Simple Diffusion in Cell Membranes

Steps:

• Identify simple diffusion as passive transport of small, nonpolar substances through the lipid bilayer without proteins.
• Recall that polar or charged substances require channel or carrier proteins for passage.
• Assume standard options: 1 (e.g., ions like Na⁺, need channels), 2 (e.g., O₂ or CO₂, nonpolar, diffuse directly), 3 (e.g., glucose, polar, needs carrier).
• Select option matching only nonpolar substance (2) for direct passage.

Why D is correct:

• Simple diffusion applies solely to small nonpolar molecules like oxygen, crossing the hydrophobic bilayer without proteins, per fluid mosaic model.

Why the others are wrong:

• A includes 1 (ions require channels, not direct diffusion).
• B includes 1 (ions cannot pass directly).
• C includes 3 (polar solutes like glucose need facilitated diffusion via carriers).

Final answer: D