How is the plasma membrane selectively permeable?

How is the plasma membrane selectively permeable?

• A. Permeability depends on temperature
• B. All substances can cross
• C. Only certain substances can cross (small, noncharged molecules)
• D. No substances can cross

Answer: (C)

The ability of the plasma membrane to allow some substances through while blocking others comes from its structure: a phospholipid bilayer with a hydrophobic interior. That oily core strongly resists polar or charged molecules, so only certain particles can slip across by diffusion. Small, nonpolar (or very small uncharged) molecules can dissolve in the lipid bilayer and move down their concentration gradient without energy input, which is why these substances cross readily. Larger polar molecules and ions, or charged particles, can’t cross by simple diffusion and thus rely on specific transport proteins—channels or carriers—or active pumps to get across. Water can cross too, but often through channels, whereas many ions and big polar molecules are effectively blocked unless a transporter is involved. So the selective permeability isn’t about temperature or “all substances,” but about the molecule’s size, polarity, and charge in relation to the membrane’s hydrophobic core and embedded transport proteins.

The ability of the plasma membrane to allow some substances through while blocking others comes from its structure: a phospholipid bilayer with a hydrophobic interior. That oily core strongly resists polar or charged molecules, so only certain particles can slip across by diffusion. Small, nonpolar (or very small uncharged) molecules can dissolve in the lipid bilayer and move down their concentration gradient without energy input, which is why these substances cross readily. Larger polar molecules and ions, or charged particles, can’t cross by simple diffusion and thus rely on specific transport proteins—channels or carriers—or active pumps to get across. Water can cross too, but often through channels, whereas many ions and big polar molecules are effectively blocked unless a transporter is involved. So the selective permeability isn’t about temperature or “all substances,” but about the molecule’s size, polarity, and charge in relation to the membrane’s hydrophobic core and embedded transport proteins.