Potato / Osmosis Experiment.

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Qasim Chaudry 10.2

Biology Coursework

POTATO OSMOSIS EXPERIMENT

Introduction

        Osmosis is the diffusion of water molecules across a partially permeable membrane with the concentration gradient. This means that water molecules move from where they are in a higher concentration, to an area where they are in a lower concentration. Therefore, osmosis is always trying to make a concentrated solution more dilute.

Why a partially permeable membrane is needed

Molecules will always spread out as evenly as they are able to. The partially permeable membrane allows the water molecules to spread out, and thus making the concentrated sugar solution more dilute; but the non-partially permeable membrane keeps the water molecules in a compact space and prevents osmosis from occurring.

AIM:

The aim of the experiment is to calculate the concentration of sucrose solution that is isotonic to the cytoplasm within the plant cells of potato samples.

Theory- Reasons for Prediction:

        I predict that the concentration of the sucrose solution will have an affect on the amount of osmosis taking place. I predict that by increasing the concentration of the sucrose solution the amount of osmosis will increase.

When osmosis occurs, the previously more concentrated sugar solution gains mass (since there is a net gain of water molecules there) and the previously more dilute solution loses mass (since water molecules are leaving it to dilute the concentrated solution). During the course of the experiment, the sucrose concentration of the solution will vary so at different stages the potato sample will be the more concentrated (when it will gain mass), and it will also be the less concentrated solution (when it will lose mass).

As the concentration of the sucrose solution becomes hypertonic to the potato sample, more and more water molecules have to leave the potato sample to dilute it – which gives the potato sample a greater loss in mass. Similarly, as the sucrose solution becomes hypotonic to the potato sample, more and more water molecules will have to leave the sucrose solution to dilute the potato sample- giving the potato sample a greater gain in mass.

This means that as the concentration of the sucrose solution increases, the mass gain of the potato sample decreases – so they are in direct inverse proportion.

PREDICTION - THE THREE STAGES ON THE SKETCH GRAPH:

STAGE 1is where the solution surrounding the potato sample is hypertonic to it. Here, the potato sample will take in water from the solution by osmosis and increase in mass. This will cause the potato sample’s cells to become turgid.

Being turgid helps plants leaves stay firm, and no-wood plants stay upright.

STAGE 2 is where the solution surrounding the potato sample is hypotonic to it. Here, water molecules will leave the potato by osmosis in order to dilute the solution. This will cause the potato sample to lose mass, and will make the potato plant cells flaccid.

If the cell membrane and vacuole shrink any further, they will rip away from the cell wall and the cell will be plasmolysed. Plasmolysis usually kills a plant cell because the cell membrane becomes damaged when it tears away from the cell wall.

STAGE 3 is where the surrounding solution is isotonic to the cytoplasm of the potato sample’s cells. Discovering this area on the actual graph, using a line of best fit, is the entire objective of the investigation. Here, diffusion of water molecules will still occur but there will be no net gain, as there is no concentration gradient for the water molecules to travel with (as the two solutions are isotonic to each other).

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Preliminary Work and Research:

        A preliminary experiment was set up in order to determine the range of values to be used in the main investigation (potato sample weight, concentration range).

The fact it was set up mainly for this reason, coupled with the time restrictions of only having two 50 minute sessions, meant that the results from the experiment were not needed to be very accurate. However, the main investigation will be completed to a larger degree of accuracy as the purpose of it is to find an exact and precise value (that of isotonic concentration).

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This report of an investigation into the effect of changing sucrose solution concentration on osmosis in potato tissue is well-structured and demonstrates an excellent understanding of the processes involved. The writer explains in detail the theory behind the experiment. He makes good use of preliminary tests to determine the exact values he will use in the actual experiment. The method is detailed although it might have been easier to follow if written in numbered stages. Anomalies are identified and the readings repeated. The conclusion is based firmly on the evidence collected and is backed up by a good level of science. All in all, an excellent effort at this level. 5 stars.