In cell biology, molecular biology and related fields, the word extracellular (or sometimes extracellular space) means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid. The term is used in contrast to intracellular (inside the cell).
According to the Gene Ontology database the Extracellular Space is a Cellular Component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid. Note that for multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism (excluding the extracellular matrix). Gene products from a multi-cellular organism are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term".
The composition of the extracellular space includes metabolites, ions, various proteins and non-protein substances (i.e. DNA, RNA, lipids, microbial products etc.) that might affect cellular function. For example, hormones, growth factors, cytokines and chemokines act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are various enzymes, including digestive enzymes (Trypsin, Pepsin), extracellular proteinases (Matrix metalloproteinases, ADAMTSs, Cathepsins) and antioxidant enzymes (extracellular superoxide dismutase). Often, proteins present in the extracellular space are stored outside the cells by attaching to various Extracellular matrix components (Collagens, Proteoglycans, etc.). In addition, Extracellular matrix proteolytic products are also present in the extracellular space, especially in tissues undergoing remodelling .
The term 'extracellular' is often used in reference to the extracellular fluid (ECF) compartment which composes about 15 litres of an average adult 70 kg human body which is assumed to contain a total of about 50 litres of water (thus, about 30% of the body's water is in the ECF compartment).
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.
Two compartments comprise the extracellular space: the vascular space and the interstitial space.