Only Small, Uncharged Molecules and Oily Molecules Can Penetrate BiomembranesWithout the Aid of Proteins
Charged particles (i.e., ions) do not pass through a pure phospholipid bilayer because of the inner» hydrophobic region of bilayer. Polar molecules (molecules with no net charge but with electrical imbalances) with a molecular weight greater than about IOO daltons are also unable to pass readily through a pure lipid bilayer, thus excluding all sugar molecules (monosaccharides), amino acids, nucleosides, as well as their polymers (polysaccharide, proteins, nucleic acids).
On the other hand, some crucially important polar molecules (e.g., water, urea) arc small enough to pass through the lipid bilayer. Small, moderate-size, and large molecules that are soluble in oily solvents readily pass through a pure lipid bilayer. Physiologically important molecules in this class include O2, N2, and the steroid hormones (see Chapters 33 and 34). However, many toxic, synthetic molecules, such as insecticides, are also in this category.
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