RP4 - investigation into effect of a named variable on permeability of cell surface membranes

Question 1

Method to investigate effect of a named variable on permeability of cell membrane

Answer 1

• Cut identical cubes of plant tissue (beetroot) of same type using scalpel
• Rinse to remove pigment released during cutting/blot on paper towel
• Add same number of cubes to 5 test tubes containing same volume of water
• Place each tube in a water bath at different temperatures
• Leave for same length of time
• Remove plant tissue and measure pigment release by measuring intensity of colour, or concentration of solution semi quantitatively or quantitatively

Question 2

2 methods to estimate pigment concentration in a solution

Answer 2

• Semi quantitative
  -> use a known concentration of extract and distilled water to prepare dilution series
  -> compare results with colour standards and estimate concentration
• Quantitative
  -> measure absorbance of light of known concentrations using colorimeter
  -> draw calibration curve - plot graph of absorbance against concentration and draw line of best fit
  -> read off sample absorbance value on curve to find associated concentration

Question 3

Why is the beetroot is washed before placing it in water

Answer 3

Wash off pigment on surface -> shows that release is only due to variable

Question 4

Why is each test tube regularly shaken

Answer 4

Ensures all surfaces of cubes remain in contact with liquid -> maintains concentration gradient for diffusion

Question 5

Why does the volume of water need to be controlled

Answer 5

• Too much water would dilute the pigment -> solution appears lighter/more light passes through in colorimeter than expected
• So results are comparable

Question 6

How to ensure beetroot cylinders are kept at the same temperature throughout experiment

Answer 6

• Take readings in intervals throughout experiment of tube temperature using thermometer
• Use corrective measure if temperature has fluctuated

Question 7

Issues with comparing to a colour standard

Answer 7

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

Question 8

What does high absorbance suggest about cell membrane

Answer 8

• More permeable or damaged
• Since more pigment leaks out, making the surrounding solution more concentrated

Question 9

How does temperature affect permeability of cell membrane

Answer 9

• As temperature increases, permeability increases
  -> phospholipids gain kinetic energy so fluidity increases
  -> transport proteins denature at higher temperatures as hydrogen bonds break, changing the tertiary structure
• At lower temperatures, permeability increases
  -> ice crystals may form, which pierce cell membrane and increase permeability

Question 10

How does pH affect permeability of cell membrane

Answer 10

• High or low pH increases permeability
  -> transport proteins denature as - – H/ionic bonds break, changing tertiary structure

Question 11

How do lipid soluble solvents (alcohol) affect permeability of cell membrane

Answer 11

• As concentration increases, permeability increases
• Ethanol may dissolve phospholipid bilayer, creating gaps