Temperature: Higher temperatures increase membrane fluidity because it adds energy for lipid molecules to move more quickly. Lower temperatures decrease fluidity as molecules slow down.
Unsaturated Fatty Acids: 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.