CP - CC3 - Determination of solute potential by measuring the degree of incipient plasmolysis

When is there no net movement of water in our out of a cell? (2)

when the water potential of plant tissue and its surroundings are equal, it is at incipient plasmolysis

What is occurring in a cell at incipient plasmolysis? (3)

cell membrane is withdrawn from the cell wall in places, cell contents exert no pressure on the cell wall, seen under a microscope

What is the practical definition of incipient plasmolysis for this experiment?

when half of the cells are plasmolysed and half are not plasmolysed

What is the equation describing water potential?

water potential of cell = solute potential + pressure potential

At incipient plasmolysis, what is the pressure potential of the cell?

0

What does this mean about the water potential of a cell at incipient plasmolysis?

water potential is equal to solute potential

What apparatus are needed for this experiment? (12)

white tile, fine forceps, fine scissors, red onion, 5 × 9 cm petri dishes or 100cm³ beakers or watch glasses, distilled water, sodium chloride solutions, stop clock, microscope slides, cover slips, microscope, dropping pipettes

What sodium chloride solutions concentrations should be used? (4)

0.2,0.4,0.6,0.8 mol dm^-3

Step 1

label 5 petri dishes containing 10cm³ of one of the concentrations of sodium chloride

Step 2 of getting a sample of red onion (4)

insert the fine forceps tip just under the upper epidermis of the onion, do not penetrate underlying mesophyll, grip the epidermis maintaining tension and pull epidermis off the mesophyll, place into distilled water

Step 3

repeat step 2 to get one square for each petri dish

Step 4

leave at room temperature for at least 30 minutes

Step 5 (2)

spread tissue out on microscope slide so it is not folded, use a scalpel to gently cut a 0.5 × 0.5 cm square

Step 6 (2)

Add two drops of bathing solution, apply a cover slip

Step 7

blot excess solution with filter paper

Step 8 (2)

use 10x then 40x objective lens to examine cells in a field of view, count the number of turgid cells and the number of plasmolysed cells

Step 9

Repeat steps using all concentrations of sodium chloride solutions

Step 10 (3)

record results in a table, plot a graph of percentage of plasmolysed cells against the concentration of the bathing solution, use the graph to read the concentration of bathing solution that would produce plasmolysis in 50% of cells

Outline the risk assessment of the hazard of scalpel blades using ‘due to’ and ‘whilst’ (5)

scalpel blades are sharp, there is a risk of cutting your skin due to using the scalpel whilst cutting the tissue into a square, ensure you keep your fingers away from the blade of the scalpel when cutting, cut away from the body, place the scalpel down on the table when not in use

Outline the risk assessment of the hazard of using electrical equipment during a wet practical using ‘due to’ and ‘whilst’ (3)

there is a risk of electrocution due to water meeting electrical equipment whilst using the microscopes, ensure hands are dry when using the microscope, mop up any spills immediately

Outline the risk assessment of the hazard of broken glass using ‘due to’ and ‘whilst’ (2)

there is a risk of cutting your skin due to cover slip braking while placing it on the sample, place a paper towel over the coverslip before pressing down on it

How can you ensure that the same cell is not counted more than once?

a tracking system across the specimen may be used

How are successful fields of view separated?

by the width of at least one cell

What should you do when cells occur on the edge of the field of view and are only partially visible? (3)

a rule should be made for scoring, like a partially visible cell is never counted, or a cell is scored if it is at least half visible

Why is a curve of best fit used in this situation? (2)

it takes into account all of the data, rather than only two data points either side of that representing 50% plasmolysis