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: -"Simple diffusion.” Simple diffusion is pretty much exactly what it sounds like – molecules move down their gradients through the membrane. Molecules that practice simple diffusion must be small and nonpolar*, in order to pass through the membrane.

Detailed explanation-3: -Small, nonpolar molecules (e.g. oxygen and carbon dioxide): These molecules can pass through the lipid bilayer and do so by squeezing through the phospholipid bilayers. They don’t need proteins for transport and can diffuse across quickly.

Detailed explanation-4: -Oxygen and carbon dioxide and most lipids enter and leave cells by simple diffusion.

Detailed explanation-5: -Nonpolar molecules such as CO₂ and O₂ can diffuse through the lipid bilayer in each direction. Polar molecules cannot diffuse through the bilayer. Polar molecules need polar protein channels to diffuse through the membrane. This is “facilitated diffusion".