permeability of beetroot membranes

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Georgia Barnett

An investigation into how the concentration of a solution affects the rate of osmosis in a potato

Definitions of important terms:

Osmosis:

Osmosis is the movement of water through a partially permeable membrane from a low concentration into a high concentration. This continues until both solutions are of equal concentration. Osmosis is a natural occurrence that can be simulated with an artificial membrane such as a visking tube. The rate of osmosis can be determined by the difference in water potential between the two substances on either side of the partially permeable membrane.

Water potential (symbol ψ):

Water molecules possess kinetic energy, which means that they are continually moving around when in a gas or liquid state. The higher the concentration of water molecules in a system, the greater the total kinetic energy of water molecules in that system and also the higher its water potential. Water potential is a measure of the energy available in an aqueous solution to cause the migration of water molecules across a semi-permeable cell membrane during . Water moves from areas of high (less negative) to areas of low (more negative) potential. Pure water is given the value zero.

Aim of My Experiment:

I am going to study whether or not the amount of water that may be taken into a potato cell by osmosis is affected by the concentration of a solution on one side of the partially permeable (cell) membrane. For this experiment I am going to use slices of potato as my partially permeable membranes and glucose solutions.

MY PRELIMINARY EXPERIMENT

For this investigation it was necessary to carry out a preliminary experiment in order to both gain a rough idea of what my result might be for the final experiment and to refine the method and techniques used in this experiment. This first experiment was carried out in class and the method used is a great deal less accurate than I hope to make my final experiment.

Factors affecting rate of Osmosis:

  • Temperature:

Particles can gain energy by heat transfer. Therefore if the temperature of one or both of the concentrations increases then the particles will gain more energy and move a lot faster. They will thus collide more frequently will the partially permeable membrane and so the speed at which osmosis takes place.

  • Concentration:

Concentration has a great effect on plant cells; if the concentration of the glucose solution is higher than the concentration inside the potato cells, then water will pass out of the potato cells and into solution by osmosis. If the glucose solution were less concentrated than the cell contents, then water would pass into the cell through osmosis. However, if the concentration inside and outside the cell are exactly equal, then water will pass out of and into the cell at the same rate through random collision with the partially permeable membrane. This means that the cell will be unaffected by being placed in a glucose solution.

  • Surface area of partially permeable membrane:

The surface area of the potato is both very hard to measure accurately and very difficult to keep at a constant level. The greater the surface area of the potato is, the greater the area of partially permeable membrane there is available for osmosis to take place. Therefore the greater the surface area of the potato, the greater the amount of osmosis which can take place.

We decided to measure how changing the concentration of the glucose solutions affected the rate at which osmosis takes place in potato tissue. Our controls for this experiment were the amount, size and shape of potato, the apparatus used and the volume of solution used for each concentration.

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My Predictions

As the concentration of the glucose solution increases, the potato slices will lose increasing amounts of water.

This is because of the negative water potential values of the potato and the solutions. The potato tissue has a certain negative value. If the glucose solution has a greater negative value than the potato tissue, and is therefore of a higher concentration, the potato tissue will lose water by osmosis until the concentrations are equal or until the potato has lost all its water. Therefore the water will diffuse by osmosis out of ...

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