Water is a very low-energy molecule and is thus very stable; it can serve as an electron donor only when a large amount of energy from an external source is spent on splitting it into protons, electrons, and molecular oxygen. This is exactly what happens in oxygenic photosynthesis, where the external energy source is the sun, as we shall see later in the section on chloroplasts (p. 782).
the Energy Derived from Oxidation Is Stored as an Electrochemical Gradient
In mitochondria, the process of electron transport begins when two electrons and a proton are removed from NADH (to regenerate NAD+). These electrons are passed to the first of about 20 different electron carriers in the respiratory chain. The electrons start at a large negative redox potential (see Panel 14–1, p. 765)— that is, at a high energy level—which gradually drops as they pass along the chain. The proteins involved are grouped into three large respiratory enzyme complexes, each composed of protein subunits that sit in the inner mitochondrial membrane. Each complex in the chain has a higher affinity for electrons than its predecessor, and electrons pass sequentially from one complex to the next until they are finally transferred to molecular oxygen, which has the highest electron affinity of all.
The net result is the pumping of H+ out of the matrix across the inner membrane, driven by the energetically favorable flow of electrons. This transmembrane movement of H+ has two major consequences: 1.
It generates a pH gradient across the inner mitochondrial membrane, with a high pH in the matrix (close to 8) and a lower pH in the intermembrane space. Since ions and small molecules equilibrate freely across the outer mitochondrial membrane, the pH in the intermembrane space is the same as in the cytosol (generally around pH 7.4).
2.
It generates a voltage gradient across the inner mitochondrial membrane, creating a membrane potential with the matrix side negative and the crista space side positive.
The pH gradient (∆pH) reinforces the effect of the membrane potential (∆V ), because the latter acts to attract any positive ion into the matrix and to push any negative ion out. Together, ∆pH and ∆V make up the electrochemical gradient, which is measured in units of millivolts (mV). This gradient exerts a protonmotive force, which tends to drive H+ back into the matrix (Figure 14–14).
force due to proton-motive force due to INTERMEMBRANE membrane potential + + + + + + + + + pH 7.2 inner mitochondrial membrane pH 7.9
Figure 14–14 The electrochemical proton gradient across the inner mitochondrial membrane. this gradient is composed of a large force due to the membrane potential (∆V) and a smaller force due to the h+ concentration gradient—that is, the ph gradient (∆ph). Both forces combine to generate the proton-motive force, which pulls h+ back into the mitochondrial matrix. the exact relationship between these forces is expressed by the Nernst equation (see panel 11–1, p. 616).
The electrochemical gradient across the inner membrane of a respiring mitochondrion is typically about 180 mV (inside negative), and it consists of a membrane potential of about 150 mV and a pH gradient of about 0.5 to 0.6 pH units (each ∆pH of 1 pH unit is equivalent to a membrane potential of about 60 mV). The electrochemical gradient drives not only ATP synthesis but also the transport of selected molecules across the inner mitochondrial membrane, including the import of selected proteins from the cytoplasm (discussed in Chapter 12).