Mitochondrie.svg|thumb|300px| Mitochondria structure :
1) Inner membrane
2) Outer membrane
3) Crista
4) Matrix ]]

The outer membrane refers to the outside membrane of an organelle which surrounds the rest of the organelle components. Both the chloroplast and the mitochondria have an outer membrane.

Gram-negative bacteria also possess an outer membrane, of unusual structure. The outer leaflet of the membrane comprises a complex lipopolysaccharide and the lipid portion acts as an endotoxin. If endotoxin enters the circulatory system it causes a toxic reaction with the sufferer having a high temperature and respiration rate and a low blood pressure. This may lead to endotoxic shock, which may be fatal.

Membrane biology