# Intracellular

In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".[1]

It is used in contrast to extracellular (outside the cell). The cell membrane (and, in many organisms, the cell wall) is the barrier between the two, and chemical composition of intra- and extracellular milieu (Milieu intérieur) can be radically different. In most organisms, for example, a Na+/K+ ATPase maintains a high potassium level inside cells while keeping sodium low, leading to chemical excitability.[2][3]

A commonly used notation is ${\displaystyle [S]_{i}}$ to denote the intracellular concentration of a substrate ${\displaystyle S}$. For example, ${\displaystyle [Ca^{2+}]_{i}}$ is the intracellular concentration of Calcium.