AP BIOLOGY

Detailed explanation-1: -The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion. During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane.

Detailed explanation-2: -Answer and Explanation: Polar molecules cannot cross the membrane by simple diffusion because they cannot interact with the hydrophobic tails of the phospholipids. The cell membrane is made of a phospholipid bilayer.

Detailed explanation-3: -Oxygen and carbon dioxide and most lipids enter and leave cells by simple diffusion.

Detailed explanation-4: -Small, moderately polar molecules are able to passively diffuse across the cell membrane. To transport larger, more polar compounds such as most sugars, amino acids, peptides, and nucleosides, membrane transporters are utilized.

Detailed explanation-5: -Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins.