Osmosis is a special type of diffusion involving only water molecules.

Shane Sinclair Biology

Biology Coursework

Plan

Background Knowledge

Osmosis is a special type of diffusion involving only water molecules. The definition of osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. If a plant cell is surrounded by a solution of water that has a higher water potential than that of inside the cell, the water will enter the cell and push the contents of the cell against the cell wall. If the plant cell was surrounded by lower water potential the water will pass out of the cell. This will make the protplast pull away from the cell wall and shrink. The cell has been plasmolysed.(class notes-osmosis and water potential). I have done a preliminary experiment called "The change in mass of a potato after being in different types of water potential." In this experiment a piece of potato was placed in sucrose solutions of different water potentials and weighed, this weight would be compared to the weight of the potato before being placed in the solution and the difference would be plotted on a graph. What I found out was when there was a higher potential of the sucrose solution the potato gained weight as osmosis takes place the solution moves into the potato. When the sucrose solution was of lower potential than the potato cell it lost weight as water was moving out of the cell.

The direction in which the water moves, in or out of the cell will depend on the relative water potential inside the cells (the cytoplasm) and in the surrounding solution (sucrose solution). If the net movement of water were into the cell then it would become turgid, the cytoplasm and vacuole pushed up against the cell wall, making the cell stiffer and wider. Water moving out of the cell will have the opposite effect and so the cell will become flaccid. Water will continue to move across the membrane until equilibrium is reached i.e. the water potential is the same either side of the membrane.

Hypothesis

From this background knowledge I can see that my experiment will be very similar to that of the potato practical. Like in the potato practical I will expect that when my sweet potato is left in a sucrose solution such as distilled water with a high water potential the sweet potato will gain weight as due to osmosis, water will move into the cell. If the cell was placed in a sucrose solution with a low water potential the sweet potato will lose weight as during osmosis water moves from high water potential to low water ...

This is a preview of the whole essay

Hypothesis

Apparatus

6 equal pieces of sweet potato (x3)- these must be equal to ensure fair results in the experiment. I will need the six pieces of sweet potato 3 times as I will repeat the experiment three times.
1 Cork borer with a radius of about 10mm.
6 test tubes.
Test tube rack.
Measuring cylinders and syringes.
1 marker pen
Filter Paper
25 ml of sucrose solution with water potentials of 0.4 ,0.5, 0.6, 0.7, 0.8 and 1.0
25ml of distilled water.
Electronic balance to three decimal places- this is to give accurate results.
Stop watch- This is to ensure each piece of sweet potato is in the solution for the same amount of time ensuring fair results.

Method

The first thing I shall do is cut out 6 pieces of sweet potato using my cork borer all of equal length (3cm). Each of the six test tubes will then have a different sucrose solution placed in it of about 25 ml. The six sucrose solutions are of 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0.

Before placing each piece of sweet potato in the sucrose solution I will weigh them. I will then place the pieces of sweet potato in each of the sucrose solutions and leave them for 24 hours. Once the 24 hours is up I shall take the pieces of sweet potato out of the sucrose solution and weigh them on the electronic balance. After that I will calculate the weight loss of each piece of sweet potato. The experiment will be carried out three times to ensure that there was no mistakes made. A mean result will be take from these results, then the standard deviation will be worked out.

Controls and Variables

The factors that I shall keep the same during this experiment will be the size of the pieces of sweet potato, the amount of sucrose solution and the time in which the experiment will take. The only variable in this experiment will be the different types of solution i.e. 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0.

Safety

During the carrying out of this experiment I shall have to take some safety precautions. One of these safety precautions will be to ensure that the electronic balance is kept well away from any wet substances and when weighing the sweet potato the potato must be dry. Another precaution that should be taken is in the cutting of the pieces of sweet potato. Great care and concentration should be taken whilst carrying out the cutting of the pieces of sweet potato.

Results

Firstly my results will be recorded in a simple table showing the weight of the sweet potato at the beginning of the experiment, after the experiment, the total change in the weight of the sweet potato and a mean value for the results. This table will then be converted into a line graph showing the percentage change in mass as well as the error margin for each result. I will also have an intercept graph.

Analysis

A table showing the amounts of distilled water and sucrose solution required to make up our sucrose concentration.

Sucrose Concentration (mold m`3)Volume of Distilled H2O (cm3)Volume of sucrose solution0.415100.512.512.50.610150.77.517.50.85201.0025

A table showing the weight of each sweet potato before going into the sucrose solution.

Weight of each potato (grams)Sucrose Solution (mold m`3)1230.41.3841.431.4460.51.591.4141.4530.61.4051.4441.390.71.4651.5281.5180.81.4741.6111.5351.01.4891.4591.404

A table showing the weight of each sweet potato after being in thier sucrose solutions.

Weight of each potato (grams)Sucrose Solution (mold m`3)1230.41.51.4851.5350.51.6151.5081.5280.61.3981.4311.370.71.3851.481.420.81.3581.4911.4691.01.2171.2321.237

A table showing the change in mass for each sweet potato.

Change in massSucrose Solution (mold m`3)1230.40.1160.05500890.50.0250.0940.0750.6-0.007-0.013 -0.020.7-0.11-0.048 -0.0980.8-0.116-0.12-0.0661.0-0.272-0.227 -0.167

A table showing the percentage change in mass for each sweet potato.

Percentage Change in massSucrose Solution (mold m`3)1230.48%4%6%0.52%7%5%0.6-0.5%-1%-1%0.7-8%-3%-6%0.8-8%-7%-4%1.0-18%-16%-12%(nearest whole number)

A table showing the mean percentage change in mass and the Standard Deviation.

Mean PercentageStandardSucrose Solution (mold m`3)change in massDeviation0.46%20.55%2.5490.6-1%0.50.7-6%2.5490.8-6%1.7321.0-15%2.517

Conclusion

There aren't any results that really stand out apart from the standard deviation for the sucrose solution 0.6 in which the standard deviation is fairly low showing that the results for this part of the experiment where fairly accurate and similar. All of the other standard deviations had a slightly larger difference showing that the results where not as accurate.

In my experiment we can see that at the beginning the potato's weight increased slightly and this was due to the higher sucrose potential moving into the cell as osmosis moves from a region of higher water potential to region of lower water potential. Thus causing the potato to gain in weight. As the experiment continued and the water potential was becoming less of that than inside the cell the potato began to loss weight as the water was moving from the cell into the outside solution. What I have concluded from this is that when the sucrose solution is higher than that of the sweet potato the sweet potato will gain weight and when the sucrose solution is less than that of the sweet potato the sweet potato will lose weight.

Evaluation

There are a few areas in which my experiment could be improved but not many as there were not many variables in the experiment that had to be controlled. One thing that could be improved is the slight variation in weight of each piece of sweet potato. Each potato should have the exact same surface area allowing the experiment to be fairer so that there would be the same area for the water to diffuse across. The amount of sucrose solution could have also caused the experiment to have slight problems. The transfer of the solutions from the measuring cylinders to the test tubes may cause some of the solution not be have transferred as some droplets may be left in the measuring cylinder. Another possible factor that could have being changed is the recording of temperature. Temperature should have been kept constant because an increase in temperature will make the water diffuse faster. Although a few things in the carrying out of my experiment could have been changed I would still have came to the same outcome.

I found that my results were fairly accurate as they all had a standard deviation of around 2 which shows that although my results weren't perfectly accurate they were all of around the same standard deviation. I didn't have many anomaly results as all of my results were around the same region. There were some differences in results, which could have been caused by the inaccuracy of my drying of the potato before weighing or the possibility of a difference in amount of sucrose solution due to the inaccuracy of my measuring.