Practical 6 - Determination of water potential by measuring changes in mass or length

Question 1

What are the different ‘solutions’ that have different water potentials in this experiment?

Answer 1

The cytoplasm and the cell sap of a cell

Question 2

What separates cytoplasm and cell sap within in a cell?

Answer 2

A selectively permeable membrane

Question 3

Whats the selectively permeable membrane between cytoplasm and cell sap?

Answer 3

The cell membrane

Question 4

What is the cell membrane to the cytoplasm and the cell sap within a cell?

Answer 4

The selectively permeable membrane

Question 5

Within a cell, where will water move through a selectively permeable membrane (between what?)?

Answer 5

Into the solution with the lowest water potential
(Cell sap or cytoplasm)

Question 6

Which substance do we use which consists of different concentration levels?

Answer 6

Sodium chlorie

Question 7

What do we use as our tissue sample?

Answer 7

A vegetable - eg potato

Question 8

Why do we use potato in this experiment?

Answer 8

Contains millions of cells which would have an increase in mass if it gained water by osmosis
+Cells will stretch a small amount = increased length

Question 9

How many cylinders of tissue do we chop in this experiment + describe these?

Answer 9

15, of equal length

Question 10

What do we use to remove any periderm from the potato?

Answer 10

A scalpel

Question 11

Periderm

Answer 11

Skin

Question 12

Word for the skin on a potato

Answer 12

Periderm

Question 13

Why do we have to remove the periderm (skin) off of the potato?

Answer 13

Its Suberin makes it waterproof and would prevent osmosis

Question 14

What may prevent osmosis in the potato?

Answer 14

The periderm (skin), as its Suberin makes it waterproof

Question 15

How much of each solution do we place in a test tube?

Answer 15

30cm^3

Question 16

What do we need to do to the potato samples before placing them within the test tubes?

Answer 16

Either…
Measure the length to the nearest mm
Measure the mass to the nearest 0.01g

Question 17

What do we measure the mass to the nearest?

Answer 17

0.01g

Question 18

What do we measure the length to the nearest?

Answer 18

mm

Question 19

How long do we leave the potato samples in the solutions?

Answer 19

At least 45 minutes or overnight at 4 degrees Celsius

Question 20

At what temperature would we leave the potato samples in their solutions if let overnight?

Answer 20

4 degrees Celsius

Question 21

What do we do to the potato samples after removing them from their solutions?

Answer 21

Gently blot them and re-measure/re-weigh and record in a table

Question 22

What do we plot on the graph?

Answer 22

Mean percentage change against concentration of the solution

Question 23

How do we figure out the solute potential of the tissue?

Answer 23

Using a standard table
Convert from where the concentration of NaCl solution causes no change in mass (hits the x-axis)

Question 24

Which point on the graph shows where the concentration of NaCl solution causes no change in mass?

Answer 24

Where the line hits the x-axis

Question 25

What would be the results of this experiment and why?

Answer 25

Potato in lower concentrations of salt: The water potential of the outside of the potato is higher than the inside of the cell tissue, therefore water moves into the cell via osmosis, causing it to increase in mass/size

Question 26

What is the change in mass calculated as?

Answer 26

A % change

Question 27

Why is the change in mass calculated as a percentage change?

Answer 27

As the potato pieces are at different lengths and masses to start - we need to be able to make a comparison

Question 28

How can we ensure that the we can make comparisons in the changes in mass of the potato pieces?

Answer 28

Calculate it as a % change

Question 29

% change of mass formula

Answer 29

Change in mass ( end-start ) ————————————— Start mass or length

Question 30

What do we need to remember to write with the % change in mass or length if it’s LOST mass or length?

Answer 30

Negative value

Question 31

When would the mass or length of the potato sample decrease?

Answer 31

In high concentrations of NaCl, as the water potential of the outside of the cell is lower, so water diffuses out of the cell via osmosis

Question 32

Where does the line cross the x-axis on the graph?

Answer 32

Where there’s no change in mass or length

Question 33

What goes along the x and y axis on the graph?

Answer 33

x - concentration of bathing solution y - % change

Question 34

What’s actually occurring when there’s no change in mass or length?

Answer 34

There’s no net movement of water in or out of the potato tissue, as the water potentials of the tissue + bathing solution are equal

Question 35

What are equal when there’s no net movement of water in or out of the potato tissue?

Answer 35

The water potentials of the tissue and the bathing solution

Question 36

What else is equal when the water potentials of the tissue and bathing solution are equal?

Answer 36

The solute and water potentials

Question 37

When are the solute and water potentials equal?

Answer 37

When the water potentials of the tissue and bathing solution are equal

Question 38

What are the cells in when the water potentials of the solute and water are equal?

Answer 38

Incipient plasmolysis

Question 39

When are cells in incipient plasmolysis?

Answer 39

When the solute and water potentials are equal

Question 40

Whats the pressure potential when the cell is in incipient plasmolysis?

Answer 40

0kPa

Question 41

When is the cells pressure potential 0kPa?

Answer 41

When cells are in incipient plasmolysis

Question 42

How would this experiment be different using a sweet potato and why?

Answer 42

No decrease in mass (Very high sucrose concentration - very low water potential)

Question 43

How would we alter the experiment for a sweet potato?

Answer 43

Increase the sucrose concentration of the external medium until a point is reached where water will flow out of the sweet potato