Practice question

Cholesterol's structure with rigid tetracyclic ring system and short iso-octyl tail allows it to insert into phospholipid bilayers with hydroxyl near phospholipid carbonyls and ring system interacting with upper acyl chains. At low temperatures it disrupts tight packing of saturated chains, preventing transition to gel phase and preserving lateral diffusion essential for protein function, while at high temperatures its rigid ring restricts chain motion, decreasing permeability to small polar molecules, protons, and sodium ions, maintaining electrochemical gradients. Within membranes cholesterol drives formation of liquid-ordered lipid rafts that concentrate GPI-anchored proteins and Src kinases for signaling. Beyond barrier modulation, cholesterol is substrate for mitochondrial P450scc that cleaves side chain to pregnenolone, precursor for glucocorticoids, mineralocorticoids, sex steroids, bile acids, and vitamin D. Passive transport of glucose through GLUTs or water through aquaporins does not require cholesterol as facilitator; proteinaceous pores and carriers mediate downhill movement independent of sterol content, so cholesterol does not act as transport facilitator.