Required practical 4

Question 1

Describe a method to investigate the effect of a named variable (eg. temperature) on the permeability of cell-surface membranes

Answer 1

1. Cut equal sized / identical cubes of plant tissue (eg. beetroot) of same age / type using a scalpel
2. Rinse to remove pigment released during cutting or blot on paper towel
3. Add same number of cubes to 5 different test tubes containing same volume of water (eg. 5cm3)
4. Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50
   oC)
5. Leave for same amount of time (eg. 20 mins)
6. Remove beetroot and measure intensity of colour of surrounding solution:
   ○ Measure absorbance (of light) of known concentrations using a colorimeter
   ○ Draw a calibration curve → plot a graph of absorbance (y) against conc. of extract (x) and draw a line / curve of best fit
   ○ Absorbance value for sample read off calibration curve to find associated extract conc.

Question 2

What are the issues with comparing to a colour standard?

Answer 2

● Matching to colour standards is subjective
● Colour obtained may not match any of colour standards

Question 3

Why wash the beetroot before placing it
in water?

Answer 3

● Wash off any pigment on surface
● To show that release is only due to [named variable]

Question 4

Why regularly shake each test tube
containing cubes of plant tissue?

Answer 4

● To ensure all surfaces of cubes remain in contact with liquid
● To maintain a concentration gradient for diffusion

Question 5

Why control the volume of water?

Answer 5

● Too much water would dilute the pigment so solution will appear lighter / more light passes through in colorimeter than expected
● So results are comparable

Question 6

How could you ensure beetroot cylinders
were kept at the same temperature
throughout the experiment?

Answer 6

● Take readings in intervals throughout experiment of temperature in tube using a digital thermometer / temperature sensor
● Use corrective measure if temperature has fluctuated

Question 7

What does a high absorbance suggest about the cell-membrane?

Answer 7

● More permeable / damaged
● As more pigment leaks out making surrounding solution more concentrated (darker)

Question 8

Explain how temperature affects permeability of cell-surface membranes

Answer 8

● As temperature increases, permeability increases
○ Phospholipids gain kinetic energy and fluidity increases
○ Transport proteins denature at high temperatures as H bonds break, changing tertiary structure
● At very low temperatures, permeability increases
○ Ice crystals can form which pierce the cell membrane and increase permeability

Question 9

Explain how pH affects permeability of cell-surface membranes

Answer 9

● High or low pH increases permeability
○ Transport proteins denature as H / ionic bonds break, changing tertiary structure

Question 10

Explain how lipid-soluble solvents eg. alcohol affect permeability of
cell-surface membranes

Answer 10

● As concentration increases, permeability increases
● Ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer (gaps form)