BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.2. Oxidative Phosphorylation of ADP

16.2.2. Transport of ATP and ADP across Mitochondrial Membranes

In most Eukaryotic Cells, the bulk of ATP synthesis occurs inside the Cell/35.html">Mitochondria, whereas the primary ATP consumers are located outside them. Conversely, an adequate concentration of ADP must be maintained within the mitochondrial matrix. These charged molecules cannot spontaneously cross The Lipid Bilayer of the membranes. The inner membrane is impermeable to charged and hydrophilic substances, yet it contains A number of transporters that selectively carry such molecules from the Cytosol into the matrix and from the matrix into the cytosol.

The membrane features an ATP/ADP antiporter protein that mediates The transport of these metabolites across the membrane (Fig. 16.12). An ADP molecule enters the mitochondrial matrix only upon the simultaneous exit of an ATP molecule from the matrix.

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Fig. 16.12. Schematic representation of transmembrane substance transport driven by ΔμΗ⁺ energy: substances (ATP, ADP, H₃PO₄, Ca²⁺) pass through specific transporters, utilizing the energy of the membrane Electrochemical Potential

The driving force for this exchange is the Membrane Potential generated by Electron transport along the ETC. Calculations indicate that about a quarter of the Free energy of the proton motive force is expended on the Transport of ATP and ADP. Other transporters can also utilize the energy of the electrochemical gradient. This is precisely how inorganic phosphate (P_i), required for ATP synthesis, is transported into the mitochondrion. Furthermore, the direct source of free energy for the transport of Ca²⁺ into the matrix is the proton gradient rather than ATP energy.