AP BIOLOGY

Detailed explanation-1: -Thus, gases (such as O2 and CO2), hydrophobic molecules (such as benzene), and small polar but uncharged molecules (such as H2O and ethanol) are able to diffuse across the plasma membrane. Other biological molecules, however, are unable to dissolve in the hydrophobic interior of the phospholipid bilayer.

Detailed explanation-2: -Only small uncharged molecules can diffuse freely through phospholipid bilayers (Figure 2.49). Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes.

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.

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

Detailed explanation-5: -Only the smallest molecules like water, carbon dioxide, and oxygen can freely diffuse across cell membranes. Larger molecules or charged molecules often require an input of energy to be transported into the cell. Even when equilibrium is reached, particles do not stop moving across the cell membrane.