Membrane Fluidity (HL) (HL IB Biology)

• Phospholipids contain glycerol, a phosphate group, and two fatty acid chains
• Fatty acids can vary in two ways:
  • Length of the hydrocarbon chain
  • The fatty acid chain may be saturated or unsaturated
• Saturated fatty acids
  • Every carbon atom is bonded to 4 other atoms, meaning that each carbon in the chain is linked to 2 hydrogen atoms
    • The chain can be said to be 'saturated' with hydrogens; it contains as many hydrogen atoms as it possibly can
  • Saturated fatty acids are straight, allowing the molecules to pack together tightly
  • They therefore have higher melting points, so their presence in cell membranes allow membranes to maintain stability at higher temperatures
• Unsaturated fatty acids
  • Contain one or more double bonds between carbon atoms
    • One double bond - mono-unsaturated
    • More than one double bond = polyunsaturated
  • Unsaturated fatty acids have bends, or kinks, in the chain, meaning that they cannot pack together so tightly
  • Unsaturated fatty acids have lower melting points so they allow membranes to be fluid and flexible

Fatty acids can be saturated (top) or unsaturated (bottom); this affects the shape, and therefore the properties of the fatty acid

Fatty acids & regulating membrane fluidity

• Bacteria do not regulate their internal temperature, so their cell membranes are subject to temperature change
  • This means that they require mechanisms to overcome temperature fluctuations
  • Some bacteria species produce enzymes called fatty acid desaturases which increase the number of double bonds within a fatty acid as part of the membrane; this helps to maintain membrane fluidity, particularly during exposure to colder temperatures
• Deep-sea marine organisms have to contend with extreme temperatures
  • Correlations have been found between sea temperature and membrane-fluidising lipid components, such as polyunsaturated fatty acids
• Plants, such as Arabidopsis thaliana, have shown fatty acid unsaturation pathways that appear to have key roles in the acclimatisation of membranes to high temperature

• Cholesterol is an important membrane lipid
• Just like phospholipid molecules, cholesterol molecules have hydrophobic and hydrophilic regions
  • Their chemical structure allows them to exist within the bilayer of the membrane
• Cholesterol affects the fluidity and permeability of cell membranes
  • It maintains membrane fluidity at low and high temperatures
    • It disrupts the close-packing of phospholipids, increasing the flexibility of the membrane at low temperatures
    • It holds the fatty acid tails together, providing increased membrane stability at high temperatures
  • It acts as a barrier, fitting in the spaces between phospholipids
    • This prevents water-soluble substances from diffusing across the membrane

Cholesterol structure diagram

The structure of a cholesterol molecule gives it a hydrophobic region and a hydrophilic region