required practical 4

Question 1

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

Answer 1

1. cut equal size/identical cubes of plant tissue (e.g. beetroot) of same age/type using a scalpel
2. rise to remove pigment released during cutting or blot on paper towel
3. ass same number of cubes to 5 different test tubes containing same volume of water (e.g. 5cm^3)
4. place each test tube in water bath at a different temperature (e.g. 10,20,30,40,50°C)
5. leave for same length of time
6. remove beetroot and measure pigment release by measuring intensity of colour or concentration of surrounding solution semi-quantitively or quantitively

Question 2

Describe two methods to estimate pigment concentration in a solution

Answer 2

1. 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
2. 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

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

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

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

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

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 e.g.. 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)