Syllabus Edition

First teaching 2023

First exams 2025

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Membrane Fluidity (HL) (HL IB Biology)

Revision Note

Cara Head

Author

Cara Head

Expertise

Biology

Fatty Acid Composition

  • 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

Saturated fatty acid example (1)Unsaturated fatty acid example (1)

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

  • 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

Cholesterol structure_1

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

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Cara Head

Author: Cara Head

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding