1.3.1 Phospholipid Bilayer Properties

Phospholipids

• Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
• 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 (hydrophilic) and therefore soluble in water
• The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water

The generalised molecular structure of a phospholipid

• Due to their amphipathic properties, phospholipids display an emergent property when placed into water
• The hydrophilic phosphate heads orientate towards the water and the hydrophobic hydrocarbon tails orientate inwards (away from the water)
  • They form a phospholipid monolayer

A phospholipid monolayer

• If phospholipids are mixed/shaken with water they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing inwards
  • This is called a micelle

A micelle

• Alternatively, when there is a sufficient concentration of phospholipids present then two-layered structures may form
• These sheets are called phospholipid bilayers – this is the basic structure of the cell membrane

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

• The two layers of phospholipids are loosely held together by weak hydrophobic interactions between the hydrocarbon tails allowing some membrane fluidity
• The amphipathic properties result in the phospholipid bilayer acting as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)
• This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in

• Phospholipids and cholesterol are the two main components of animal cell plasma membranes (cholesterol is absent in plant membranes)

Cholesterol

• Cholesterol is a lipid that belongs to the steroid group
• It is amphipathic, with the majority of the cholesterol molecule being hydrophobic and therefore attracted to the hydrophobic hydrocarbon tails of the phospholipid
• The hydroxyl group of the cholesterol molecule is hydrophilic. It is attracted to the phosphate heads of the phospholipid
• Therefore in the plasma membrane cholesterol is positioned between phospholipids

The molecular structure of cholesterol

• The plasma membrane is fluid, meaning the components are free to move
• The fluidity of the membrane needs to be controlled:
  • If it was too fluid the cell could not regulate what moved in and out
  • If it was not fluid enough then the cell would not be able to move and substances would be unable to move into or out of the cell

• Cholesterol helps with the regulation of the membrane fluidity and permeability
  • Interaction between cholesterol and phospholipid tails stabilises the plasma membrane at higher temperatures by stopping the membrane from becoming too fluid
    • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
• At colder temperatures cholesterol increases the fluidity of the membrane, stopping it crystallizing and becoming too rigid
  • This occurs because cholesterol stops the phospholipid tails packing too closely together

• The impermeability of the membrane to hydrophilic ions (e.g. sodium and hydrogen) is also reduced by cholesterol