6. Determine the water potential of a plant tissue.

Question 1

What organism is used when determining the water potential of a plant tissue?

Answer 1

An Onion bulb (Onion epidermis)

Question 2

What does ‘turgid’ mean?

Answer 2

When a plant cell is in a HYPOTONIC solution, it takes up water by osmosis and starts to swell, but the cellulose cell wall prevents the cell from bursting.

Question 3

What does ‘plasmolysed’ mean?

Answer 3

When a plant is in a HYPERTONIC solution, so much water leaves the cell that the vacuole will be reduced and the protoplasm will shrink away from the cell walls.

Question 4

Background information:

What is ‘incipient plasmolysis’?

Answer 4

When plant cells are placed in a range of solutions, the concentration that has the same OSMOTIC potential as the cell sap is just concentrated to cause sufficient water loss so that the plasma membrane begins to separate from the cell wall (incipient plasmolysis).

Question 5

How do we measure ‘incipient plasmolysis’?

Answer 5

It is difficult to measure this, because plasmolysis occurs in different plant tissues at different rates, so the OSMOTIC POTENTIAL of the solution that causes visible plasmolysis in 50 percent of the cells is therefore used. This is the concentration equivalent to the solute potential of cell sap.

Question 6

What variables are controlled when measuring the water potential of plant tissue?

Answer 6

• Plant tissue size, length, width, diameter.
• plant age, variety and storage time (use the same onion)
• time left in the watch glass with the salt solution (20 mins)
• Temperature.

Question 7

Hypothesis:

Answer 7

As the concentration of the salt solution increases, there will be a higher percentage of cells plasmolysed.

Question 8

Method:

Answer 8

1. Cut six, thin, 1cm squared sections of plant tissue. One cell thick.
2. Label six watch glasses with the different concentrations of salt solution: 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 mol/dm cubed.
   Transfer some of each salt solution into the appropriate watch glass with a pipette. Place one of the five tissue sections onto each of the watch glasses and leave for 20 minutes.
3. Remove each tissue section with forceps and place on a slide labelled with the appropriate concentration. Put a drop of the corresponding solution on the slide and float the tissue on to it.
4. Cover each section with coverslip, observe under microscope.
5. Observe 25 cells and record how many show plasmolysis. Record findings in a table. Include column recording percentage of cells showing plasmolysis.

Question 9

Risks:

Answer 9

Glass equipment:

be cautious when handling glass. Avoid touching broken glass. Inform teacher.

Question 10

Analysis of results:

Answer 10

• Work out percentage of cells showing plasmolysis at each conc.
• Draw a line graph of the percentage of cells plasmolysed against the concentration of the solution.
• From graph, estimate the percentage at which exactly 50 percent of the cells would be plasmolysed. This can be taken to represent the concentration at which incipient plasmolysis occurs.

Water potential of the experimental solution= osmotic potential of the cells.

Question 11

Explain the expected results:

Answer 11

The higher the concentration of the cell, the lower its water potential will be.

This means that in more concentrated salt solutions, more water will move out of plant cells, down the concentration gradient by osmosis, so more cells will be plasmolysed.