Find out what effect altering the concentration of sucrose solution would have on the occurrence of osmosis in potato chips.

BIOLOGY

An Experiment to Investigate Osmosis

Aim

To find out what effect altering the concentration of sucrose solution would have on the occurrence of osmosis in potato chips.

Background

Osmosis is the net or overall movement of solvent molecules (often water) from an area of higher concentration to an area of lower concentration through a semi-permeable membrane (often called selectively permeable or partially permeable membrane).

Substances often move from an area of higher concentration to an area of lower concentration by diffusion, the result of random particle movement.

In osmosis, two solutions are separated by a membrane that will only let the solvent particles pass through. This means that the solvent continues to move freely along a concentration gradient. However, the other substance involved (solute) cannot move freely because the particles are too large and cannot pass through the membrane. This means that although there is random movement of the solvent molecules in both directions across the membrane, there will be a net movement of solvent into the area where it is at the lowest concentration, in other words, into the solution containing the highest levels of solute.

Terms related to osmosis are:

Turgid and Flaccid. Turgid cells contain lots of water due to the diffusion through the vacuole. The cell becomes a lot firmer or even swells because it has taken in so much water, the cytoplasm and cell membrane push up against the cell wall. These types of cells have a rigid firm structure so it is difficult to burst from the pressure.

Flaccid cells have lost too much water due to osmosis. The vacuole shrinks so the cytoplasm and cell membrane come away from the cell wall. This causes the plant to loose its turgidity or firmness so it wilts, these cells are plasmolysed. Other terms are: Isotonic is when two concentrations are balanced.

Hypertonic is when one concentration is higher than the other.

Hypotonic is when one concentration is lower than the other.

If two solutions are isotonic, osmosis is taking place but there is no net movement. Net movement is when the same amount of molecules diffuses into each solution but they remain at the same concentration.

ISOTONIC OSMOSIS

Osmosis is the way in which many living organisms, especially plants, absorb water. There will be six variable factors affecting osmosis in my experiment, which are:

What Affects the Rate of Osmosis?

Temperature and Humidity - can affect the rate of osmosis this is because if more evaporation were to take place due to high temperature thus leaving less water for the plant.

Pressure - can affect the rate of osmosis this is because if there is a high amount of force acting on the lower concentration part of the plant then there will be more water molecules then there will be more water molecules entering the higher concentration part of the plant.

Concentration - affects osmosis because, different concentrations of solute molecules leads to different concentrations of molecules on either side of the membrane. On the side of the membrane with higher free water concentration (i.e. a lower concentration of solute), more water molecules will strike the pores in the membrane in a given interval of time. More strikes equates to more molecules passing through the pores, which in turn results in net diffusion of water from the compartment with high concentration of free water to that with low concentration of free water.

Type of Potato - affects ...

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Concentration - affects osmosis because, different concentrations of solute molecules leads to different concentrations of molecules on either side of the membrane. On the side of the membrane with higher free water concentration (i.e. a lower concentration of solute), more water molecules will strike the pores in the membrane in a given interval of time. More strikes equates to more molecules passing through the pores, which in turn results in net diffusion of water from the compartment with high concentration of free water to that with low concentration of free water.

Type of Potato - affects osmosis as the potato rods have to be similar in genetic makeup must be as different types of potato may differ in cell number therefore using the same type of potato the result of my investigation will be more accurate as the potatoes used are similar.

Mass of Potato – the size and length of the potato must be exactly the same to make my experiment a fair test as I am using the same type of potato and cutting from the same cross section this should increase my chances of the mass of the potato being the same.

Time Given - affects the rate of osmosis because the speed of diffusion or time taken to osmose may differ.

An obstructing factor in my experiment; is the un-availability of necessary equipment to accurately control the temperature, pressure and the number of cells occupying a particular mass of a potato chip. Thus, I will study the phenomena of Osmosis by investigating the concentration of sucrose solution, rather than any other related factor/s.

I will investigate sucrose concentration by increasing it over a range of 0-1 mol/L. I will measure the mass of each potato chip before and after placing it in sucrose solution for a period of 20 minutes; I will repeat the experiment twice calculating the average to come to a conclusion.

Prediction

I predict varying the key factor (concentration of sucrose in solution) will affect the turgidity of the potatoes chip, i.e. how firm the chip is due to the amount of osmosis that has taken place.

I think that in a low concentration of sucrose solution (0.0 molar), the potato rod will increase in length. I think this because the solution is pure water so has a higher concentration of water than the cells in the potato rod. Osmosis will take place and the potato will increase in size as the water enters the cells through diffusion. When this happens the cell is called turgid.

I think that in a high concentration of sucrose solution, (1.0 molar); the potato rod will decrease in size. This will happen because the cell in the potato will have a higher water concentration than the solution. Osmosis will take place and the water will be lost from the potato cells and diffuse into the solution, this will cause it to decrease in size. This is called a flaccid or plasmolysed cell.

In a solution of 0.4, 0.5 moles, I think the potato rod will remain about the same size. This will happen because there is the same concentration of water in the solution as in the cell. Osmosis will take place but there will be no net movement of water because the levels of water are isotonic, therefore, the rod will remain the same.

Reason

Osmosis is a process in which water moves through a partially permeable membrane, from a higher concentration of water to a lower concentration, for example if there was one side of a partially permeable solution with 8/10 parts water and the other side was 4/10 parts water, the side with 8/10 would flow from its higher concentration to the lower concentration in an attempt to even out the concentration on both sides (diffusion). The reason the water molecules are able to pass through and even out the concentration is that they are small enough to fit through the membrane whereas the sucrose molecules are too large to fit through.

Above is a diagram to show osmosis across a partially permeable membrane:

Water molecules are smaller than the sugar molecules, they are able to fit/pass through into the cell. But the movement of the water cells becomes restricted nearer the extreme concentrations due to a rigid cell wall. This is not like a cell membrane as it neither expands nor decreases. The reason they put on weight is that they are filled with extra water and the vacuole is removed in one state. When the cell is expanding from its original shape due to excess water this state is called turgid. When it is decreasing due to water traveling out of the cell, this state is called cremating.

Fair test

To keep my experiment a fair I must keep a control of all the equipment used and use the same method, making sure that the variable factors I have explained earlier remain at a constant. To ensure that the genetic make up of the potato chip is as similar as possible I will use the same type of potato from the same bag assuming they have similar membranes. So the potato chips have a similar surface area I shall cut them from the same cross section of each potato. I am going to make sure that the volume of the sucrose solution is the same at 25ml. I will change the concentration of this 25ml solution. I will keep the size of the potato the same. The concentrations of sucrose solution I will be working with are 0.0mol/litre, 0.2mol/litre, 0.4mol/litre, 0.6mol/litre, 0.8mol/litre and 1.0mol/litre. Any excess solution on each individual potato chip will be removed using the same method.

Preliminary Trial for Osmosis Experiment

Apparatus:

Boiling tube
Measuring Cylinder
Still Water
Stop watch
Ruler
Test tube rack
Test tubes
Cork borer
Knife
White tile
Forceps
Potato- 5cm in length

Method: To begin with I will set up all my equipment correctly. I will then use a cork borer to cut out a rod from a potato which will be 5cm in length. I will remove all the skin from the potato as this can affect the rate of osmosis. After, I will measure 25 ml of distilled water with a measuring cylinder. I will then pour the measured water into a test tube which will already contain the potato rod. As soon as I pour the water into the tube, I will simultaneously start the stop watch. After 5 minutes, I will stop the watch and remove the potato rod with the forceps. I will then measure the potato rod with a ruler on the white tile. I will record the new length and put the potato rod back into the test tube and start the stopwatch again. I will do this whole test again for another two sessions of 5 minutes.

Results:

The purpose of the preliminary trial is to discover how long the actually experiment should last.

Osmosis Experiment

Apparatus I will use:

Test tubes
Test tube rack
White tile
Forceps
Stop watch
Knife
Ruler
Sucrose solution
Distilled water
Cork Borer
Measuring cylinder
Potato- 5cm in length

Safety Procedures: To ensure that this is a safe test, I will wear an apron to prevent any of the solution getting onto my clothes. I will also be very cautious when using the knife and cork borer. I made sure all the glassware was away from the edge of the table to avoid knocking them off and smashing them. Also I was careful carrying to and from the tray.

Method: I will start this experiment by getting six test tubes and placing them into a test tube rack. I will write out six labels on masking tape and stick it on the different levels of concentrations of the sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8, 1.0) so as I will not mix the solutions. I will then measure 25ml of distilled water with the measuring cylinder and pour it into the test tube labelled 0.0 moles. I then will pour 25ml of sucrose solution into the other 5 test tubes. After, I will use a cork borer to cut out six 5cm long potato rods out of a potato. I will cut off all the skin as this can affect the rate of osmosis. Then, I will put them into each test tube simultaneously with the help of a partner and start the stopwatch. After 20 minutes, I will stop the watch and remove each potato rod with the forceps. I will measure each potato rod with a ruler and record all the lengths. I will repeat the whole test twice to make my results more reliable.

Results:

I have tried to make the results as reliable as possible by using distilled water to eliminate any sort of variation in the water. Also the measuring cylinder was washed and cleaned between measuring to prevent the next mixture being inaccurate. I used two 25ml measuring cylinders rather than larger ones so there was little room for error. Also the same type of potato etc. I repeated the experiment twice and took the average measurement in an attempt to get the most reliable and accurate results possible.

Analysis: From the graph, I can see that my prediction at the start of the experiment was correct. I predicted that the lower the concentration of the sucrose solution, the more the potato rod would increase in length for example, at 0.0 molar on my graph, the potato rod increased by 0.15 cm. This happened because at this hypotonic concentration, there is a higher concentration of water in the solution than in the potato cells. Then, through the process of osmosis (see background information) the water osmosed from the solution into the potato causing it to increase in size.

I also predicted that at a high concentration of sucrose solution, the potato rod would decrease in size. This has also been proved as true from my results/graph. I know this because at 1.0 molar, the potato rod decreased by 0.2 cm. The potato rod decreased in length because that hypertonic concentration had a lower level of water than the potato cells. Again, osmosis took place and the water molecules diffused from the potato cells into the sucrose solution.

I predicted that when the sucrose solution was about 0.45, molar, the potato rod would remain the same length. From my graph I can see that this is true. This occurred at 0.46 moles, this is the concentration of sucrose solution in the vacuoles of the cells before the experiment started. The solutions are isotonic, so osmosis will take place but there will be no net movement of water molecules.

Evaluation: I am pleased with the way the experiment went and I am confident about the results produced. However, to make the test more accurate, I could have used a different method of cutting and measuring the potato rods, other than a cork borer and ruler. If I had used a scalpel for example, this would have given me a greater degree of accuracy. I could have also used the intermediate concentrations in my experiment. These are: 0.1, 0.3, 0.5, 0.7, and 0.9. Adding these would have made my test more accurate. I noticed that at the concentrations 0.8 and 1.0 moles, the potato rods began to float. This affected the accuracy of the investigation as the whole rod was not completely submerged. I think that I should have used a utensil to push down the potato rod and hold it down for the duration of the experiment. If I had made these adjustments, I would have produced a more precise measurement.