Which substance can diffuse across the membrane phospholipids?

Question

BlackTom AI · Accepted Answer

In membrane physiology, the ability of a substance to diffuse through the lipid bilayer depends largely on its polarity and size. 
Option a: Dihydrogen oxide (water). Water is a polar molecule and typically crosses membranes via aquaporin channels or through small amounts by simple diffusion, but not readily through the lipid core alone. So it’s not primarily diffusing directly through the phospholipid interior without assistance. 
Option b: Glycine. Glycine is a small amino acid with polar characteristics and can carry a charge at physiological pH, which makes it poorly soluble in the hydrophobic core of the lipid bilayer; thus, it does not diffuse readily through the membrane’s lipid interior without help from transporters. 
Option c: Carbon dioxide. This is a nonpolar, small molecule that can dissolve in the lipid bilayer and diffuse passively down its concentration gradient without requiring channels or transporters. This makes it a classic example of a substance that diffuses directly through membrane phospholipids. 
Option d: Chloride ion. Cl− is an aqueous, charged ion; its charge prevents it from dissolving in the hydrophobic core of the membrane, so it cannot diffuse through the phospholipid bilayer on its own and typically requires channels or transporters to cross. 
Summarizing the contrast: nonpolar, small molecules (like CO2) diffuse readily through the lipid portion, whereas polar molecules, ions, or larger hydrophilic substances generally need channels, carriers, or other mechanisms. Therefore, among the options, carbon dioxide best fits the criterion of diffusing directly through the phospholipid membrane.}

Which substance can diffuse across the membrane phospholipids?

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