Membrane Permeability

What 3 factors affect membrane permeability ?

1. Temperature

2. Solvent types

3. Solvent concentration

How can one investigate how temperature affects membrane permeability ?

1. Cut five equal sized pieces of beetroot and rinse them to remove any pigment released during cutting.

2. Place the five pieces in five different test tubes, each with 5 cm' of water.

3. Place each test tube in a water bath at a different temperature, e.g. 10°C, 20 °C, 30 °C, 40°C, 50 °C, for the same length of time.

4. Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.

5. Now you need to use a colorimeter . The higher the permeability of the membrane, the more pigment is released, so the higher the absorbance of the liquid

How does temperature affect membrane permeability ?

Temperatures below 0^oC

• Phospholipids have little energy so don’t move much

• Packed closely together and membrane is rigid

• Channel and Carrier proteins in membrane deform , increasing permeability of membrane

• Ice crystals form and pierce membrane so it is highly permeable when it thaws

Temperatures 0^oC to 45^oC

• Phospholipids not packed as tight so can move , making membrane partially permeable

• As temperature increases , phospholipids can move more as they have more energy , increasing permeability of membrane

Temperatures above 45^oC

• Phospholipids start to melt and break down so membrane gets more permeable

• Water inside cell expands, putting pressure on membrane

• Channel and Carrier proteins deform so they can’t control what enters or leaves the cell , increasing permeability of membrane

How do solvents affect membrane permeability ?

• Increase permeability of cell membranes as solvents dissolve the lipids in the membrane so it loses its structure

• Different solvents increase permeability more than others (E.g: ethanol increases permeability more than methanol )

• Increasing concentration of solvent increases permeability ( Increasing alcohol concentration percentage increases permeability )