Core practical 6

Question 1

What is the aim of this practical?

Answer 1

To determine the water potential of plant cells

Question 2

What is water potential?

Answer 2

how likely it is for water molecules to diffuse out of or into a solution.

Question 3

What is the water potential of pure water?

Answer 3

Pure water (i.e. water with no solutes) has a water potential value of 0.As you add solutes (i.e. sugar or salt) to water, the water potential value is lowered. This therefore means that the water potential value for any solution is always negative

Question 4

Method for determining the water potential of plant epidermal cells

Answer 4

1. Take 5 thin sections of red part of red onion (1cm2)
2. Label 5 watch glasses and add 5 different concs of sucrose solution
3. Add a piece of tissue to each glass and leave for 20 mins
4. Put a drop of each solution on slide
5. Remove each tissue piece using forceps and float each the matching slice
6. Cover each section with coverslip and observe
7. Count 25 cells and record how many show plasmolysis, record findings in table

Question 5

Why do we take thin sections of onion?

Answer 5

So it is easy to view under microscope

Question 6

What are the Control variables

Answer 6

-time left in solution
-number of cells counted in total (25)
-part of onion used
-same onion

Question 7

What is the independent variable

Answer 7

Conc of sucrose solution

Question 8

What is the dependent variable

Answer 8

% plasmolysed cells

Question 9

Equipment

Answer 9

-measuring cylinders
-5cm3 syringes
-filter paper
-scalpel
-labels
-microscope and slides
-coverslips
-forceps
-pipettes

Question 10

Evaluation issues

Answer 10

-difficult to obtain a single layer of red stained cells
-difficult to accurately measure just 25 cells by looking down the microscope
-the pigment varies between different parts of the onion

Question 11

Calculations and outcome

Answer 11

-work out % of cells showing plasmolysis at each conc, repeat and calculate mean
-as you increase conc, more cells become plasmolysed
-at incipient plasmolysis there is no more turgor pressure as cell membrane is no longer being pushed against the cell wall
-water potential = osmotic potential

Question 12

What is plasmolysis

Answer 12

Plasmolysis is when the protoplasm (inner contenets of cell-cytoplasm, nucleus, mitochondria etc) of a plant cell begins to shrink away from the cell wall.

Question 13

What is incipient plasmolysis

Answer 13

Incipient plasmolysis is the point at which this first begins to occur. At this point there is no turogor pressure anymore but the cell also hasnt started to shrivel yet- its ‘neutral’

This occurs
when water potential and osmotic potential are equal - at the point where the
contents of the plant cell are isotonic to the surrounding environment.

Question 14

How do we meaure incipient plasmolysis?

Answer 14

Because incipient plasmolysis is very difficult to observe with a microscope, it can also be interpreted as the point at which 50% of cells in a tissue are plasmolysed.

Question 15

The osmotic potential of the cells is equal to the water potential only at the point of incipient
plasmolysis. Use the equation ψ = P + π to explain why water potential and osmotic potential
are equal at this point.

Answer 15

Water potential is described by the equation ψ = P + π. At the point of incipient plasmolysis the
cell membrane is just beginning to peel away and exerts no pressure on the cell wall, so P = 0.
Therefore ψ must equal π. There is no net movement of water by osmosis at this point.

Question 16

Safety

Answer 16

Take care with glassware, mounting needles and cutting equipment.