Substances can pass through the cell membrane in three ways. If a favorable concentration gradient is present, substances such as water or those soluble in lipids can pass through the membrane via simple diffusion. Substances, such as simple sugars or amino acids, that are too big to move through the membrane's pores and are non-lipid soluble, can pass through the membrane through a process called facilitated diffusion or passive transport. If a favorable concentration gradient is present, proteins, called permeases, can bind to substances and transport them across the membrane without the expenditure of energy. Substances can even pass through the membrane against the concentration gradient via a process called active transport. In active transport, the cell must expend energy in order for the permeases to bind with the substance and pump it through the membrane. Cells must use active transport to maintain the proper concentration of sodium and potassium ions in their cytoplasm, for example, since the concentration gradients favor too much sodium and too little potassium entering the cell.
Substances can enter a cell without ever passing through the cell membrane through the process of endocytosis. The two types of endocytosis include phagocytosis, or the invagination of large particles via the flow of the cell membrane around the particle, and pinocytosis, or the invagination of small particles via the formation of narrow channels in the cell membrane directly beneath the particle after the particle has bound to the surface of the membrane. Following either type of
