Required Practical 4

Question 1

Describe a method to investigate the effect of temperature on the permeability of cell-surface membranes.

Answer 1

1. Cut equal-sized/identical cubes of plant tissue (e.g. beetroot) of same age and 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 tubes containing same volume of water.
4. Place each test tube in a water bath at a different temperature (e.g. 10, 20, 30, 40, 50 °C).
5. Leave for same amount of time and occasionally shake test tubes to ensure surfaces of cube in contact with liquid.
6. Remove beetroot and measure intensity of colour of surrounding solution:
   • SEMI-QUANTITATIVELY
     
     • Use a known concentration of extract and distilled water to prepare dilution series (colour standards).
     • Compare results with colour standards to estimate concentration.
   • QUANTITATIVELY
     
     • Measure absorbance of light of known concentrations using colorimeter.
     • Draw a calibration curve - plot a graph of absorbance (on y-axis) against concentration of extract (on x-axis) and draw a line/curve of best fit.
     • Absorbance value for sample read off calibration curve to find associated extract concentration.

Question 2

What are issues with comparing to a colour standard?

Answer 2

• Matching to colour standards is subjective.
• Colour obtained may not match any 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 the variable being tested.

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 will pass through in colorimeter.
• It ensures results are comparable.

Question 6

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

Answer 6

• Taking readings in intervals throughout the experiment of temperature using a digital thermometer/temperature sensor.
• Use corrective measures 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 membrane.

Answer 8

As temperature increases, permeability increases.

• Phospholipids gain kinetic energy and fluidity increases.
• Transport proteins denature at high temperatures as hydrogen 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 membrane.

Answer 9

High or low pH increases permeability.

• Transport proteins denature as hydrogen/ionic bonds break, changing tertiary structure.

Question 10

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

Answer 10

As concentration increases, permeability increases.

• Ethanol may dissolve phospholipid bilayer forming gaps.