It is the outer boundary of the cell and provides structural integrity for the cell. The phospholipid bilayer is composed of phospholipids, proteins, cholesterol, and carbohydrates. It has the characteristics of structural fluid; hence, the term fluid mosaic. It controls the movement of substances in and out of the cell: semi-permeable. Provides recognition (glycoproteins), communication (receptors) and adhesion between cells. Passive transport: movement across the membrane that doesn"t require energy, molecules move down their concentration gradients. Simple diffusion - gases like o2 and co2, steroid hormones. Active transport: movement across the membrane that requires energy, molecules move against their concentration gradients. Diffusion is the process whereby a substance moves down its concentration gradient. Molecules move from high concentration to low concentration. Equilibrium occurs when the concentration gradient is dissipated. Movement of a substance from a region of higher concentration to a region of lower concentration with the aid of a membrane protein.