AP BIOLOGY

Detailed explanation-1: -In simple diffusion, the substance passes between the phospholipids; in facilitated diffusion there are a specialized membrane channels. Charged or polar molecules that cannot fit between the phospholipids generally enter and leave cells through facilitated diffusion.

Detailed explanation-2: -Sometimes molecules need the help of special transport proteins to move across the cell membranes. The movement of molecules across a membrane without the input of energy is known as passive transport.

Detailed explanation-3: -Transport proteins in the cell membrane allow for selective passage of specific molecules from the external environment. Each transport protein is specific to a certian molecule (indicated by matching colors).

Detailed explanation-4: -Transport proteins make passage possible for molecules and ions that would not be able to pass through a plain phospholipid bilayer. Some transport proteins have a hydrophilic tunnel through them which allows polar molecule or ions to pass. Others actually bind to the molecules and move them across the membrane.