AP BIOLOGY

Detailed explanation-1: -Although ions and most polar molecules cannot diffuse across a lipid bilayer, many such molecules (such as glucose) are able to cross cell membranes. These molecules pass across membranes via the action of specific transmembrane proteins, which act as transporters.

Detailed explanation-2: -Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer.

Detailed explanation-3: -Polar molecules are less free to diffuse across membranes. In order for a polar molecule to cross a membrane, it must break its interactions with water as it enters the hydrophobic interior of the bilayer. Breaking these interactions is energetically unfavorable and requires an input of energy.

Detailed explanation-4: -The charged particles can move through the aqueous solutions, but they cannot move through the lipid environment of the cell membrane. For ions to move through the membrane, they need help in one form or another from the proteins that span the membrane.

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.