AP BIOLOGY

Detailed explanation-1: -Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot.

Detailed explanation-2: -Because the cell membrane is semipermeable, only small, uncharged substances like carbon dioxide and oxygen can easily diffuse across it. Charged ions or large molecules require different kinds of transport.

Detailed explanation-3: -Gases, hydrophobic molecules, and small polar uncharged molecules can diffuse through phospholipid bilayers.

Detailed explanation-4: -3 – Simple Diffusion Across the Cell (Plasma) Membrane: The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.