Osmosis

• Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
• A partially permeable membrane is a membrane with very small holes in it. So small in fact only tiny molecules like water can pass through them and bigger molecules e.g. sucrose can’t.
• In osmosis water molecules actually pass both ways through the membrane. This happens because water molecules move about randomly all the time. But because there are more water molecules on one side than on the other, there’s a steady net flow of water into the region with fewer water molecules.
• Osmosis is a type of diffusion - passive movement of water particles from an area of higher water concentration to an area of lower water concentration.
• You can observe how sugar solutions affect plant tissue by using an experiment. The first two steps to this would be:

  1. You cut up a potato into identical cylinders, and get some beakers with different sugar solutions in them. One should be pure water and another should be a very concentrated sugar solution (e.g. 1 mol/dm^3). Then you can have a few others with concentrations in between (e.g. 0.2 mol/dm^3, 0.4 mol/dm^3, 0.6 mol/dm^3, etc.).
  2. You measure the mass of the cylinders, then leave one cylinder in each beaker for 24 hours or so.
• When carrying out the experiment to observe how sugar solutions affect plant tissue. Once you’ve measured the mass of the cylinders and left them for 24 hours. The next steps would be to take the cylinders out of each beaker, dry them with a paper towel and measure their masses again. If the cylinders have drawn in water by osmosis, they’ll have increases in mass. If water has been drawn out, they’ll have decreased in mass.
• In the experiment observing how sugar solutions affect plant tissue, the dependent variable is the chip mass and the independent variable is the concentrations of the sugar solution. All other variables (vol of solution, temperature, time, type of sugar used, etc.) must be kept the same in each case (control variable) or the experiment won’t be a fair test.
• In the experiment to observe how sugar solutions affect plant tissue, like any experiment, you need to be aware of how errors may arise. Sometimes they may occur when carrying out the method e.g. if some potato cylinders were not fully dried, the excess water would have given a higher mass, or if water evaporated from the beakers, the concentration of the sugar solutions would change. You can reduce the effect of these errors by repeating the experiment and calculating a mean percentage change at each conc.