Topic 2 : 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 of plant tissue (eg. beetroot) of same age 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. 5 cm3
   )
4. Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50
   oC)
5. Leave for same length of time (eg. 20 minutes)
6. Remove plant tissue and measure pigment release by measuring intensity of colour or
   concentration of surrounding solution semi-quantitatively or quantitatively (see below)

Question 2

Describe two methods to estimate pigment concentration in a solution

Answer 2

Semi-
quantitative

● Use a known concentration of extract and distilled water to prepare a dilution series
● Compare results with these ‘colour standards’ to estimate concentration

Quantitative :

● Measure absorbance (of light) of known concentrations using a colorimeter
● Draw a calibration curve → plot a graph of absorbance (y) against concentration of
extract (x) and draw a line / curve of best fit
● Read off sample absorbance value on curve to find associated concentration

Question 3

Explain why the beetroot is washed
before placing it in water. (2)

Answer 3

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

Question 4

Explain why each test tube containing
cubes of plant tissue is regularly
shaken. (2)

Answer 4

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

Question 5

Explain why the volume of water needs
to be controlled. (1)

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

Explain how you could ensure beetroot
cylinders were kept at the same
temperature throughout the
experiment. (2)

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

Describe the issues with comparing to
a colour standard. (2)

Answer 7

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

Question 8

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

Answer 8

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

Question 9

Explain how temperature affects permeability of cell-surface membranes

Answer 9

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

Question 10

Explain how pH affects permeability of cell-surface membranes

Answer 10

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

Question 11

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

Answer 11

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