Figure 7

The proposed scenario for the evolution of membrane bioenergetics. The scheme shows the proposed transition from primitive membranes that were leaky both to Na⁺ and H⁺ (dotted lines), via membranes that were Na⁺-tight but H⁺-leaky (dashed lines) to the membranes that were impermeable to H⁺ and Na⁺ (solid lines). Dashed arrows show symbiotic acquisitions of α-proteobacteria (purple arrow) and of cyanobacteria (green arrow). The scheme emphasises that proton tightness of the membranes was achieved in different ways in different lineages (see text).