Required practical 4 - Investigation into the effect of a named variable on the permeability of cell-surface membranes Flashcards
Describe a method to investigate the effect of a named variable (eg. temperature) on the permeability of cell-surface membranes
- Cut equal sized / identical cubes of plant tissue (eg. beetroot) of same age / type using a scalpel
- Rinse to remove pigment released during cutting or blot on paper towel
- Add same number of cubes to 5 different test tubes containing same volume of water (eg. 5 cm3)
- Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50oC)
- Leave for same length of time (eg. 20 minutes)
- Remove plant tissue and measure pigment release by measuring intensity of colour or concentration of surrounding solution semi-quantitatively or quantitatively
Describe the semi quantitative method to estimate pigment concentration in a solution
● Use a known concentration of extract and distilled water to prepare a dilution series
● Compare results with these ‘colour standards’ to estimate concentration
Describe the quantitative method to estimate pigment concentration in a solution
● 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
Explain why the beetroot is washed before placing it in water
● Wash off any pigment on surface
● To show that release is only due to [named variable]
Explain why each test tube containing cubes of plant tissue is regularly shaken
● To ensure all surfaces of cubes remain in contact with liquid
● To maintain a concentration gradient for diffusion
Explain why the volume of water needs to be controlled
● Too much water would dilute the pigment so solution will appear lighter / more light passes through in colorimeter than expected
● So results are comparable
Explain how you could ensure beetroot cylinders were kept at the same
temperature throughout the experiment
● Take readings in intervals throughout experiment of temperature in tube using a digital thermometer / temperature sensor
● Use corrective measure if temperature has fluctuated
Describe the issues with comparing to a colour standard
● Matching to colour standards is subjective
● Colour obtained may not match any of colour standards
What does a high absorbance suggest about the cell-membrane?
● More permeable / damaged
● As more pigment leaks out making surrounding solution more concentrated (darker)
Explain how temperature affects permeability of cell-surface membranes
● 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
Explain how pH affects permeability of cell-surface membranes
● High or low pH increases cell membrane permeability
○ Transport proteins denature as H / ionic bonds break, changing tertiary structure
Explain how lipid-soluble solvents eg. alcohol affect permeability of cell-surface membranes
● As concentration increases, cell membrane permeability increases
● Ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer (creating gaps)
