The Colors of Respiration and Electron Transport Reporter & Curator: Larry H. Bernstein, MD, FCAP Molecular Biology of the Cell. 4th edition Electron-Transport Chains and Their Proton Pumps http://www.ncbi.nlm.nih.gov/books/NBK26904/ Having considered in general terms how a mitochondrion uses electron transport to create an electrochemical proton gradient, we need to examine the mechanisms that underlie this membrane-based energy-conversion process. In doing so, we also accomplish a larger purpose. As emphasized at the beginning of this chapter, very similar chemi- osmotic mechanisms are used by mitochondria, chloroplasts, archea, and bacteria. In fact, these mechanisms underlie the function of nearly all living organisms— including anaerobes that derive all their energy from electron transfers between two inorganic molecules. It is therefore rather humbling for scientists to remind themselves that the existence of chemiosmosis has been recognized for only about 40 years. We begin with a look at some of the principles that underlie the electron-transport process, with the aim of explaining how it can pump protons across a membrane. Although protons resemble other positive ions such as Na+ and K+ in their movement across membranes, in some respects they are unique. Hydrogen atoms are by far the most abundant type of atom in living organisms; they are plentiful not only in all carbon-containing biological molecules, but also in the water molecules that surround them. The protons in water are highly mobile, flickering through the hydrogen-bonded network of water molecules by rapidly dissociating from one water molecule to associate with its neighbor, as illustrated in Figure 14-20A. Protons are thought to move across a protein pump embedded in a lipid bilayer in a similar way: they transfer from one amino acid side chain to another, following a special channel through the protein. Protons are also special with respect to electron transport. Whenever a molecule is reduced by acquiring an electron, the electron (e -) brings with it a negative charge. In many cases, this charge is rapidly neutralized by the addition of a proton (H+) from water, so that the net effect of the reduction is to transfer an entire hydrogen atom, H+ + e – (Figure 14-20B). Similarly, when a molecule is oxidized, a hydrogen atom removed from it can be readily dissociated into its constituent electron and proton—allowing the electron to be transferred separately to a molecule that accepts electrons, […]![]()
