The cell membrane is selectively permeable, allowing some substances to enter and leave the cell. The phospholipid molecules that make up the cell membrane have hydrophilic heads and hydrophobic tails. There are two types of cellular transport: passive transport, which moves substances down a concentration gradient without energy, and active transport, which moves substances against a concentration gradient and requires energy. The three types of passive transport are simple diffusion, facilitated diffusion, and osmosis, while active transport includes exocytosis, endocytosis, and various pumps.

1. Cellular Transport Biology I Factoids