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First exams 2025

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Lipid Bilayers (HL IB Biology)

Revision Note

Cara Head

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Cara Head

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Lipid Bilayers: Basis of Cell Membranes

  • Phospholipids form the basic structure of cell membranes, which are formed from phospholipid bilayers
  • They are formed by a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails
  • As phospholipids have a hydrophobic and hydrophilic part they are known as amphipathic
    • The phosphate head of a phospholipid is polar and therefore soluble in water (hydrophilic)
    • The fatty acid tail of a phospholipid is nonpolar and therefore insoluble in water (hydrophobic) 

Phospholipid structure diagram

The generalised molecular structure of a phospholipid, downloadable AS & A Level Biology revision notes

Phospholipids consist of a molecule of glycerol, two fatty acid tails, and a phosphate group

  • When phospholipids are placed in water the hydrophilic phosphate heads orient towards the water and the hydrophobic hydrocarbon tails orient away from the water
    • This forms a phospholipid monolayer

Phospholipid monolayer diagram

_Phospholipid monolayer, downloadable AS & A Level Biology revision notes

Phospholipids can form a monolayer in water

  • When there is a sufficient concentration of phospholipids present then two-layered structures may form
  • These sheets are called phospholipid bilayers

Phospholipid bilayer diagram

Phospholipid bilayer, downloadable AS & A Level Biology revision notes

A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails facing inwards and hydrophilic heads outwards

Lipid Bilayers: Barriers

  • The phospholipid bilayer has two regions - a hydrophobic core and a hydrophilic outer layer
  • The hydrophobic regions are attracted to each other and the hydrophilic regions are attracted to water in the cytoplasm or the extracellular fluid
  • These properties allow the bilayer to form a barrier
    • Large molecules cannot pass through the barrier as the hydrophobic region is tightly packed and has low permeability to larger molecules
    • Polar molecules and ions cannot pass through the hydrophobic tails of the phospholipid structure
      • The hydrophilic nature of these molecules and ions means that they will not interact with the hydrophobic fatty acid tails of the phospholipids
  • The bilayer forms an effective barrier so that it is able to control which molecules pass through and out of the cell

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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