Osmosis Coursework

Abdullah Jafar Chowdhury Science Osmosis Coursework

Osmosis

Planning:

Aim:

To find out the effect that osmosis has on the lengths and mass of the potato chip in different sugar solutions; distilled water, weak sugar solution and concentred sugar solution.

Background Knowledge:

Osmosis is a special type of diffusion; diffusion is the movement of particles from a high to low concentration until equilibrium is reached. It is the diffusion of water across a partially permeable membrane from regions of high water concentration (or low sugar concentration) to regions of low water concentration (or high sugar concentration).

Osmosis will continue to occur until both sides have an equal concentration. Several things affect the speed of osmosis. These include heat, solution concentration and surface area (as the membrane will be bigger, allowing for more water transfer).

“The cell surface membrane separates the cell contents from the surrounding environment. As you can see below, the membrane allows some molecules to pass through but not others.”*

Water Potential

“A ‘weak’ solution will have a high concentration of water molecules. These ‘free’ water molecules will have a tendency to move about.”* This is called high water potential.

In a potato chip the cells have a cell wall which is freely permeable allowing all molecules to pass in and out. Under the cell wall is the cell surface membrane which is known as the semi-permeable membrane. And inside the cell is another membrane is the Tonoplast. This separates the vacuole from the cytoplasm.*

Solute Potential

The concentration of dissolved substances inside the cell is called the solute potential.*

The solute potential is always in the negative, pure water is 0. It is negative because the forces of attraction between the solute molecules and the water molecules reduce the movement of the water molecules.*

This means the higher the solute molecules present, the lower the water potential.

The fewer the solute molecules present, the higher the water potential.*

Pressure Potential

Water enters a plant cell if the solute potential inside the cell is lower than the water potential outside the cell. This means that as water passes through into the cell, the cell start to swell. But when it reaches a limit the cellulose in the cell starts to physically push back. “The pressure that the cell wall develops is caused he pressure potential. The pressure potential is usual, though not always, positive.”*

At any time, the water potential of a plant cell is the sum of the solute potential and the pressure potential.*

Turgidity

If you put a plant cell into a weak solution (sometimes called a hypotonic solution)* the water potential inside the cell will be smaller than the water potential in the solution. The water will enter the cell by osmosis.

As the water passes through it causes the cell to swell producing a pressure potential. As more and more water enters the cell until it is the same as the water potential in the solution. The water potential is now 0 and no more water can enter the cell. A cell in this state is said to be Turgid.*

Plasmolysis

If you put a plant cell into a strong solution (sometimes called a hypertonic solution)* then the solution has a lower water potential than the plant cell. This will cause water to pass out of the cell by osmosis. As the water passes out of the cell, the cell surface membrane starts to shrink away from the firm cell wall. The pressure potential is now 0 and the state of the cell is known as flaccid.

As more and more water leaves the cell, the cell’s contents continue to become smaller and smaller. This time the cell membrane starts to peel away from the cell wall. When a cell is in this condition it is known as being Plasmolysed.

Turgid cell are firm because they are filled with water. They give support to the plant stem keeping it upright. If these cells lose water, they become flaccid. As a result the plant stem wilts, but the cells’ turgidity can be returned by watering the plant.

If a plant cell was placed into an isotonic solution ...

This is a preview of the whole essay

If a plant cell was placed into an isotonic solution which is a solution with the same water potential as that of the cell. There is no movement between the cell and the solution. This means the cells will be in equilibrium.

Osmosis in Animal Cells

If you put some animal cell such as red blood cells into distilled, water enters the cell by osmosis, and then the cell bursts. The reason behind this is that animal cells have no cell wall.

“The cells have negative solute potential and therefore have a low water potential. The water outside has a high potential of 0. So there is an inflow of water into the cells by osmosis. Without a cell wall to stop the expansion of the cell, water keeps entering the cell until it bursts. When this happens to red blood cells it is called Haemolysis.”*

Diagram:

Hypothesis:

I hypothesise that the mass of the potato chip will increase if the concentration of the sugar is decreased. I also predict that if the concentration of the sugar is increased then the potato chip will decrease in mass. I also predict that if the concentration of sugar is increased then the length of the potato chip will decrease.

Distilled water: I predict that in the solution with no sugar, the potato mass will increase because the higher concentration of water will be outside the potato and the water will be dilute. So the water will pass into the potato increasing the mass. I also predict that the length of the potato in the distilled water will stay the same.

Weak sugar solution: I predict that the potato in the 20% sugar solution will lose some mass because there is going to be a low concentration of water; water will come out of the potato. I also predict that the potato will become shorter because it will lose water and will shrivel up.

Strong sugar solution: I predict that this potato chip will lose mass and height because there is going to be a very low concentration of water, so water will come out of the potato chip making it lose mass. The potato chip will be plasmolysed because of the loss of water.

Apparatus:

The apparatus for the execution of the experiment are the following:

1 Potato: to get the chips from it. It is important that the same potato is use to cut out each chip to make the experiment a fair test.

1 Scalpel: to cut the potato into chips of the same size – 2.5cm long
2 Cork borers: 1 to cut the potato chips with the same diameter. The other to push the chip out from inside the first borer
3 test tubes: to put the solutions and the chips in
1 Hairdryer: to dry the chips after taking them out of the solution
1 Ruler: to measure the potato chips before and after the experiment. A ruler with mm scale should be used to take accurate readings.
1 weighing scales: to weigh the chips before and after the experiment. An electrical one should be used, to increase the accuracy of the readings.
Sugar (50 grams): to produce the Weak and Strong sugar solutions.
Water (100ml): to dissolve the sugar so as to create the weak and strong sugar solutions. Also to produce the distilled water solution.

Independent variables:

The amount of sugar concentration,

Dependant variable:

The length of the chip
The mass of the potato chip

Control variable:

The size of the chip. If the potato chips were to be of different sizes, they amount of particles that pass through the semi-permeable membrane would be affected. The mass and length of the potato chip will also change.
The size and diameter of the test tube. This will affect the amount of solution held in the beaker. This would in turn affect how much osmosis take place.

I will record the results from my dependant variables on a table, comparing the mass and size of the potato chips before and after the experiment. I will do this once. I understand that repeating the experiment provides more reliable results but due to time constraints, I am forced to carry out the experiment once.

Fair Test:

To make sure that the results of the experiment are reliable, I need to make sure that certain variables are kept constant in order for a fair test. These are:

The length of all the potato chips must be the same

The potato chips should be surrounded by the same amount of solution

The scales should read 0.00 before each weigh in

Each potato chip should remain in the solution for equal amounts of time. In this case it is 24 hours.

These factors must be kept the same throughout the experiment so these are called constant variables; however I will change the concentration for each different solution so this is called an independent variable.

Safety feature:

In this experiment I will have to be careful while using the scalpel and the cork borer as both are sharp and could potentially cut or scratch skin. To reduce the danger, I wore safety gloves so as to prevent cuts or scratches from either the scalpel or the cork borers.

Measurements and Justification:

I will be measuring how osmosis affects the lengths and masses of potato chips in sugar solutions of varying concentration. This is to see how the concentration of a solution affects the movement of particles from a region of high water concentration to a region of low water concentration through a semi-permeable membrane. I will be measuring the lengths and masses of each potato chip prior to and after the experiment so that I can see the differences in the two readings. I will do this once, 24 hours after the start of the experiment to ensure that the osmosis process has fully finished. I have chosen 24 hours because this amount of time will be sufficient for a reasonable amount of osmosis to take place, giving me a clear result as to how osmosis affects the potato chips.

Procedure:

Preparation of potato chips:

Using a size 5 cork borer bore a chip through the potato; push it out with a size 4 cork borer. Do this 3 times. Then use the scalpel and cut the potato chips so that they are all 3cm long. Prior to the experiment note down the length of the potato chip as well as the mass.

Preparation of solutions:

There are three solutions. The first is the Distilled Solution. This is prepared by pouring 50ml of normal tap water.

The second solution is the Weak Sugar Solution. This is prepared by using 10g sugar being dissolved in 40ml of water.

The third and final solution is the Strong Sugar Solution. This is prepared by dissolving 40g of sugar in 10ml of water.

Setting up experiment:

Each potato chip is placed into a different beaker. It is then marked which is in the Distilled Solution, Weak Sugar Solution, and strong Sugar Solution. They should then be left in a cool and dry place for 24 hours.

After 24 hours each chip should be taken out of its beaker with a pair of tweezers. The excess solution should be dried off by using a hairdryer. The length of the chips should be measured and noted down. After measuring the length of the chips they should be weighed on the electronic scale.

Any difference in texture should also be noted down.

Diagram of Experiment

Distilled Water Weak Sugar Solution Strong Sugar Solution

Key:

Water Molecule

Potato Chip

Sugar Molecule

Observation of results

What I have observed in this experiment has been noted down below.

I have drawn a table to show my results:

In this experiment I kept the volume of the solutions and the lengths of the potato chips the same. The one error that occurred was that although the potato chips were from the same potato and were the same length they were not of the same mass.

Analysis of results

The first graph, showing the Effect of osmosis on the lengths of potato chips shows the change in lengths from before and after the experiment. The blue column shows the length of the potato chips in different solution before the experiment. The red columns show the decrease in length of the potato chip. As I had predicted, my results show that the more sugar the more the length will decrease.

The second graph, showing the Effect of osmosis on the masses of potato chips shows the change in mass from before and after the experiment. The blue column shows the mass of the potato chips in different solution prior to the experiment. The red columns show the decrease or increase in mass of the potato chips after the experiment. I predicted that the higher the concentration of sugar, the more mass the potato chip would lose. My results show that the more sugar the more mass the potato chip will lose

Looking at my “Observation of results” table as well as the above graphs you can see that the lengths decrease as the sugar concentration increases.

This means there is a movement of water from the potato chip to the solution. Osmosis takes place until the concentrations have reached equilibrium.

Conclusion

After completing the experiment, I have found that leaving a potato chip in a distilled water solution causes its length and mass to increase.

Leaving a potato chip in a strong sugar solution causes its length and mass to decrease.

I have also found that leaving a potato chip in a weak sugar solution causes its length and mass to decrease very slightly.

Evaluation

Overall my experiment was conducted well. It was easy to carry out and I encountered no problems that impaired the ability to carry out the experiment. The method worked properly. I received reliable results from my experiment. I know they are reliable because I can see they reflect the information in my Background Knowledge. I do not know if my experiment and results are consistent because I only carried out the experiment once. However looking at previous experiments into this topic I can say that my results are similar to previous experiments.

In my experiment I was measuring how osmosis affects the lengths and masses of potato chips in different sugar solutions. The results show the difference of lengths and mass in the potato chip. This shows that my results are valid.

I am confident with my findings because they match my conclusions. The results show the more concentrated a sugar solution is, the more length and mass the chip loses.

The weakness of my experiment was the mass of the different potato chips before the experiment took place. They were not all the same. The strengths of my experiment are the uniform length of the potato chip prior to the experiment as well as the fact that it was carried out on controlled variables; i.e. the size and diameter of the test tube and the size of the chip.

To make my experiment more accurate, I could have kept the chips at a constant room temperature. I should have put all the chips in at the same time, because others could have been in the beaker for a different amount time. I would have repeated the experiment a few more times to obtain reliable results.

Bibliography

All the excerpts that are in bold and that have an asterix are from the following book: