Practice question

Fluid mosaic model postulates membrane proteins diffuse laterally, associate with cytoskeleton, function as enzymes receptors carriers and structural anchors. Transverse movement of proteins between leaflets termed flip-flop faces enormous energetic barrier because large hydrophilic extramembranous domains containing charged residues lysine arginine aspartate glutamate would need dehydration and passage through hydrocarbon core twenty to thirty angstroms low dielectric constant incurring free energy tens kilocalories plus disruption of established orientation determined during biogenesis at Sec61 translocon following signal anchor and positive-inside rule where cytosolic Lys Arg enriched. Experimental tracking using fluorescent labeled Band 3 or glycophorin in erythrocytes shows no spontaneous flipping over days; turnover occurs via endocytosis lysosomal degradation and recycling rather than flipping. Phospholipid flip-flop catalyzed by flippases floppases scramblases half-life minutes hours regulated. Therefore assertion that membrane proteins can flip between leaflets without assistance is false highlighting conservation of topology and asymmetry essential for vectorial transport and signaling orientation preserved throughout lifetime.