Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane.

Other ways that substances can cross the plasma membrane include facilitated transport, where molecules or ions must interact with a channel/carrier protein to be transported across the membrane, down a concentration gradient, which doesn't require energy. Another method is active transport, where molecules are moved via protein pumps, which requires energy in the form of ATP, and which may move up a concentration gradient. In addition, water may move across membranes via osmosis, from high to low water potential, via aquaporins or water channels. Alternatively, substances contained within vesicles may exit the cell via fusion of the vesicular membrane with the plasma membrane, leading to the release of its contents to the extracellular space. Extracellular material may also enter the cell via phagocytosis, which involves the cell membrane surrounding and engulfing the substance, to form an intracellular vesicle or vacuole.