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Studies of net translocation of various anions and cations and their corresponding acids and bases, using extensions of established techniques, imply that the M phase of the cristae membrane of rat liver mitochondria is very impermeable (i.e. not permitting net permeation) to the following ions: CH3COO, H2PO4, HPO42−, SO42−, Fe(CN)63−, Fe(CN)64−, choline+, K+, Na+ and NH4+. How-ever, the M phase appears to be permeated rapidly by SCN and slowly by Cl.

It has been confirmed that CH3COO and NH4+ probably permeate as CH3COOH and NH3 respectively, and that phosphate equilibrates across the M phase by an electrically neutral mechanism equivalent to H3PO4 translocation. The possibility that phosphate is obligatorily translocated as KH2PO4 has been eliminated.

Sulphate equilibrates by an electrically neutral mechanism equivalent to H2SO4 translocation.

The alkali metals Na+ and K+ (and possibly NH4+) equilibrate across the M phase via specific electrically neutral H+/Na+ and H+/K+ antiport, the former being the more active at neutral pH. Choline does not participate significantly in proton-coupled antiport.