The Phospholipid Bilayer
Membranes
- Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles
- Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport
- Membranes play a role in cell signalling by acting as an interface for communication between cells
Membranes of Eukaryotic Cells Diagram
Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane
Phospholipids
- Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
- They are formed by a hydrophilic phosphate head bonding with 2 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
Phospholipid Structure Diagram
The generalized molecular structure of a phospholipid
- Due to their amphipathic properties, phospholipids display an emergent property when placed into water
- The hydrophilic phosphate heads orientate toward the water and the hydrophobic hydrocarbon tails orientate inward (away from the water)
- They form a phospholipid monolayer
Phospholipid Monolayer Diagram
A phospholipid monolayer
- If phospholipids are mixed/shaken with water, they form spheres with the hydrophilic phosphate heads facing out toward the water and the hydrophobic fatty acid tails facing inward
- This is called a micelle
Micelle Diagram
A micelle
- Alternatively, when there is a sufficient concentration of phospholipids present, then 2-layered structures may form
- These sheets are called phospholipid bilayers – this is the basic structure of the cell membrane
Formation of a Phospholipid Bilayer Diagram
A phospholipid bilayer is composed of 2 layers of phospholipids; their hydrophobic tails face inward and hydrophilic heads face outward
- The 2 layers of phospholipids are held together loosely 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 nonpolar fatty acid tails prevent polar molecules or ions from passing across the membrane
- This ensures that water soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and that unwanted water soluble molecules cannot get in