Practice question

Permeability of lipid bilayer correlates closely with solute oil-water partition coefficient, molecular size, and hydrogen bonding potential in solubility-diffusion framework. Gases such as oxygen, molecular nitrogen, carbon dioxide and nitric oxide possess small molecular weight under 50 Daltons, non-polar character lacking hydrogen bond donors or acceptors and high solubility in hydrocarbon, allowing direct dissolution into hydrophobic acyl chain core and rapid transverse diffusion without assistance, permeability coefficient on order of ten to hundred centimeters per second. This explains instantaneous alveolar gas exchange, tissue oxygenation for oxidative phosphorylation, CO2 elimination and NO signaling paracrine diffusion to smooth muscle activating guanylate cyclase. Larger polar molecules like glucose with five hydroxyl groups, amino acids, nucleosides and phosphorylated metabolites face high energetic barrier for dehydration requiring transient breaking of many hydrogen bonds, drastically low permeability needing carriers. Even water despite small size diffuses slowly through pure lipid, so aquaporins accelerate flux fiftyfold for kidney water reabsorption. Charged ions face even larger Born energy barrier essentially impermeant. Therefore oxygen movement relies strictly on simple diffusion governed by Fick's law and partial pressure gradients, demonstrating that membrane protein assistance is selective for hydrophilic solutes and not required for lipophilic gases supporting respiration and signaling.