IB Biology SL

Revision Notes

1.3.1 Phospholipid Bilayer Properties

Amphipathic 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 place into water
  • The hydrophilic phosphate heads orientate towards the water and the hydrophobic hydrocarbon tails orientate inwards and so as a whole the phospholipids function differently, they form a phospholipid monolayer

_Phospholipid monolayer, downloadable AS & A Level Biology revision notes

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 in towards each other
    • This is called a micelle

Micelle, downloadable AS & A Level Biology revision notes

A micelle

  • Alternatively, two-layered structures may form in sheets
  • These are called phospholipid bilayers – this is the basic structure of the cell membrane

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

  • 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

Animal Cell Membrane: Cholesterol

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

Cholesterol

  • Cholesterol is a lipid. It 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

Cholesterol structure_1, downloadable AS & A Level Biology revision notes

The molecular structure of cholesterol

Mammalian Membranes: Role of Cholesterol

  • The plasma membrane is fluid, the components are free to move
  • The fluidity of the membrane needs to be controlled because:
    • 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 could not 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
  • 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 and cells burst)

Exam Tip

It is important to remember that cholesterol reduces membrane fluidity and the permeability of hydrophilic ions (e.g. sodium and hydrogen) in mammal membranes.

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