gap junction

Imagine two neighboring rooms with a short pipe joining them through the shared wall, so that small things can pass directly from one room to the other without going outside into the corridor. Cells often need to share signals and small molecules with their neighbors quickly and privately, without releasing them into the surrounding fluid. A gap junction is that connecting pipe: a tiny channel that links the inside of one cell directly to the inside of the next.

Each gap junction is made of proteins called connexins (in vertebrates). Six connexins arrange into a ring with a hole down the middle, forming a half-channel in one cell's membrane; this half lines up with a matching half in the neighbor's membrane, and together they make a continuous tube bridging the narrow gap between the two cells. The tube is small, letting through only ions and small molecules up to about a thousand atomic mass units — things like calcium ions, small sugars, and signaling molecules — while keeping large proteins and DNA inside each cell. The channels can open and close, so the connection is controllable, not always wide open.

Gap junctions matter most where cells must act in unison. In the heart, electrical signals pass through gap junctions from one muscle cell to the next so the whole heart contracts as a coordinated beat instead of a twitchy mob. They let liver cells share metabolites, let a nerve cell pass a fast signal to its neighbor, and let early embryo cells coordinate. A useful contrast: tight junctions seal the space between cells so nothing passes, while gap junctions punch open channels so small molecules pass freely — opposite jobs, often found in the same sheet of cells.