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 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