Lipid Bilayers (HL IB Biology)

• 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

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

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

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

• 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