Simple Diffusion Across Cell Membranes

Steps:

• Identify substances based on size, charge, and polarity for membrane permeability.
• Small, nonpolar molecules like CO2 diffuse directly through the lipid bilayer.
• Charged ions (K+, Cl-) require channel proteins due to hydrophilicity.
• Large polar molecules like glucose (C6H12O6) need carrier proteins for facilitated diffusion.

Why D is correct:

• CO2 is a small, nonpolar gas that passes via simple diffusion, as defined by the fluid mosaic model where uncharged molecules dissolve in the hydrophobic core.

Why the others are wrong:

• A includes ions (1), which cannot diffuse freely due to charge repulsion by the bilayer.
• B includes ions (1) and polar glucose (3), both needing transport proteins.
• C includes glucose (3), a hydrophilic solute too large for simple diffusion.

Final answer: D