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Vitamin E (tocopherol), a fat-soluble antioxidant, protects cell membranes by neutralizing free radicals.
Ref: NCERT Class 12 Chemistry > Chapter 14: Biomolecules > Topic: Vitamins - Types, Coenzymes and Deficiency Diseases - Part 2
Freeze-fracture technique, developed by Branton and refined by Pinto da Silva, revolutionized visualization of membrane interior by physically splitting frozen lipid bilayers. Live cells are rapidly frozen in liquid propane at -180C, fractured with cold knife under high vacuum, fracture plane preferentially passes through hydrophobic interior of membranes where van der Waals forces are weakest, splitting bilayer into exoplasmic and protoplasmic leaflets exposing intramembrane particles representing integral proteins including band 3 anion exchanger, aquaporins, connexin gap junction hexamers,
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Freeze-Fracture and Membrane Proteins.
Regulation of fluidity reflects balance between order and disorder of fatty acyl chains governing membrane physical properties and protein function. Saturated acyl chains straight all trans conformation maximize van der Waals contacts small area per lipid about forty eight square angstroms low fluidity high transition temperature. Cis double bond introduces kink thirty degree bend reducing contact area to about sixty square angstroms increasing free volume lowering transition temperature fifty degrees increasing lateral diffusion coefficient and rotational freedom measured by fluorescence anis
Ref: Singer and Nicolson, Fluid Mosaic Model and Fluidity Dependence on Unsaturation, Science 1972.
Cellular ion homeostasis requires continuous energy expenditure to maintain steep gradients. Sodium gradient high outside 145 millimolar low inside 12 millimolar and potassium opposite 4 outside 140 inside established by Na+/K+ ATPase hydrolyzing ATP forming phosphoenzyme E1P E2P cycling three sodium out two potassium in per ATP. Calcium gradient extreme low cytosolic 100 nanomolar versus 1 to 2 millimolar extracellular maintained by plasma membrane Ca2+ ATPase PMCA and sarcoplasmic endoplasmic reticulum Ca2+ ATPase SERCA, both P-type pumps transporting two calcium per ATP against ten thousand
Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Ion Distributions - Cl- Passive vs Active Transport.
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