Unsaturated fatty acid tails make membranes more fluid because their carbontocarbon double bonds produce bends that prevent phospholipids from packing closely together. Saturated fatty acid tails are straight, pack more closely and therefore form less fluid membranes.
The mechanism
More precisely, membranes contain phospholipids with fatty acid tails, rather than free “fats.” In an unsaturated fatty acid, one or more carbon to carbon double bonds are present; the cis double bonds typical of biological membranes create bends or “kinks” in the hydrocarbon chain.
These bends increase the distance between neighbouring phospholipid tails. This reduces the intermolecular attractions between them, so less thermal energy is required for the phospholipids to move relative to one another. The bilayer therefore remains fluid at lower temperatures and has a lower melting point.
A saturated fatty acid has no carboncarbon double bonds. Its straight hydrocarbon chain can align closely with adjacent tails, producing stronger intermolecular attractions. More energy is therefore required to separate or move the phospholipids, giving the membrane a higher melting point and lower fluidity at the same temperature.
| Fatty acid tails | Effect on membrane |
|---|---|
| Unsaturated; one or more double bonds; bent chains | Less close packing, weaker intermolecular attractions, lower melting point and greater fluidity |
| Saturated; no double bonds; straight chains | Closer packing, stronger intermolecular attractions, higher melting point and lower fluidity |
The biological significance is that cells can alter fatty acid composition to maintain suitable membrane flexibility at different temperatures. However, excessive fluidity can make a membrane less stable, so membrane composition must be regulated.
IB exam technique
This is Additional Higher Level (AHL) content from B2.1 Membranes and membrane transport. For an “explain” question, link the complete sequence: double bonds → bent tails → reduced packing → weaker attractions → lower melting point → greater membrane fluidity. A common misconception is that double bonds make individual phospholipids move faster; the key effect is reduced packing between their fatty acid tails.
