Phospholipids (Cambridge (CIE) AS Biology): Revision Note

The vital role of phospholipids

Structure

• Phospholipids are a type of lipid, therefore they are formed from the monomer glycerol and fatty acids

• Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO₄^3-)

• As the phosphate is polar it is soluble in water and described as hydrophilic

• The fatty acid ‘tails’ are non-polar and therefore insoluble in water and described as hydrophobic

• Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts)

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

Role of phospholipids

• They are the main component (building block) of cell membranes

• Due to the presence of hydrophobic fatty acid tails, a hydrophobic core is created when a phospholipid bilayer forms

  • This acts as a barrier to water-soluble molecules

• The hydrophilic phosphate heads form hydrogen 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

• 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