The aim of my experiment is to investigate the effect of osmosis via potato tubers.

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Biology Coursework Investigation

The aim of my experiment is to investigate the effect of osmosis via potato tubers.  

I have chosen to investigate how the concentration of salt solution affects the rate of osmosis via potato tubers.  I have chosen this factor because it is a relatively simple experiment to conduct and there is minimal risk of something going wrong, compared to other variables.  Also from my research I have been informed that this experiment produces substantial results.  

To ensure my experiment is a fair test the following factors must be considered:

  • Concentration of the solution
  • Size of the potato tuber (and mass)
  • Volume of the solution
  • Duration of time left in the solution
  • Surface area of the potato tuber
  • Age and type of potato used
  • Temperature of the solution

To create a fair test all aspects of the experiment will have to be kept the same whilst one key variable is changed.  For this experiment that means the concentration of the solution that the potato tubers are left in.  If any of the non-variables (factors written above) are not kept constant it would mean it would not be a fair test.  For example if one of the potato tubers was slightly longer, then the surface area of the tuber would be larger and there would therefore be more space for osmosis to occur.

Other non-variables can be monitored, for example the temperature of the solution in which the tubers are left.  Doing all the tests at one temperature; room temperature (around 22-25˚C) would control it.  The volume of the solution can be controlled using measuring cylinders and the duration of the experiment can be controlled using a stopwatch.  I will also be using the same balance throughout the experiment.  This is because the measurements may slightly vary between scales.  All these controls must be made in order to get accurate and reliable results.  

The science involved in this investigation will be osmosis and plant cells. The definition of osmosis: Osmosis is the diffusion of water through a selectively/partially permeable membrane from a dilute solution to a concentrated solution along a concentration or diffusion gradient until the two solutions become an equal concentration.  An isotonic solution is the same concentration as the cell contents.  When the solution is isotonic it stays the same, the potato chip will not be affected.

A hypertonic solution contains a high concentration of solute relative to another solution (e.g. the cell's cytoplasm).  When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel.

A hypotonic solution contains a low concentration of solute relative to another solution (e.g. the cell's cytoplasm).  When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode.

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 makes the green parts of the plant "stand up" into the sunlight.

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When plant cells are placed in concentrated salt 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.

Prediction

From the above information I predict that the higher the concentration of salt solution, the more water will be lost from the potato tuber through osmosis which therefore means the mass of it ...

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