In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". It 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. Cell biology (also called cellular biology or cytology, from the Greek kytos, container) is an academic discipline which studies cells. ... Molecular biology is the study of biology at a molecular level. ... Cells in culture, stained for keratin (red) and DNA (green) The cell is the structural and functional unit of all living organisms, and are sometimes called the building blocks of life. ... In cell biology, molecular biology and related fields, the word intracellular means inside the cell. It is used in contrast to extracellular (outside the cell). ... Drawing of a cell membrane A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... A cell wall is a more or less solid layer surrounding a cell. ... ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and a free phosphate ion. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ...

The term 'extracellular' is often used in reference to the extracellular fluid (ECF) which composes about 15 litres of the average human body. In some animals, including mammals, the two types of extracellular fluids are interstitial fluid and blood plasma. ... The litre (spelled liter in American English) is a unit of volume. ...