The composition of the extracellular space includes metabolites, ions, proteins, and many other substances that might affect cellular function. For example, neurotransmitters "jump" from cell to cell to facilitate the transmission of an electric current in the nervous system. Hormones also act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are the digestive enzymes.
The term 'extracellular' is often used in reference to the extracellular fluid (ECF) which composes about 15 litres of the average human body. The cell membrane (and, in plants and fungi, the cell wall) is the barrier between the two, and chemical composition of intra- and extracellular milieu can be radically different. In most organisms, for example, a Na+/K+-ATPase pump maintains a high concentration of sodium ions outside cells while keeping that of potassium low, leading to chemical excitability. Many cold-tolerant plants force water into the extracellular space when the temperature drops below 0 degrees Celsius, so that when it freezes, it does not lyse the plants' cells.