When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid". (This is the opposite of "turgid"). If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and the cell membrane will appear to be pulled away from the cell wall. In this state the plant cells they are said to be plasmolysed.
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.
Osmosis in plant cells
Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.
When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight.
Apparatus:
- McCartney bottles
- Stopwatch
- Scalpel
- Cork borer
- Sucrose solution 2Molar concentration
- Kumara (sweet potato)
- Filter paper
- Weighing scales
- Syringe
Variables
The variables in this experiment are: size, temperature, volume of solution, surface area of kumara, concentration, maturity of kumara and pressure. The variable I will change is the concentration of the sucrose solution.
Results
Prediction
If the solutions water potential is lower than the sucrose solution then the chip mass will also lower but to find the water potential of the kumara is very difficult but we know that a kumara has a high sugar content so the water potential will be low.
I predict that the mass of the chip will increase in low molar sucrose solution because I know that that the kumara has a high sugar content and that using osmosis the water in the sucrose solution will diffuse from an area of high concentration to an area of low concentration. I think this will happen in 0.3 molar and 0.6 molar. In the higher molar solution the kumara will decrease in weight because the water will diffuse out of the kumara using osmosis to get to an area of lower concentration.
Analysis
The results that I obtained were not as accurate as they could have been this is shown by the large range of results in the graph, However they display the principle of the results accurately the fact that the graph displays the percentage weight loss of a Kumara.
Evaluation
I think my experiment was as accurate as it could be in a class laboratory however there was a large margin of human error. The Kumara had to be changed which was a problem because it meant that the new Kumara had a different water potential to the first 1 affecting the amount of water that could pass either in or out of the Kumara. I cannot see an anomaly, this is most probably as the experiment was comprised of 3 sub experiments giving a fair and accurate average. An anomaly could also have been the result of the Kumara being changed at that point so the molar of sucrose in the new Kumara could have been different.
The results give enough evidence to let me conclude that the Kumara gains weight in low molar sucrose solution because I know that that the kumara has a high sugar content and that using osmosis the water in the sucrose solution will diffuse from an area of high concentration to an area of low concentration.
To improve the experiment I could try to do the whole experiment with the same kumara and weigh them more accurately. To further the investigation I could use potatoes or another fruit and compare those results to that of the Kumara and in doing so find out which has the highest sugar content and water potential.
