Fatty acid saturation changes membrane fluidity because unsaturated tails kink and reduce packing while saturated tails pack more tightly; Temperature responses depend on how tightly the fatty acid tails can pack.
Coverage 2022–2025 · Updated 15 Jul 2026
Cis-unsaturated fatty-acid tails have lower melting points and increase bilayer fluidity, whereas saturated tails pack closely, have higher melting points and strengthen membranes at warmer temperatures.
Cis double bonds create kinks that prevent tight packing. Cells can change chain length and degree of unsaturation to keep membrane viscosity within a functional range as habitat temperature changes—a homeoviscous adaptation.
At the same temperature: more cis unsaturation usually means looser packing and greater fluidity; more saturation usually means tighter packing and lower fluidity. Temperature and cholesterol must also be considered.
Lake sturgeon acclimated to colder water can increase unsaturated membrane lipids so their membranes remain flexible rather than becoming too rigid.
Unsaturation is not the only control. Compare membranes at the same temperature and consider chain length and cholesterol before predicting fluidity.
This objective is assessed through structured response, commonly using Identify / Outline.
Identify / Outline
Build the answer around this relationship: Unsaturated fatty acid tails increase membrane fluidity by reducing tight packing.
Reversing the effects of saturated and unsaturated fatty acids on membrane fluidity.
Representative question
Outline the effect of fatty acids on the fluidity of membranes.
The HL extension asks how membrane structure becomes dynamic cell behaviour. Fatty acid saturation and cholesterol tune fluidity. Fluid membranes form and fuse vesicles. Gated ion channels and sodium-potassium pumps create nerve-cell gradients and electrical responses. Sodium-dependent glucose cotransport uses a sodium gradient to move glucose indirectly against its gradient. Adhesion molecules organize tissues.
