Practice question

Membrane fluidity describes lateral mobility, rotation, and flexing of components, parameter influencing permeability, fusion, and signaling complex formation. Phospholipids primarily determine fluidity through acyl chain composition: presence of cis double bonds as in oleoyl 18:1 and arachidonoyl 20:4 introduces kinks preventing close packing, lowering melting temperature Tm and increasing lateral diffusion coefficient about 10^-8 cm2/s measured by fluorescence recovery after photobleaching and fluorescence anisotropy. Shorter chains also increase fluidity. Cholesterol buffers fluidity by intercalating with rigid ring near chains, reducing motion at high temperature while preventing crystallization at low temperature, broadening phase transition. Proteins and carbohydrates can locally restrict diffusion via cytoskeletal corrals and lectin crosslinking, but bulk fluidity originates from lipids. Ribosomes associated with rough ER translation do not influence plasma membrane lipid order. Regulated desaturase enzymes SCD1 introducing double bonds adjust fluidity in response to cold, diet, and insulin signaling, essential for maintaining receptor tyrosine kinase activity and cold tolerance in poikilotherms.