Determining the water potential of potato cortex cells.

Determining the water potential of potato cortex cells

An experiment to investigate the effect of different sucrose concentrations on potato tissues.

Biological Background knowledge:

Biological background knowledge on osmosis:

Osmosis is a special form of diffusion, which involves the movement of solute molecules. Osmosis can be defined as the passage of water from a region where it is highly concentrated to a region where its concentration is lower, through a partially permeable membrane.

Biological background knowledge on water potential:

Water potential is a measure of the free kinetic energy of water in a system, or the measure of the tendency of water to leave a solution. The more concentrated the solution the more negative will be its water potential. Water diffuses from a region of higher to a region of lower water potential.

The water potential of a system is the difference between the chemical potential of the water in the system and the chemical potential of pure water under standard conditions. The water potential of pure water is zero.

Water molecules move randomly. When water is enclosed by a membrane, living or artificial, some of the moving water molecules will hit the membrane, exerting pressure on it. This pressure is known as water potential.
As the number of water molecules increases, the number of collisions between the molecules and the surrounding membrane increases. This causes the pressure on the surrounding membrane to increase so the water potential will increase.
Water potential is represented by the symbol Ψ (Greek letter, "psi"). It is measured in units of pressure, usually kilopascals (kPa). Pure water has a water potential of zero. A solution will have a lower concentration of water molecules so it will have a negative water potential.

Hypothesis:

Independent variable: concentration of sucrose

Dependant variable: Mass of potato discs

As the concentration of sucrose increases, from 0 to 0.5M, the percentage change of mass of potato discs immersed in a sucrose solution will decrease.

Planning:

Apparatus and Materials:

A range of dilutions of sucrose: 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0M
11 50ml beakers
Cork borer, size 6 (use a size 5 cork borer to push out the potato cylinder)
Razor
White tile
Top pan microbalance
Weighing boats
Large fresh potato
Bench cloth/ paper towel
1 25ml ...

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Hypothesis:

Independent variable: concentration of sucrose

Dependant variable: Mass of potato discs

As the concentration of sucrose increases, from 0 to 0.5M, the percentage change of mass of potato discs immersed in a sucrose solution will decrease.

Planning:

Apparatus and Materials:

A range of dilutions of sucrose: 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0M
11 50ml beakers
Cork borer, size 6 (use a size 5 cork borer to push out the potato cylinder)
Razor
White tile
Top pan microbalance
Weighing boats
Large fresh potato
Bench cloth/ paper towel
1 25ml measuring cylinder
Tea strainers

Procedure:

20ml of 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0M sucrose solutions were measured out using a 25ml-measuring cylinder and placed into 11 50ml beakers that were appropriately labelled with the sucrose concentrations.

Using a cork borer (size 6) several potato cylinders were cut out of a large fresh potato and then using a razor they were cut up into 110 discs about 2mm thick. The skin of the potato was avoided, as it is a waterproof layer that prevents water getting in and out and if some of the potato discs had skin on and others didn’t, it would mean that the experiment would not be a fair test.

The prepared potato discs were then quickly rinsed by putting them in tea strainers and pouring distilled water over them.

To remove any excess water, the potato discs were blotted dry with bench cloth/ paper towels, using a standardised blotting technique (they were blotted for the same amount of time and with the same amount of pressure exerted on them). A top-pan microbalance was then used to find the masses of groups of 10 potato discs, this was done by placing a weighing boat on the top-pan microbalance, it was then set to zero and then the potato discs were added in the weighing boat to find the masses, which were recorded. One group of 10 individual potato discs were then added to each beaker and left for an hour.

After the hour, the potato discs were removed from the solutions, blotted dry (again using the standardised blotting technique) and reweighed to find if there had been any change in mass, which was recorded.

Results:

From my results I can plot a graph showing the percentage change in mass compared to the molarities. This will show me how the molarity affects the percentage change in mass: this will be graph 1.

I can also convert the molarities into values for pressure and plot a graph for it. This will help me to determine the water potential of my potato tissues: this will be graph 2.

Analysis:

In graph 1, there is a pattern: the line declines as the concentration of sucrose increases; this shows me that as the concentration of the sucrose solution increases, the percentage change in mass of the potatoes decrease. There is one large evident anomaly in my results: at when the molarity is 0.4. We can see that this result does not follow the same pattern as the other results; this is probably due to errors that happened during the experiment (which are discussed in the evaluation).

I can see also see on my graph that when the concentration is from 0.0 – 0.5M at first the line is steep and then it starts to level out. This suggests that when the molarity is from 0.0 – 0.5M, as the concentration of sucrose increases then the percentage change in mass gradually decreases.

I can use my biological knowledge to understand and explain why my experiment produced the results that I got. When the potato discs are placed in a high sucrose concentration solution (1.0M), water moves from the potato to outside of the potato (as when osmosis occurs, water moves from a region where it is highly concentrated to a region where it is lower.) This means that water leaves the potato and it looses mass. This is represented in my results as when the potatoes loose mass, and therefore produce negative numbers of percentage change of mass.

When potato discs are placed in pure water (0M), water moves from outside of the potato to inside the potato. This means that the water moves into the potato and it gains mass. This is represented in my results as when the potatoes gain mass, and therefore produce positive numbers of percentage change of mass.

I can also explain why the line is steep to begin with and gradually levels out, by using my biological knowledge. Osmosis does not occur when equilibrium has been reached, this is when the concentration inside of the potato cells is the same as the concentration out of the potato cells, therefore there is no region where water is of higher concentration and osmosis does not occur. If there is a little difference between the two concentrations then osmosis will occur, but it will be at a slower rate than if there is a larger difference between the two concentrations. This is what has happened at when the line on my graph starts to level out: the rate of osmosis occurring is happening at a slower rate and therefore, less mass is gained when the potato discs are immersed in the different sucrose solutions for the same amount of time.

In graph 2: I can also see a very similar pattern: the line declines as the water potential becomes more negative; this shows me that as the concentration of water decreases (there are less and less water molecules in the solution) then the percentage change in mass of potato will decrease.

Evaluation:

It is evident that there were sources of error within my experiment. These is due to the following factors:

The surface area of the potato discs: Although we aimed to get each potato disc 2mm thick, we cannot be sure that we cut them to that exact measurement, this means that the surface area on each potato disc was different and was not a fair test.
Other factors that affect the rate of osmosis: There are other factors that affect the rate of osmosis occurring and could therefore have affected the results. Temperature, an increase in temperature increases the kinetic energy that the particles have and this means that the rate of osmosis will increase. We did not monitor the temperature of our experiment, although this would not have dramatically affected our results, if we were to redo the experiment and make sure it was fair then we need to consider this.

Ethical Implications:

The potato that was used was not a rare species, it is commonly found.

Risk Assessment:

Care must be taken when using the cork borers and the razors to cut up the potato discs.

Disposal Methods:

All waste that was disposed of did not cause any threat to the environment; potatoes are biodegradable and could be thrown in the bin. No chemicals were used in this experiment.