then washed out the beakers and put them away safely.

When I finished cleaning, I placed each potato on the top-pan balance and recorded all the results in my table.

As I had time after I finished the first investigation, I decided to repeat it. This gave me more results for an accurate conclusion and theory.

Fair Test

It is very important to make sure the results are as fair as possible

To make my investigation fair, I did the following things:

I used a size 5 borer on all the chips to ensure they were all the same weight.
I measured all the chips with the same ruler.
I weighed all the chips on the same scale.
I made sure all the potato chips went in to the beakers at the same time using a couple of friends to help me.
I also made sure that none of the chips were in the solution longer than the others.
All the beakers were kept in the same room, to ensure they were all in the same room temperature seeing as temperature affects the rate of osmosis.
All the chips were cut into the same lengths using a scalpel.
I used the same molar salt solution in all the beakers.

Safety Measures

Many safety measures were taken into consideration whilst doing this investigation. This was to ensure that nothing harmed anybody. For example, I took extra care whilst cutting the potatoes. I also took care that all the liquids were correctly labelled. Everything was cleared away before I started the investigation.

Results

First Investigation Results:

I did not think that only doing one investigation would give accurate and reliable results, so I decided to repeat it.

Second Investigation Results:

Average Results of Both Investigations:

Graph Analysis

Graphs 1 and 2:

These two graphs show the results of my first investigation. The first one is of the mass of the potato chips before and after the experiment. I recorded the results in line graphs. On the Y axis the mass/length is plotted in g/cm. On the X axis the beakers containing solutions of varying concentrations are plotted.

The straight lines plotted on 2 and 5 on the Y axis shows the masses and lengths of the potato chips before osmosis.

The second line of each graph is the plotted points of the mass and length of each potato chip after osmosis. The last line is the trend line. These three lines occur in all the graphs.

From these graphs, a trend can be identified that the more the solution is concentrated the less the potato chips will weigh and measure after osmosis.

This occurs because the concentration of the solution outside the chip is less than the concentration of the water inside the potato. This causes osmosis to take place which means the water molecules move from the potato chip into the solution. This results in the potato chips losing mass and length.

Graphs 3 and 4:

These two graphs show the mass and length of the potato chips in my second investigation. The graphs are set out in the same way as the first two; however, some of the results are different to the ones in the first two graphs.

Graphs 5 and 6:

These graphs show the average masses and lengths of both the investigations for each beaker. The axes are labelled in the same way as the other graphs. These graphs show that the trend of all the graphs is the same.

I worked out the average of the results by adding them together and then dividing them by two.

Conclusion

My main prediction for this experiment was that the higher concentration solution would always move to a region of a lower concentration of solution. By doing this investigation, I proved my prediction right, as the higher the concentration of the salt solution, the smaller the mass of the potato chip. For example, in Beaker E the concentration of the salt solution was highest and the mass of the potato chip that had been placed inside it decreased by 0.55g.

If the concentration of water in the solution is less than the concentration of water in the potato chip, the mass and length of the chip may decrease or remain unchanged.

The potato chips in Beakers A, B and C all increased in mass whereas the chips in Beakers D and E decreased in mass. The length of the potato chips only increased in Beaker A by a minute amount.

Therefore, I conclude that the statement, ‘the higher the concentration, the lower the mass and length of the chip’ is correct.

Evaluation:

I feel that this experiment has gone very well. I have obtained a good set of results from which I have worked out the average mass and length of all the chips and drawn six graphs including a key to explain them. The time that I left the potato chips in the salt solution for was sufficient, as clear results were obtained from the experiment. My aim for this investigation was to study osmosis closely using a peeled potato and different concentrations of salt solution. I feel that I have succeeded in fulfilling my aim as I have experimented and recorded results showing what happens to the potato chip when it is placed in a concentration of salt solution.

In my opinion, the evidence obtained was generally quite accurate, as I could see a clear and distinctive pattern in my results. Overall, my results were accurate but would have been more so if I had weighed the potatoes correct to two decimal places instead of one decimal place.

If I were to carry out his experiment again, the main changes I would make would be to use better equipment, such as a computerised weighing scale, which would give results that are more accurate. Instead of results, 0.0g I would have results 0.000g etc.

Also I would have extended my investigation by experimenting with different temperatures and how they affect the rate of osmosis. I already know that temperature causes the particles to gain more energy so I would like to look further into this.

I found that cutting the potatoes was the most time consuming seeing as you had to do it several times to make sure all the potatoes were cut equally.

However, I am pleased with the set of results that I obtained and feel that this experiment went well.