Which substances usually require membrane transport proteins to cross the cell membrane?

Which substances usually require membrane transport proteins to cross the cell membrane?

• A. Hydrophobic non-polar molecules
• B. Hydrophilic, polar or charged, or large molecules
• C. Gases such as oxygen and carbon dioxide
• D. Water can diffuse through the membrane without channels

Answer: (B)

The key idea is that the cell membrane’s core is a hydrophobic, oily layer. Nonpolar, small molecules can slip through this layer on their own, which is why hydrophobic, non-polar substances and small gases like oxygen and carbon dioxide diffuse across the membrane without help. Water can also cross by diffusion, though it often moves more quickly through aquaporin channels when rapid transport is needed. In contrast, hydrophilic, polar, or charged substances—and especially large molecules—do not dissolve well in the lipid interior. They face a big energy barrier if they try to pass by themselves. To cross, they rely on membrane transport proteins that provide a hydrophilic pathway or that bind and move the solute across the membrane. These proteins enable selective, regulated transport, either down the concentration gradient (facilitated diffusion) or against it (active transport). So substances that are polar, charged, or large usually require membrane transport proteins to cross the membrane, making them the best answer.

The key idea is that the cell membrane’s core is a hydrophobic, oily layer. Nonpolar, small molecules can slip through this layer on their own, which is why hydrophobic, non-polar substances and small gases like oxygen and carbon dioxide diffuse across the membrane without help. Water can also cross by diffusion, though it often moves more quickly through aquaporin channels when rapid transport is needed.

In contrast, hydrophilic, polar, or charged substances—and especially large molecules—do not dissolve well in the lipid interior. They face a big energy barrier if they try to pass by themselves. To cross, they rely on membrane transport proteins that provide a hydrophilic pathway or that bind and move the solute across the membrane. These proteins enable selective, regulated transport, either down the concentration gradient (facilitated diffusion) or against it (active transport).

So substances that are polar, charged, or large usually require membrane transport proteins to cross the membrane, making them the best answer.