Step 5:
Put cylinders into sucrose solutions.
Step 6:
Leave solutions for 25 minutes.
Step 7:
Take cylinders out, re-measure and re-weigh each potato cylinder.
Step 8:
Redo steps 1-7 two more times.
Step 9:
Pack up equipment
Safety:
I will make my experiment safe by following the class rules throughout the experiment such as not doing an experiment sitting down or tucking in all chairs whilst doing the experiment. To protect my eyes from the different sucrose solutions I will wear goggles throughout the experiment. I will be very careful with the handling of the solutions and be careful to not spill any or use excess. The equipment for my experiment will be used responsibly and safely.
To make it a fair test:
In order to make the test fair I will make sure that the lengths of each potato cylinder are equal before putting the cylinder into the solution. I will do this by using the same cork borer to obtain the different cylinders. I will also use a timer to have all five sucrose solutions timed for exactly 25 minutes at the same time; so started and stopped together. I will try to have the weights of the potato cylinders near as equal as they can be. The same amount of each sucrose solution will be poured into each of the beakers (50ml). The ends of each cylinder of potato will not be cut. Each of the potato cylinders will be dried with paper towels after coming out of the solution; before being re-weighed. The scales will be dried before weighing the cylinders at both times to take off any excess water on them. This will be done to add no more weight to the reading of the scales.
Results:
Conclusion:
The conclusion of my experiment is that the potato cylinders in the distilled water gained in weight on average. This is because the distilled water had a region of higher concentrated water than the cells of the potato. The water molecules were therefore able to flow both in and out of the cell membrane, but after the 25 minutes had passed, more water had entered into the cell of the potato than left it. The potato cylinder that was in the distilled water shortened in size a little on th1 1st and 3rd attempts. This is because the cell membrane had become saturated with water molecules that had come into the cell and the potato cylinder had shrunk as a result of that. The 0.25 concentration showed that it had on average lessened in size but grown in weight. This is because it was not pure water but had a little sucrose solution in it which stopped more water from entering the potato cylinders. The 0.5 concentration had the same water concentration as the cell and so there was a basic no change in size. The 0.75 concentration was relatively low in terms of the amount it shrunk in size. This had quite a low region of water concentration because it had larger molecules in it (sucrose) which didn’t pass through the cell membrane. Its weight on average shrunk because of the water molecules already inside of the potato cylinder passed out into the solution. The 1.0 concentration had a low concentration of water which meant that osmosis still took place but the cell lost water by it; therefore losing weight. Also because more water molecules left than entered the cell shrunk.
Evaluation:
My evaluation of the experiment is that there were a few problems involved in it. In planning I do not think that I did enough preparation for the experiment. I could have improved the final result of it by looking more closely at the preliminary results and analysing them also. My measurements I feel could have been more accurate and the range in grams between the potato cylinders could have been smaller. The amount of sucrose solution in put into each beaker (50ml) could have been more accurate. This is important because if excess sucrose solution is put into the beakers then not as many molecules will be able to pass either in or out of the cell. The graph it is mainly showing my predictions – that with a solution with a high region of water concentration the cylinder in the beaker will shrink in length but will gain in mass. The solution with a low region of water concentration will shrink in size and length. The graph does still have some anomalies in it.
What is Osmosis?
Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
Semi-permeable membranes are very thin layers of material (cell membranes are semi-permeable) which allow some things to pass through them but prevent other things from passing through.
Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through. A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water.
A region of low concentration of water is a concentrated solution of something like sucrose. In this case there is much less water from a dilute solution through a semi-permeable membrane to a more concentrated solution.
When you put an animal or plant cell into liquid containing water one of three things will happen.
- If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis.
Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The overall result is that water enters the cell. The cell will swell up.
- If the medium is exactly the same water concentration as the cell there will be no net movement of water across the cell membrane.
Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
- If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis.
The water crosses the cell membrane in both directions, but more water leaves the cell than enters it and so the cell shrinks.
