RP4 - Investigation into the effect of a named variable on the permeability of cell-surface membranes.

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

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

● More permeable / damaged

● As more pigment leaks out making surrounding solution more concentrated (darker)

control variables

- same size/shape cubes

- same age/type/variety of plant tissue

- same volume of water

- length of time left in water bath

What are the issues with comparing to a colour standard?

● Matching to colour standards is subjective

● Colour obtained may not match any of colour standards

Why wash the beetroot before placing it in water?

● Wash off any pigment on surface

● To show that release is only due to [named variable]

Why regularly shake each test tube containing cubes of plant tissue?

● To ensure all surfaces of cubes remain in contact with liquid

● To maintain a concentration gradient for diffusion

Why control the volume of water?

● Too much water would dilute the pigment so solution will appear lighter / more light passes through in colorimeter than expected

● So results are comparable

How could you 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

explain how temperature affects permeability of cell-surface membranes

● As temperature increases, permeability increases

○ Phospholipids gain kinetic energy and fluidity increases ○ Transport proteins denature at high temperatures as H bonds break, changing tertiary structure

● At very low temperatures, 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 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 of alcohol increases, permeability increases

● This is because ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer, and cause gaps to form