Intermembran

Intermembran, or intermembrane space, is the aqueous compartment found between two lipid bilayers in cells that possess double-membrane organelles. The term is used to describe spaces between membranes such as the outer and inner envelopes of mitochondria and chloroplasts, as well as similar spaces in other enveloped organelles. The exact width and composition of these spaces vary among organelles and species, but they share the common feature of separating two membranes that regulate transport and signaling.

In mitochondria, the intermembrane space lies between the outer mitochondrial membrane and the inner mitochondrial membrane. Here, the electron transport chain pumps protons into this space, contributing to the proton motive force that drives ATP synthesis across the inner membrane. The intermembrane space contains soluble enzymes and small molecules involved in energy metabolism, redox balance, and metabolite exchange. It is also a key player in apoptosis: release of cytochrome c and other pro-apoptotic factors from the intermembrane space into the cytosol activates a caspase cascade leading to programmed cell death.

Proteins are targeted to and transported through the mitochondrial membranes, with specific pathways delivering proteins to the matrix or to the intermembrane space. Import and maturation processes help maintain the distinct composition of this space, including disulfide bond formation for certain, small intermembrane space proteins.

A similar intermembrane space exists between the outer and inner envelopes of chloroplasts and other plastids, where it participates in metabolite exchange and protein trafficking. Although the concept is most often discussed in mitochondria, intermembrane spaces broadly serve to separate and regulate the functions of adjacent compartments within double-membrane organelles.