Practice question

Liquid ordered raft stability arises favorable packing between sphingolipid and sterol driven enthalpy entropy compensation. Cholesterol small polar hydroxyl rigid planar tetracycle inserts into voids between long saturated amide linked acyl chains sphingosine backbone maximizing van der Waals contacts permitting hydrogen bond between sphingosine C3 hydroxyl cholesterol hydroxyl and amide carbonyl to amide donor. This promotes condensed complex formation increasing bilayer thickness four angstroms decreasing permeability solutes and creating diffusion distinct from liquid disordered with FRAP coefficients point one micrometer squared per second versus one. Methyl beta cyclodextrin extraction collapses order increasing disorder and lateral mixing. Cholesterol does not thin membranes nor generically increase permeability; low concentrations increase permeability slightly gel phase but fluid phase reduces it significantly. Proteins with saturated GPI double palmitoyl anchors partition into ordered domains enriching signaling kinases Src Lyn. Describing stabilization sphingolipid cholesterol interaction distinguishes physical chemistry rafts from simple protein clustering and explains why raft size regulation affects immune receptor triggering BCR TCR activation and signaling thresholds examined in cell biology immunology questions about domain formation mechanism and biophysics of phase separation.