2.5.9 Practical: Investigating Water Potential

Practical 1: Investigating water potential using potato cylinders

• 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

Results

• The percentage change in mass for each potato cylinder is calculated

To find the percentage change in mass, the change in mass must be divided by the initial mass and then multiplied by 100

• 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
  • The gain of water makes 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)
  • 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

• 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
• 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

Analysis

• 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.

Practical 2: Investigating water potential using onion cells

• Evidence of osmosis occurring in plant cells can be shown when the cells undergo plasmolysis:
  • If a plant cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution), water will leave the cell through its partially permeable cell surface membrane by osmosis
  • As water leaves the vacuole of the plant cell, the volume of the cell decreases
  • The protoplast (living part of the cell inside the cell wall) gradually shrinks and no longer exerts pressure on the cell wall
  • As the protoplast continues to shrink, it begins to pull away from the cell wall
  • This process is known as plasmolysis – the plant cell is plasmolysed

• This process can be observed using epidermal strips (sections of the very thin outer layer of tissue in plants)
  • Plants with coloured sap (such as red onion bulbs, rhubarb petioles and red cabbage) make observations easier

• The epidermal strips are placed in a range of molarities of sucrose solution or sodium chloride solutions, of gradually decreasing water potential
• The strips are then viewed under a light microscope and the total number or percentage of onion cells that have undergone plasmolysis can be counted
  • Plasmolysis may take several minutes to occur

Light micrograph of normal red onion cells alongside those that have plasmolysed (artistic impression). The cells on the left are epidermal cells that have been immersed in distilled water, whilst the cells on the right are epidermal cells that have been immersed in 1.0 mol dm⁻³ sucrose solution.