Internalization of the proto-mitochondrion cell. A1) Initial stage before proto-cytoplasmic cell has begun to surround proto-mitochondrion cell. A2) Intermediate stage at which proto-cytoplasmic cell has largely surrounded proto-mitochondrion cell. A3) Full internalization; proto-mitochondrion cell is now a mitochondrion; the two cells are now a single cell that possesses a respiratory organelle. The cell membrane of the proto-mitochondrion becomes the inner mitochondrial membrane and the outer mitochondrial membrane is derived from the cell membrane of the proto-cytoplasm cell. These are one-dimensional slices through the cells; in three dimensions the proto-cytoplasm cell surrounds the proto-mitochondrion cell on all sides. Filled arrows in A1-A3 represent non-respiratory (e.g., Na/K ATPase) pump activity. These pumps pump Na and Ca out of the cytoplasm into the external medium or lumen, and K out of the external medium or lumen into the cytoplasm. As the proto-cytoplasm cell surrounds the proto-mitochondrion cell, these pumps would thus automatically (i.e., without any change in their orientation in the membrane) work so as to maintain an external-like (high Na and Ca, low K) ionic environment in the lumen separating the cells. “L” shaped lines represent cell anchoring proteins. B1) Electron micrograph showing how the inner membrane enclosed entity of contemporary mitochondria can divide separately from the outer mitochondrial membrane, just as would a prokaryote enclosed by the membrane of another prokaryote. Figure taken with permission from . B2) Tracing showing the membrane disposition in B1.