Extracellular
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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).
The cell membrane (and, in plants, 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 sodium level outside cells while keeping potassium low, leading to chemical excitability.
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 cell receptors. 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.
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