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
• Cholesterol?increases the mechanical strength and stability of membranes?(without it membranes would break down causing cells to burst)