Which molecules can diffuse through the phospholipid bilayer readily?

Which molecules can diffuse through the phospholipid bilayer readily?

• A. Hydrophobic/non-polar molecules (fats and lipids)
• B. Large ions
• C. Hydrophilic polar molecules
• D. Water molecules that are tightly bound

Answer: (A)

The main idea is that the interior of the phospholipid bilayer is hydrophobic, like an oily layer, so substances that are nonpolar can dissolve in it and move across the membrane without help. Hydrophobic, non-polar molecules such as fats and lipids can dissolve in this lipid core and diffuse directly through the membrane down their concentration gradient. In contrast, large charged ions and hydrophilic polar molecules don’t dissolve well in the oily interior, so they can’t cross readily without specific transport proteins or channels. Water can cross too, but it doesn’t do so as readily as nonpolar molecules and often moves through water channels rather than by simple diffusion across the lipid core. So the molecules that diffuse through the bilayer most readily are the hydrophobic, non-polar ones.

The main idea is that the interior of the phospholipid bilayer is hydrophobic, like an oily layer, so substances that are nonpolar can dissolve in it and move across the membrane without help. Hydrophobic, non-polar molecules such as fats and lipids can dissolve in this lipid core and diffuse directly through the membrane down their concentration gradient. In contrast, large charged ions and hydrophilic polar molecules don’t dissolve well in the oily interior, so they can’t cross readily without specific transport proteins or channels. Water can cross too, but it doesn’t do so as readily as nonpolar molecules and often moves through water channels rather than by simple diffusion across the lipid core. So the molecules that diffuse through the bilayer most readily are the hydrophobic, non-polar ones.