Phospholipids (AQA AS Biology): Revision Note

Phospholipids: structure & function

• Phospholipids are a type of lipid molecule that are a major component of cell membranes. They are amphipathic, meaning they contain both:

  • a hydrophilic head

  • hydrophobic tails

Structure

• A typical phospholipid consists of:

  • one glycerol molecule

  • two fatty acid chains

    • These are non-polar and hydrophobic

  • one phosphate group

    • This is polar and hydrophilic

    • Sometimes attached to an additional polar group (e.g. choline)

• The fatty acids are bonded to glycerol by ester bonds, while the phosphate group is attached to glycerol via a phosphoester bond

Monolayers and bilayers

• As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers in water

Function

• Hydrophobic fatty acid tails create a hydrophobic core when a phospholipid bilayer forms

  • This acts as a barrier to water-soluble molecules

• The hydrophilic phosphate heads form H-bonds with water, allowing the cell membrane to be used to compartmentalise

  • This enables the cells to organise specific roles into organelles, helping with efficiency

• The composition of phospholipids contributes to the fluidity of the cell membrane

  • If there are mainly saturated fatty acid tails, then the membrane will be less fluid

  • If there are mainly unsaturated fatty acid tails, then the membrane will be more fluid

• Phospholipids control membrane protein orientation

  • Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer