Practice question

Selective permeability originates from continuous oily interior separating cytosol from extracellular milieu. Phospholipid tails containing fourteen to twenty four carbon hydrocarbon chains exclude water creating low dielectric slab with Born energy penalty greater than one hundred fifty kilojoules per mol for moving sodium potassium chloride through effectively blocking ions and large polar molecules like glucose nucleotides sucrose. Small nonpolar gases oxygen carbon dioxide nitrogen dissolve readily and cross by solubility diffusion while water crosses slowly via transient pores permeability about ten minus three centimeters per second. Ion movement depends entirely on integral proteins channels carriers pumps providing hydrophilic pathways regulated by gating. Pure liposomes lacking proteins demonstrate identical barrier confirming lipid core sufficient. Glycolipid carbohydrate and asymmetry provide recognition not permeability. This core hydrophobicity underlies Nernst potentials generation secondary active transport driving nutrient uptake by SLC transporters and predictive rules for drug logP absorption. Appreciating barrier energetics explains why transporter mutations cause channelopathies like cystic fibrosis and why lipophilic drugs cross blood brain barrier efficiently for central action.