Estimating Water Potential in Plants (CIE A Level Biology)

• It is possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate the water potential of the plant tissue itself
• The most common osmosis practical of this kind involves cutting cylinders of potato and placing them into solutions with a range of different water potentials (usually sucrose solutions of increasing concentration – at least 5 different concentrations are usually required)

Method

• The required number of potato cylinders are cut (one for each of the solutions you are testing – or more than one per solution if you require repeats)
• They are all cut to the same length and, once blotted dry to remove any excess moisture, their initial mass is measured and recorded before placing into the solutions
• They are left in the solutions for a set amount of time (eg. 30 minutes), usually in a water bath (set at around 30^o)
• They are then removed and dried to remove excess liquid
• The final length and mass of each potato cylinder is then measured and recorded

You will need to use apparatus appropriately to measure out the volumes of your solutions and record your measurements

Analysis

• The percentage change in mass for each potato cylinder is calculated
• If there is a potato cylinder that has neither increased nor decreased in mass, it means there was no overall net movement of water into or out of the potato cells
• This is because the solution that this particular potato cylinder was in had the same water potential as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient and therefore no net movement of water into or out of the potato cells
• The concentration of sucrose inside the potato cylinders can be found if a graph is drawn showing how the percentage change in mass changes with the concentration of sucrose solution – the point at which the line of best fit crosses the x-axis is the concentration of sucrose inside the potato cylinders

A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution had a higher water potential than the potato. A negative percentage change suggests the opposite

• A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution had a higher water potential than the potato
  • This would make the potato cells turgid, as the water exerts turgor pressure (or hydrostatic pressure) on the cell walls – the potatoes will feel hard

• A negative percentage change suggests the opposite, that is, the solution had a lower water potential than the potato
  • The potato cylinder in the strongest sucrose concentration will have decreased in mass the most as there is the greatest concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)
  • This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the potato cylinder – the potato cylinders will feel floppy
  • If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall