The cell membrane is a selectively permeable or semipermeable structure that separates the internal environment of a cell from its surroundings. It is mainly composed of a phospholipid bilayer containing proteins, cholesterol and carbohydrates. One of its most important functions is to regulate the movement of substances into and out of the cell. This regulation allows the cell to obtain nutrients, remove waste products, maintain proper concentrations of ions and molecules, and preserve a stable internal environment. The transport of molecules across the cell membrane occurs through several mechanisms, which can broadly be classified into passive transport, active transport and vesicular transport. Passive transport is the movement of substances across the membrane without direct expenditure of cellular energy in the form of ATP. It generally occurs down a concentration or electrochemical gradient, from a region of higher concentration to a region of lower concentration. Simple diffusion is the simplest form of passive transport. Small, non-polar and lipid-soluble molecules such as oxygen, carbon dioxide and certain steroid molecules can pass directly through the hydrophobic interior of the phospholipid bilayer. The rate of diffusion depends on factors such as the concentration gradient, temperature, size and lipid solubility of the molecule, and the surface area and thickness of the membrane. Osmosis is a special form of passive transport involving the movement of water across a selectively permeable membrane. Water moves from a region of higher water potential, generally having a lower concentration of dissolved solutes, toward a region of lower water potential, having a higher concentration of dissolved solutes. Although water can cross the lipid bilayer to some extent, most rapid water movement in biological membranes occurs through specialised channel proteins called aquaporins. Osmosis is important in maintaining cell volume and water balance. In plant cells, it also contributes to turgor pressure, while in animal cells excessive water movement can cause