AQA AS Biology

Revision Notes

1.2.3 Phospholipids

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  • Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO43-)
  • As the phosphate is polar it is soluble in water (hydrophilic)
  • The fatty acid ‘tails’ are non-polar and therefore insoluble in water (hydrophobic)

Phospholipid structure, downloadable IGCSE & GCSE Biology revision notes

Phospholipids are the major components of cell surface membranes. They have fatty acid tails that are hydrophobic and a phosphate head, that is hydrophilic, attached to a glycerol molecule

  • 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

Phospholipid monolayer and bilayer, downloadable IGCSE & GCSE Biology revision notes

In the presence of water due to the hydrophobic and hydrophilic parts phospholipids will form monolayers or bilayers

Role

  • 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 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
  • 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

Phospholipids v Triglycerides Table

Differences between Phospholipids and Triglycerides table, downloadable AS & A Level Biology revision notes

Exam Tip

Ensure you know the difference between phospholipids and triglycerides!

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