L2 - membrane permeability

• All membranes are permeable to water. They control the passage of different substances into and out of the cells (and organelles)
• Those that are permeable to water and some other solutes are described as; semi-permeable or partially permeable membranes. If membranes lose their structure, they lose control of what enters and leaves the cell, and cell processes will be disrupted
• There are two key factors that affect membrane permeability:

  -temperature

  -concentration of solvent
• Temperature and permeability:

  -phospholipids in a cell membrane move constantly

  -as temperature increases, the phospholipids have more kinetic energy and so move more

  -this increases fluidity and the membrane loses its structure until the temperature increases so much that the membrane structure fully breaks down

  -this increases permeability making it easier for particles to cross

  -carrier and channel proteins denature as the temperature rises and this also affects permeability
• Solvent concentration and permeability:

  -water is a polar solvent and necessary for membranes to form correctly to form a hydrophobic core in the lipid bilayer

  -solvents are less polar than water (alcohols) or non polar (eg. benzene)

  -these solvents dissolve membranes disrupting cells

  -the alcohol molecules can enter the membrane and sit between the phospholipids and this disrupts the membrane structure

  -this makes it more fluid and therefore permeable
• When neuronal membranes are disrupted by alcohols they can no longer transmit nerve impulses correctly. This causes behavioural changes after people have consumed alcohol
• Effect of alcohol on membrane permeability:

  -a larger absórbanme value indicates a more permeable membrane. This is due to the level of pigments that can exit the membrane into the solution

  
• Colorimeter:

  -a device that shines a beam of light through a sample and detects (via a photoelectric cell) how much light has passsed through

  -this is recorded as either amount of light absorbed or transmitted

  -you need to choose a filter (red, green, blue) or wavelength to transmit