To find the osmotic pressure of potato cells.

Osmosis

Aim

To find the osmotic pressure of potato cells. I can find this out, when there is no net movement (change of mass in potato cylinder) when placed in different concentrations of sugar solution.

Background Information

Arrangement and movement of particles in solids, liquids and gases.

Solids have strong forces of attraction between molecules. The molecules are held in fixed positions, in a very regular lattice arrangement. They keep a definite shape and volume. Solids can’t be compressed because the molecules are very close together. Solids are generally very dense.

Liquids have some force of attraction between molecules. The molecules are free to move. Liquids don’t keep a definite shape. The molecules are constantly moving in random motion. Liquids can’t be compressed because the molecules are already packed closely together; liquids are quite dense.

Gases do not have a force of attraction between their molecules. The molecules are free to move. Gases don’t keep a definite shape or volume. The molecules are constantly moving at random. Gases can be compressed and have very low densities.

Diffusion

This is the movement of a substance from a region where it is at a high concentration to a region of lower concentration. i.e. down a concentration gradient.

Particles that make up a gas or a liquid always move around. If there is a lot of one type in one place and few of them in another, then particles move to where there are fewer. Eventually the particles will be spread about evenly. They move from where there is a high concentration to where there is a low concentration. This is diffusion.

Osmosis

Osmosis is a special type diffusion, it is the movement of water molecules from a region of high water concentration (a dilute solution) to a region of low water concentration (concentrated solution) through a semi-permeable membrane.

A semi-permeable membrane is a very thin sheet of material. Semi-permeable membranes have very tiny holes in them letting particles smaller than the hole through but not larger particles.

Water molecules are smaller then sugar molecules because water molecules are made of three atoms (two Hydrogen and one Oxygen). While sugar molecules are made of 44 atoms (11 Carbon, 11Oxygen and 22 Hydrogen). Water particles can go through but not large particles like sugar and proteins. When a membrane separates a strong and a weak sugar solution water will pass through until they are the solution concentrations are the same.

Osmosis does not require energy to work. How quick osmosis happens depends on the concentration and temperature of the solutions, and surface area of the reactants or particles.

When the temperature is increased, all particles gain more energy and start to move quicker. When they move faster they collide more frequently with each other and with the semi-permeable membrane. As a result, there is more chance that the small particles would go through the membrane leaving behind larger molecules. This shows that increasing the temperature of the solution increase the osmosis.

If the solution is made more concentrated it means, there are more particles of reactant that collides about each other, so there are going to be more collisions and more chances of the water molecules going through the semi permeable membrane leaving behind the large particles (e.g. sugar molecules). This means when there is a high concentration osmosis will increases.

If one reactant is a solid then breaking it up into smaller pieces will increase its surface area. This means the particles around it in the solution will have more opportunities to “have more useful collisions”. So there are more chances of the water molecules to diffuse through the semi permeable membrane leaving behind the larger particles (sugar molecules).

Examples of diffusion

Diffusion occurs in all types of living things

E.g. Carbon dioxide diffuse from muscle cells into blood in humans.

Carbon dioxide diffuse into palisade into for photosynthesis.

Oxygen diffuses from the alveolus in red blood cells in humans.

Osmosis in cells

The cell membrane is a semi permeable (selectively permeable) membrane. This means that it lets small molecules e.g. water in, so osmosis occurs as long as the cell membrane is not damaged.

The cytoplasm of cells contains dissolved sugar and salt so we could think of it as a weak solution. The direction of osmosis depends on the concentration of the fluid surrounding the cell.

Plant Cells

In plants the cells the Cell Wall will limit the amount of water that enters the vacuole by osmosis. When a cell is swollen with water it is called turgid because the cell membrane is pushes against the cell wall, the pressure is at maximum when the wall stretches and cannot take any more water. If water leaves cell, the volume of the cell would decrease as water flows out of the vacuole by osmosis. Then the cytoplasm starts to pull away from the cell wall, leaving g gap between the wall and plasma membrane. This withdrawal of the cytoplasm from the cell wall is the called plasmolysis. The cell will be come squashy and flaccid, but the basic shape stays the same due to the rigid cell wall. If many cells are plasmolysed the plant will wilt.

Animal Cells

In animal cells there is no cell wall. So the amount of water entering the cell is not limited. The red blood cell will swell until it bursts in fresh water because the fresh water is more concentrated than the blood cell so it moves into the lower concentration the blood cell to balance it out, but seen there is no cell wall the amount of water in entering is not limited so it burst. But if the blood cell was in a concentrated solution the water leaves the cells by osmosis it will shrink and shrivel because the blood cell is more concentrated than the concentrated solution so the water moves out of the cell and shrinks.

Prediction

In the experiment I predict that when I put the potato cylinder in pure water, the potato will swell because water enters the vacuole by osmosis, making the cell wall stretch until it can take no more. I also predict that when the potato cylinder is placed in water it would expand because the water leaves the vacuole by osmosis. The direction of the water movement will be going in to the potatoes cell.

The concentration that would be the osmotic pressure of the potato is the concentration 3M, because ...

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Prediction

The concentration that would be the osmotic pressure of the potato is the concentration 3M, because the concentration would be equal and there would be no net movement, because both of the sides are equal. The water potential of a solution is the capacity of a solution to lose water so pure water will have a high water potential. The osmotic pressure of a solution depend on its solute in concentration, the greater the osmotic pressure. The osmotic pressure is the tendency of a solution to gain water across a semi permeable membrane. When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The contents of the potato cells shrink and pull away from the cell wall. These cells are said to be plasmolysed.

Low concentration

If the potato is in a water solution which is less concentrated, water moves in the potato pieces and should expand because of the water will continue to move into there a of higher sugar concentration until the two concentration are equal. Therefore the solution concentration increases and the potatoes concentrations decreases. At the same time the potatoes volume increases and the sugar solution decreases due to the movement of water

High concentration

High concentration of water molecules on the outside and a lower concentration of water molecules on the inside so water would move in by osmosis. If a potato is placed in high concentration sugar solution I think it will shrink. This is because the sugar solution has a high concentration and the potato has a low concentration so to balance it out the water molecule in the potato move out into the sugar solution, making the potato shrink. So in my experience if the potato shrinks that solution does not tell us the potato’s osmotic potential.

If the potato is in a high sugar solution, which is more concentrated than the potato, the water moves out of the potato, which should then shrink.

Same concentration

If the potato is placed in the same concentration of solution I think there would be no net movement of water. This is because if the concentration of the potato and the sugar solution were equal there would be no need of water movement.

If the potato is in a solution of the same concentration there should be no net movement of water and the weight of the potato should not change.

Safety

In order to make my experiment safe I had to follow safety rulers.

I had to wear safety goggles to protect my eyes from accidentally spillages and from the knife.
When using a knife I used hand glove and cut very carefully.
When glassware is accidentally broken, I would immediately inform the teacher.

Fair testing

Fair testing should play a big part in this experiment. If this experiment isn’t a fair test, we will be obtaining the wrong results, which could lead us to the wrong conclusions.

First of all, and most importantly, we will have to get the measurements and the weights of the solutions and the potatoes as exact, and as accurate as possible. We will try and get the measurements of the potatoes as accurate as possible for every single potato, evenly cutting the potato pieces, and making a record of the length to the nearest millimetre.

But I believe one of the most important steps in the fair testing is to make sure that the potato is fully covered by the solution. This is because the potato should fully submerge, by having total contact with the solution.

When using the balance, we will make sure that the balance is reading zero, before we put our potatoes on it, to make sure all of the weight of all the potatoes are all the same. This is so that we don’t get a false reading, with the weight of our potato with the reading it had before. And after the experiment, we will measure the potatoes that should be dried as possible, and weigh it the same way, taking the reading to the nearest 1 decimal places.

Carrying out the experiment in a constant temperature for the 24 hours of experiment is important. And to avoid temperature change, which might affect our results, we will take the temperature of the solution into account.

Another important factor of a fair test is to have same length of time as quickly as possible. This meaning that we should start as soon as the potato is put inside the test tube, and stop as soon as 24 hours have passed. If we where to leave the potatoes in for an extra 24 hours, then our results which be immediately affected by our delay. So we leave the potatoes in the test tube for 24 hours, the afterward we should take them out immediately to obtain a fair results.

Getting and experimenting with the exact measurements of different molars and water is vital to this task. If the volume of one solution in a test tube is higher or lower than another, will affect the pattern of results later on. We will also make sure that the potato is fully covered by the 5 different kinds of moles. This is because, is the potato isn’t covered up by the solutions, the effect of osmosis might not occur to the fullest.

Range and Accuracy

Although my measurements were accurate, if I were to repeat the experiment in a laboratory I would use more precise instruments to measure surface area to as close a degree as possible to make sure that they are the same. I would also weight the potatoes to a closer degree maybe within one tenth of a gram or what the equipment allows me. I would also try to find a way of measuring the starch concentrations in the potato pieces to ensure that they are the same.

A range of sucrose sugar solutions will be prepared with concentrations 0 molar, 0.1 molar, 0.2molar, 0.3molar 0.4molar and 0.5molar.
Sections of potato will be cut using a scalpel and will be measured using a ruler. I will be using a scalpel in every potato, to produce the same diameter

of the potatoes and a fair test. This part of the preparation must be done very accurately as a change in the surface area may allow more or less osmosis to occur.
The mass of each chip will be measured as well so that more results can be obtained. Three chips will be placed in each test tube each time so that I can take an average for each tube.
I will use 10 ml of each concentration of sugar solution and once in the test tubes they each will be labelled.
The potato pieces will then be placed in the different test tubes and then left for 24 hours.
Then the potato pieces will be removed, the surface solution removed by gently blotting paper towels over surface at the same pressure to keep it a fair test and then they will be re-weighed.

Constant variables

The starting weight of the potato
Volume of solution (30ml)
Same length of time
Same conditions- temperature

Input variables:

Same equipment
Size and shape i.e. surface area of potato cylinder.

Concentration of the sugar solution

Output variables

Change in the weight of potato pieces

The importance of keeping the variable the same is vital, because it might effect our results. Also I’m going to explain whys its important to keep this variables and explain how this would effect my set of results.

Temperature = the temperature must be kept constant, because If I increases the temperature, it enabling the water particles to move faster, which makes the osmosis process faster, which will effect the overall result.

The volume of the solution should be kept the, because the higher the volume, the more water, which makes the solution dilute on the other hand the less volume, the less volume which makes the potato’s to have more solution

If the length if times are not kept the same, then the water molecules will have more or less, time then the other experiment, therefore increases the volume of the potatoes.

If the weights are not the same the same then this would mean that the number of particles in the potatoes are not equal and therefore the concentrations inside the potatoes might not be the same. This would lead to different amount of osmosis taking place, as the starting concentrations need to be the same inside the potatoes to allow the solution concentrations to be varied

If the potato pieces are not the same exact shape this will mean that the surface area of the pieces is not equal and this would lead to a different of osmosis taking place with more occurring where the surface area is larger.

The starch molecules in potato are bigger and can’t go through the semi permeable membrane and to the pure water solution. But the water molecules are much smaller and can go through and swell the potato.

To make my experiment fair I will have to keep the volumes of the solutions the same in all tests at 30ml. I will use a graduated measuring cylinder to measure the volume of each solution and make my measurements accurate. I will have to keep the weight, size and length of the potato pieces the same. I will use a digital top pan balance, a ruler and core borer to help me achieve equal weight size and length of these pieces. I will use a core borer to keep the potato size the same so the surface area is the same and does not affect the experiment. I will try to keep a constant temperature (average 20°c) so it does not affect each the results of each test. I will use a thermometer to measure the room temperature before each test. This is because a higher temperature there will be more energy so the particle will move faster and diffuse will happen faster. I will use six different concentrations so I could see a clear pattern in my results and I will do the experiment three times and calculate the average % change in mass to make my experiment more reliable. Each test was timed for 24 hours so osmosis could take place. A bung was placed on each boiling tube to stop any evaporation. To make my weighting measurements more accurate I will use a digital top pan balance to weigh each potato cylinder and recording their weights to one decimal place. I will measure the effect by measuring the change in mass of the potato and the calculating the % change in mass of each potato cylinder.

The only input variable that I will change is the concentration of the sugar solution.

My Secondary Sources

The table above is my secondary sources result table, which I have copied from the Internet .I have copied it to prove my prediction weather its correct or faults. as you can see above I have my results are pretty similar to my original results that I have do as you read the essay. The website which I got this setoff results are form www.studentcental.com

Equipment List

Potato
Cork borer
Distilled water
Weighing scale
Boiling tube rack
Boiling tubes
Bungs
Filter paper
Sugar in water (0.1M, 0.2M, 0.3M, 0.4M, 0.5M)
Graduated measuring cylinder
Thermometer
Filter paper

Method

I will use a cork bore to cut the potato in 6 pieces this, helps make the diameter the same.
Then I will measure the length of the potatoes pieces and cut each of them to same size (30mm), I will cut the potatoes by using a knife.
I will then dry the potato pieces on the filter paper
I will have to weigh the 6 potato pieces separately to weigh 2.1g. If it is was over weight I will cut a bit off, if it was under weight I will cut a new piece using the core borer.
Label 6 boiling tubes with your initials and pure water, 0.1M, 0.2M, 0.3M 0.4M, and 0.5M
I will fill the tube marked pure water with 30ml of distilled water.
After I will measure out 30 ml of each sugar solution using a graduated a measuring cylinder.
Pure each solution in 6 different boiling tubes which are labelled.
Put a potato piece in each boiling tube
Then put a bung on the boiling tubes. This is to stop any evaporation, which may affect my results.
Put the 6 boiling tubes in a boiling tube rack.
After 24 hours remove the potato pieces from the tube and dry them on filter paper, so excess solution that has not enter by osmosis will not affect my results.
Weigh each potato piece and record it weight.
I will then repeat the experiment three times to have a more reliable result.

Equipment

The only equipment that I used to obtain a precise and accurate set of results was a ruler and weight measurer.

Results

Graph

Conclusion

I have noticed a pattern in my results from my scatter graph. I have come to the conclusion, as the sugar concentration increases in the solution, the mass of the potato decreased; this is what I predicted. This shows that the more concentrated the solution the more water leaves the potato that resulted in potato shrinking in size. Therefore my predictions were correct and matched my theory because I predicted that if the potato were put in pure water it would swell and expand. If it were put in a sugar solution it would shrink. The weight of the potato increase from its original weight in distilled water and in concentrations of 0.1M and 0.2M the weight decrease at the concentration of 0.4 M and 0.5M.

This is what the graph shows the mass of soaked potato decreases to a limiting value, which is 0.3 M. This value is called the Osmotic Potential of the potato.

Early on in my prediction I stated that:

Low concentration

High concentration

If the potato is in a high sugar solution, which is more concentrated than the potato, the water moves out of the potato, which should then shrink.

Same concentration

If the potato is in a solution of the same concentration there should be no net movement of water and the weight of the potato should not change.

I think that from my results I can safely say that my prediction was right

Evaluation

First of all I believe my results are reliable because in each test I took three measurements and found an average to make it more consistent.

Secondly, I believe that my measurements are more precise because I used a digital top pan balance to keep the weight of the potato piece the same; graduated measuring cylinder was used to measure the volume of the solutions and core borer to keep the same shape of the potato.

Thirdly my results are generally accurate but I could have made them more accurate if I took more care in my measurements and had control over my input variables (For example, the temperature in the room varied between 19°C and 21°C. If I had control of the temperature I could make it more reliable. (Cause the temperature is one factor, which affects osmosis). I could have could have set my experiment longer than 24 hours (e.g. 48) and compare results. If I changed the potato surface area it would have affected my results, if the surface area were larger the more potato cell membrane will be available for osmosis and the faster osmosis will take place.

I think I could have improved my experiment by using a large range of solutions and could have used other cells like apple, pear onion, and orange.

I think that I have enough evidence to be sure of my conclusion because the results of my experiment validated theory and my conclusion as shown in my analyses and in the graph above. The experiment was very successful in my opinion. I obtained a large quantity of very accurate results from which I was able to create informative graphs.

I think I took easily enough results for the amount of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 0.5,0.4,0.3,0.2,0.1and pure water.

The problem with my experiment was, cutting of the potatoes which in my opinion was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine to cut the potato as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the solutions and to determine the molar concentrations

There were not any out of the ordinary results, but some were not as close to the line as others. When the potato chips were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. However with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.

If we were to this experiment again we would have larger pieces of potatoes to get a more precise measurement by using s ruler or we would need a microscopic measurements (a length smaller than a mm) to get an accurate size so that all the length are the same. we would do proper repeats rather than putting three in one boiling tube and counting it as a three repeat. I would also get a weight measurer that is more effective (as in measures very light weigh).

After comparing my graphs to a couple of graph’s graphs, I found out that we had the corrected and very reliable graph drawn, with a lines of best fit. This ensures me that my sets of results were very much corrected.

On my graph I have drawn a straight line form my correct set of results. I have also seen others ‘s group’s graphs and I have figured that I have drawn the correct line on the graph.

I believe we gained accurate and sufficient enough results, shown on our graph and tables that concludes the experiment, and to prove our hypothesis. My final results were very reliable, due to the precautions I took to make this a fair test.