Refers to the ability of molecules within a substance to move freely. In relation to cell membranes, it describes how easily lipids can move laterally within the phospholipid bilayer.
Higher temperatures increase membrane fluidity because it adds energy for lipid molecules to move more quickly. Lower temperatures decrease fluidity as molecules slow down.
Lipid molecules containing at least one double bond in their carbon chain. Unsaturated fatty acids increase membrane fluidity because the kinks in their structure prevent tight packing.
Phospholipid Flip-Flop: A rare event where a phospholipid molecule switches from one leaflet of the bilayer to the other. This process is slow and requires energy.