Outward Movement of ATP Is Favored over Outward ADP Movement

The charge on ATP at pH 7.2 or so is about —4, and the charge on ADP at the same pH is about —3. Thus, net exchange of an ATP (out) for an ADP (in) results in the net movement of one negative charge from the matrix to the cytosol. (This process is equivalent to the movement of a proton from the cytosol to the matrix.) Recall that the inner membrane is positive outside (see Figure 21.22), and it becomes clear that outward movement of ATP is favored over outward ADP transport, ensuring that ATP will be transported out (Figure 21.32). Inward movement of ADP is favored over inward movement of ATP for the same reason. Thus, the membrane electrochemical potential itself controls the specificity of the ATP-ADP translocase. However, the electrochemical potential is diminished by the ATP-ADP translocase cycle and therefore operates with an energy cost to the cell. The cell must compensate by passing yet more electrons down the electron transport chain.

What is the cost of ATP-ADP exchange relative to the energy cost of ATP synthesis itself? We already noted that moving 1 ATP out and 1 ADP in is the equivalent of one proton moving from the cytosol to the matrix. Synthesis of an

FIGURE 21.32 • Outward transport of ATP (via the ATP/ADP translocase) is favored by the membrane electrochemical potential.

ATP results from the movement of approximately three protons from the cytosol into the matrix through F0. Altogether this means that approximately four protons are transported into the matrix per ATP synthesized. Thus, approximately one-fourth of the energy derived from the respiratory chain (electron transport and oxidative phosphorylation) is expended as the electrochemical energy devoted to mitochondrial ATP-ADP transport.

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