Figure 3.

Stoichiometric mediation of conflict. In the initial endosymbiosis (a), the proto-eukaryote takes up substrate and uses an unspecified molecule, X, as a terminal electron acceptor. The proto-mitochondrion takes up the reduced form of this electron carrier, XH, and oxidizes it, excreting X. Both partners obtain energy in this fashion. The proto-mitochondrion replicates and the host increases in size (b), eventually dividing (c). This simple life cycle continues with conflicts mediated by stoichiometry.