Grade 11 cell membranes

Fluid Mosaic Model (Singer Nicholson)

Structure of Phospholipids:

Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic

Arrangement in Membranes:

Phospholipids spontaneously arrange into a bilayer
The hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively

Properties of the Phospholipid Bilayer:

The bilayer is held together by weak hydrophobic interactions between the tails
Hydrophilic / hydrophobic layers restrict the passage of many substances
Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)

Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane

Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)
Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane

Cholesterol

Cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane:

Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity
It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
It functions to separate phospholipid tails and so prevent crystallisation of the membrane
It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Comparison of the two models of Protein Structures

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