To investigate the effect of varying concentration of glucose solutions on the osmotic activity between the solution and potato slices.

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INVESTIGATING OSMOSIS IN PLANT CELLS

AIM:

To investigate the effect of varying concentration of glucose solutions on the osmotic activity between the solution and potato slices.

SCIENTIFIC THEORY:

Osmosis is defined as the net movement of water molecules from a region of high concentration to a region of low concentration of water, through a partially permeable membrane, which only lets smaller molecules through. This can also be described as movement down the concentration gradient. The molecules continue to diffuse through the membrane until both sides reach a state of equilibrium. This is also known as the isotonic point when the molecules are equally distributed on both sides of the membrane so that no one area has a higher or lower concentration than the other.

Plant cells have a strong cell wall and a partially permeable cell membrane around them. When cells are surrounded by a solution more dilute than their own, (hypotonic solution-with more water than solute molecules), the cell swells up and becomes "turgid". This is because the water molecules diffuse from the solution to the plant to equal out the concentration levels. The strong cell wall prevents them from bursting.

Figure 1- A turgid plant cells in more dilute surroundings (from "Biological

Sciences Review")

When they are surrounded by a solution more concentrated than their own, (hypertonic- less water molecules), they shrink and become "plasmolysed". This is because the water molecules are highly concentrated in the plant, so they diffuse out into the solution to create a state of equilibrium.

Figure 2- A plasmolysed plant cell in more concentrated solution (from

"Biological Sciences Review")

For this particular investigation potato slices will play the role of the plant cell. I will place the potato slices into six different glucose sugar concentrated solutions and record the percentage increase or decrease in the mass of the potatoes.

PRELIMINARY EXPERIMENT:

Preliminary work was carried out to help clarify the method of the experiment and make changes. It also helped to me decide the range of observations to be made so that the results obtained are as accurate as they could possibly be.

Two unknown glucose sugar solutions were given, Solution A and Solution B. 3 Potato slices were kept for 24 hours in each solution. The results were as follows:

Table 1- Results of preliminary experiment

Mass before (g)

Mass after (g)

% change

SOLUTION A

2.1

2.4

4.3

2.1

2.5

9

SOLUTION B

2.1

.5

-28.6

2.1

.6

-23.8

As there was a percentage increase in the mass of the potatoes slices in Solution A, I know that Solution A had a high concentration of water. Solution B had a low concentration of water, as there was a percentage decrease in the mass of the potatoes.

WHAT I LEARNED FROM MY PRELIMINARY

Two potato slices were put into each solution. The percentage change of the potatoes in the same solution differed slightly. This was due to an inaccuracy in the method. This means that it was not as accurate as it should have been. As a result of the preliminary the following changes were made to my method:
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> Measure and weigh the potato slices more accurately. Do not lean on the worktop when taking readings from the mass balance as this sometimes affects the reading.

> Put three potato slices into each solution rather than two so the results are more accurate.

> Use separate measuring cylinders for the different solutions to prevent the solutions from being mixed and therefore reducing the accuracy of the results.

> Measure the volume of the solution as accurately as possible by making sure that the bottom of the meniscus is parallel and as close to ...

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