How varying one factor effects the rate of osmosis between potato cylinders and Sucrose solution

Biology Coursework

PLAN

Introduction

I am doing an investigation, to find out on how varying one factor effects the rate of osmosis between potato cylinders and Sucrose solution.

Osmosis is the diffusion of solvent (usually water) particles through a selectively permeable membrane from a region of high solvent concentration to a region of lower solvent concentration.

Examples of Selectively permeable membranes are (a) the cell membrane, and (b) visking (dialysis) tubing. Such membranes are thought to have tiny pores that allow the rapid passage of small water particles, but restrict the passage of larger solute particles.

Since the membrane is Selectively permeable, Osmosis is important in the passage of water into and out of cells and organisms, and depends on Osmotic pressure.

Osmotic pressure

The pressure exerted by the osmotic movement of water that can be demonstrated in an Osmometer. Water moves into the solution in the visking bag by Osmosis, causing the liquid level in the tube to rise. Osmotic pressure depends on the relative solute concentrations of the solution s involved. The Osmotic pressure that a solution is capable of developing is called its osmotic potential, but is only realised in an Osmometer.

Cell membranes

All living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is a complex structure that is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell, and for interacting with the environment surrounding the cell.

There are two ways that the molecules move through the membrane: passive transport and active transport. Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such an energy expenditure, and occurs spontaneously.

The principle means of passive transport is diffusion. Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated.

It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout the system (the net movement will then therefore be zero).

Osmosis is a special case of diffusion. In this case, sucrose solution is used. The sucrose molecules are too large to pass through the pores in the membrane of the potato, so it cannot diffuse from one side of the membrane to the other. The water molecules can, and do, pass through the membrane. Hence the membrane is said to be semi-permeable, since it allows some molecules to pass through but not others. However, on the side of the membrane with the sucrose, the sucrose molecules interfere with the movement of the water molecules, preventing them from leaving as rapidly as they enter. Thus, more water flows into the side with the sucrose than flows out, and the solution gets diluted.

If the sucrose (or some other large molecule like a protein) is in a cell, the water moves into the cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon, and if too much water enters the cell, the cell can burst, which kills the cell. So cells usually have some kind of mechanism for preventing too much water from entering or pumping the water out or simply making a tough outer coat that will not rupture.

As I said earlier on, I am doing an investigation to find ...

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As I said earlier on, I am doing an investigation to find out on how varying one factor affects the rate of osmosis. The factors that I can vary and how they will have to be controlled are as follows:

Factors and Variables

Several factors affect how fast a molecule will diffuse. The first of these is the kinetic energy of the molecule, which is most frequently measured as the temperature of the system. Molecules in a system at a higher temperature will have more energy and will move faster, and hence diffuse faster, than molecules of the same type in a low-temperature system. The temperature will not be varied as it would have to be kept constant using an electronically controlled water bath.

Also the cell membrane is made up of phospho-lipids and proteins. So, if the temperature rises to about 40ºc the protein will start to denature. If the protein denatures then there will be no barrier for diffusion to take place.

The surface area of the potato cylinder is another factor that I can vary, and this is the factor that I will vary in the actual experiment.

The type of potato is a varying factor that makes a difference, as different potatoes may have different water potentials.

The surface area and volume ratio of the potato cylinders will not be varied in my investigation as it involves a lot of accurate measurements, which would take a lot of time which I haven’t got.

The size of the molecule also affects how rapidly it will diffuse. At the same temperature, smaller molecules will move more rapidly than larger molecules because it takes more energy to get the larger molecule moving.

Other factors include any charges on the molecule (positive or negative) and the nature of the material that the molecules are moving through.

Concentration of sucrose solution has to be kept constant throughout the experiment (kept at 1 molar). If the concentration is lowered, there would be a major affect on the rate of osmosis.

This effect would be that part of the experiment would be carried out at a lower concentration meaning that the rate of osmosis would be low, and if the other part was carried out at a higher concentration, the rate of osmosis would be high. So keeping the concentration constant would mean that the rate of osmosis would be equal throughout the whole experiment. Therefore giving me reliable results.

Apparatus

Sucrose solution (1 molar)

Potatoes (Estima)

Beaker/Cup

Electronic balances

Measuring Cylinder

Cork borers

Vernier Callipers

Knife

White tile

Stopwatch

Diagram of experiment

Risk & Safety Assessment

Take care whilst cutting the potato cylinders, as the cork borers are sharp.

When cutting the cylinders, make sure you are cutting downwards towards the tile. Be careful not to keep your hands at the bottom of the potato whilst cutting.

Keep ties tucked in. this is because it may interfere with the experiment, and may also knock the cup and its contents over.

Make sure the experiment is taking place in the middle of the bench, just incase it falls off the bench.

Experimental procedure

Clean and rinse the measuring cylinder, cup and other apparatus with distilled water.

Using the cork borer cut out potato cylinders and cut the skin off using the knife.

Add the potato cylinders to a cup of distilled water and leave over night for pre-treatment. This will keep the water potential in each potato cylinder equal, for the experiment on the next day.

Next day. Measure out and cut the required lengths and widths of potato cylinders needed, using a vernier calliper.

Measure out and add 30cm³ of sucrose solution to each of the cups.

Blot dry, then weigh the potato cylinder that is to be used in the experiment and note it down in the table.

Now place it into the cup of Sucrose solution for its allotted amount of time (30mins). I have made the time 30 minutes because, if the cylinders were left in the water for too long, the concentration of the solution would change. Meaning that the results would be unfair, as the experiment would have been carried out in a changing environment (as the environment was meant to be kept constant).

Blot dry, and weigh the potato cylinder again, and note down the weight in the table.

Clean and rinse the apparatus with distilled water, and do the same thing as above except with the different sized potato cylinders.

The potato cylinder will be cut up in the following way:

The results from the experiment will be recorded in the form of a table, which will look like as follows:

How to keep a fair test

Surface area of potato cylinder
Type of potato
Concentration of solution
The temperature
The volume of the potato cylinder
The time the potato is kept in the solution

The type of potatoes themselves will have to be kept the same all the way through the experiment. This is because each potato may have a different membrane and a different amount of pores.

Also the water content in each potato could vary. This can be explained by saying that the potatoes in the middle of the sack will have a higher water content, because it is more humid in the middle of the sack which is opposite to the potatoes on the outside of the sack.

To get rid off this unfairness, I will pre-treat all the potatoes that I will use in my experiment. This means that I will keep all my potato cylinders in the same environment over night in a beaker full of sucrose solution.

When plant cells (potatoes) are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.

When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis.

Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. So this is why I place my cylinders in sucrose solution over night.

I have to keep the concentration of the solution constant throughout the experiment, otherwise it will affect the water potentials.

Water potential is a measure of tendency for a system to absorb water. Free water molecules in the solution have kinetic energy. So by dissolving any solute in the water it decreases the energy of free water molecules.

Therefore, Distilled water has the highest water potential (0). A dilute solution of solute will slightly decrease the water potential. A more concentrated solution of solute will decrease the water potential even more:

The surface area of the potato cylinders is the factor that I am varying in this experiment. This will change the rate of osmosis drastically, if one potato cylinder was to be cut up into small pieces. The rate of osmosis would be slower if the surface area was 15cm² compared to another cylinder with a surface area of 25cm². This is because there is more area for osmosis to occur in the solution and potato.

The time that the potato is kept in the solution will have to be not too short and not too long. This is because if the potato is kept in the solution for a long time for example 24 hours, the osmosis taking place will dilute the solution around it. This would occur as water diffusing out of the potato will dilute the sucrose solution. Therefore the sucrose solution will not be kept at the same concentration, which is a major rule that has to be followed in my investigation. This will give me unreliable and inaccurate results. So, keeping it in for about 30 mins would be a good time for osmosis to occur and a too short time for the solution to become diluted.

To keep the experiment a fair test each surface area will be repeated as many times as possible, which will most probably be three times as there is not much time to carry out the experiment. Three repeats will be the minimum amount of repeats as it is just about possible to spot any anomalies in my results and just enough to take an average from.

Prediction

I predict that the rate of osmosis will increase as the surface area of the potato increases. This can be explained by using diagrams, the first diagram has a small surface area, so obviously the rate of osmosis would be low. The second diagram has a larger surface area, therefore the rate of osmosis would be greater as there is more area for osmosis to occur.

Bearing in mind that osmosis is the diffusion of water from a high concentration to a low concentration through a permeable membrane. So, the larger area of potato there is, the rate at which the water diffuses would be quicker, than a potato with a smaller surface area.

As we can see that the rate of osmosis increases as the surface area increases. Therefore, we can also say that if we double the surface area from its original area, then the rate at which osmosis occurs must take half the time. The rate can be worked out as 1/T. From this evidence I can say that the surface area of the potato is directly proportional to the rate of osmosis. Therefore, I predict that my results will look like as follows:

Surface area ∝ the percentage mass change of potato

And

Surface area ∝ Rate of Osmosis

Therefore the graph for rate against surface area will look like this:

The number of points on my graph will be approximately 10. This is because I need a good range of results to obtain accurate and reliable results.

There will be 10 points, which means that 6 different surface areas will be used to obtain these results. The surface areas will not be too close to each other, as I need a wide spread and a good range of results. If the surface areas I used in my experiment were very similar to each other, my graph will have a clump of results in the middle of the graph. This will lead to not drawing a good line of best fit, and will not illustrate what happens to the rate of osmosis as the surface area increases etc.

Therefore 10 wider surface areas will give me points that are slightly more further apart and allow me to draw a better line on my graph, which will enable me to show what happens as the surface area and rate of osmosis increase. The surface areas that I will use in my experiment will be:

17.3cm²
18.8cm²
19.8cm²
22.0cm²
23.6cm²
27.3cm²
28.8cm ²
29.8cm²
32.0cm²
37.3cm²

As you can see, I have a wide range as the surface area ranges from 17.3cm² to 37.3cm². Therefore, the surface area is over double from what it started out as. This wide range will give me points on the graph that aren’t cramped together, but nicely spread out so that I can draw a line of best fit.

Secondary sources

- Key Science Biology by David Applin (pages 143-144)

- GCSE Biology revision guide by ‘The Science Co-ordination group’ (page 5)

- Microsoft Encarta