Predicted equilibrium Ca²⁺ accumulation depending on mode of Ca²⁺ transport in energized mitochondria. The scheme highlights the net direction of Ca²⁺ flux for transport via a Ca²⁺ uniporter (A), a H⁺–Ca²⁺ exchanger (B), a 2H⁺–Ca²⁺ exchanger (C), and a 3H⁺–Ca²⁺ exchanger (D) in respiring mitochondria. The predicted equilibrium Ca²⁺ accumulation was calculated as described in the Appendix and refers to the matrix/cytosol concentration ratio so that a negative figure means that Ca²⁺ is more concentrated in the cytosol. The squiggle denotes the redox-coupled H⁺ pumps. For further explanation, see text and Appendix.