Investigation of the effect of surface area on the rate of osmosis of a potato

Introduction

I have been asked to investigate the factors that may affect the rate of reaction in osmosis. 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.
I have chosen to investigate the effect of the surface area of potato chips. In this experiment is to find out if osmosis occurs between the cut surface of potato and sodium chloride solution, how surface area affects this.

Last year we were asked to do a series of experiment about Osmosis. The method I used then is similar to this time. The only difference from last time will be, I’m using test tubes to place the chips and sodium chloride in. The reason for this is because; the chips need to be fully covered with sodium chloride solution. From last year experiment my conclusion was, the more surface area the chips have got, the faster and the more osmosis occurs. And I believe I will get the same result this time.

There are other factors involved and these are;
¨ Temperature – Higher temperature will make the protein denatured
¨ SIZE of potato – more or less surface area for osmosis to occurs
¨ Mass of potato
¨ Volume of solution potato chip is in
¨ Type of potato – different rate of absorption
¨ Concentration – Slower or faster the process of osmosis
¨ Time left in solution
¨ Surface area of potato chip
¨ Use the same balance to measure chip

In order to conduct a fair test, I have to control these other factors. Doing all the tests at one temperature will control the temperature. For the purpose of my experiment I am going to do all the experiments at room temperature.
The water potential of the potato initially will be kept the same by using the same type of potato, which have been treated in the same way

Planning

Before actually planning the experiment, I will do some research to find out about osmosis, and matters related to it, so that I can make predictions, and find out a way to make this investigation fair and safe.
Planning ahead would help me find out how to do what, when, which should lead me to good results at the end of the experiment. The size of the potato is very important. This is because if the size of the potato varies, so will the amount of mass is looses or gains. I will cut all the pieces of potatoes, to the same size. I will use the borer to get all the potato chips with the same width and dimensions, and for safety reason, we should always keep the borer’s pointing side away from us, and not to point to our palms.

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The mass of the potato is a dependent variable, and this means that it will be measured throughout the experiment. I will measure the mass in grams. The potato chip will be measured before it is put in the solution, and after. This will allow us to see whether osmosis has taken place, and to what extent.
The volume of the solution that the potato chips are kept in must be equal. They must be totally covered in the solution, and the amount of solution will be kept the same because all the potato chips are the same size. The amount of solution which cover each chip will be 25ml3.
The types of potatoes we use are going to be kept the same, because different potatoes may absorb at different rates. For this experiment I am going to use the same variety. The time the potato chip is left in the solution must be kept the same in each experiment. This is because more or less solution may be absorbed depending on time. I am going to keep each potato chip in each solution for 10 minutes, in a dish. The surface area of the potato chips will be kept the same by having all the chips the same size. This must be kept the same because the amount of surface area exposed to the solution may affect the rate of osmosis.

This is a diagram to showing the net movement.

Apparatus

· Stop Clock – to time our experiment
· Test tube rack – to place the test tube in
· Test tube – to place the chips and sodium chloride in
· White tile – to place the potato on
· Ruler – to measure the length of the chips
· Knife – to cut the potato
· 25cm Measuring cylinder – to measure the solution
· Sodium Chloride – to start the Osmosis
· Potatoes – part of the experiment
· Tissue paper – to dry the potatoes after the osmosis takes place
· Balance – to weigh the potatoes
· Size 6 borer – to cut out potato cylinders

Safety

Walk, don’t run
Don’t eat or drink anything in the lab
Listen to the teacher
Be careful of the knife, scalpel, blade or borer something that shape

Method

We assume that this means that the pressure and temperature in each case is the same, as these are factors which could affect osmosis, and we know that the volume, size and surface area of each cylinder is the same, and as they are all from the same potato, the only variable that we are altering is the concentration of the solution

We get all the potato chips in cylinder shapes by borer first, and then we use the cutting knife to chop the beginning and the end, so they will be even. After that, we weight all the chips (*for safety reason, we should cut on a white tile, and we should always keep out fingers on the other end of the potato chip, so we don’t cut ourselves); to see whether they are all in the same mass or not. We then put the first chip into the test tube, when we are going to put the first chip into the second test tube, we chop it in half, and on the third test tube we chop it into three pieces, so on. Finally we pour 25ml of sodium chloride into the dishes and put the bung on. Wait for 10mins, and then we take the chips out and dry them and weigh them.

This is a diagram showing potatoes covered with sodium chloride solution in the test tube.

Prediction

Osmosis is defined as “the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration”.

In a high concentration of water the amount of solute (e.g. sugar) is low. This could be called a weak or dilute solution. In a low concentration of water the amount of solute (e.g. sucrose) is high. This could be called a strong or concentrated solution. When a semi-permeable membrane divides two such solutions will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium).

Knowing that osmosis will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of them, and thus change their volume, we want to test the hypothesis that:
If the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less than that level it will expand. This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.

The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.

However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.

So I will predict after 10mins, the potato chips will lose their mass. They will get lighter.

Result

In this table, we can see as the surface area increases, masses will be lighter, so we can predict, rate of osmosis is affected by the surface area.

In order to obtain a accurate experiment, I have repeated my experiment with new set of equipment and potato chips. These are the results:

Analysis

From my result table and graphs, the evidence obtained from this investigation supports the prediction I made that the potato decreases in mass in solutions with a low water concentration.

In my diagram, I saw that the weight of the potato is decreasing when I increase the surface area, which fits my prediction. High concentration molecules move to low concentration. In this experiment, I believe that I have collected enough data to support my hypothesis. This investigation was, I think, successful. Successful meaning my results collaborated my predictions. Having the results, which backs up my hypothesis, proves that this investigation was fully accomplished, and was an achievement.

In the table seen on the last page, the percentage shows a steady decrease, telling us that the percentage of the difference decreases as the water concentration decreases. Which means my prediction was right, the water concentration in the potato in higher then outside, to the solution inside the potatoes escaped until it reach equilibrium. So the chips finally is lighter then before.

This experiment helped me find out that osmosis occurs between two solutions with a partially impermeable membrane, and that the lower the water concentration is the larger the decrease in grams (mass). This means that the percentage will also be lower, decreasing with the grams.

Evaluation

I believe we gained sufficient and accurate enough results, shown on our graph and tables that prove our hypothesis. My final results were very reliable, due to the precautions I took to make this a fair test. And I have checked my result with the others to see whether they got a same sort of result.

Looking at the overall experiment I have thought of a number of improvements to give me more and better accurate results. Firstly the concentrations were measured using a measuring cylinder and could be made more accurate using titration. Also when the potato was dried to remove surface liquid it was not necessarily done the same on each potato, a more accurate and uniform way of drying would improve the accuracy further. Another thing I could have done to improve my result was to measure the diameter change of the potato, which would have helped me to find out the volume before and after of the potatoes, which would also help me to explain the results obtained.

If we want a better result, we can set one for a longer period of time, for each set, which would lead us to better results, because the osmosis action would reach equilibrium its maximum ability, and therefore tell us how much water could be transferred for each solution.

Finally, I’m pleased with my initial prediction because osmosis has happened.