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First exams 2025

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Phospholipids (CIE AS Biology)

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Cara Head

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Cara Head

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  • Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO43-)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

Phospholipid Structure Diagram

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

Monolayer and Bilayer Diagram

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 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

Phospholipids vs Triglycerides Table

  Phospholipid Triglyceride
Number of fatty acid tails 2 3
Presence of phosphate Yes No
Polar or non-polar Polar phosphate head Non-polar
Number of water molecules released during formation 3 3
Function Cell membrane component  Energy storage

Examiner Tip

Ensure you know the difference between phospholipids and triglycerides!

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Cara Head

Author: Cara Head

Expertise: Biology

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