Why does the fermenter cell want to harvest energy from glucose oxidation in small amounts? how does this slow energy release allow the cell to build up a pmf?
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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 chemiosmotic 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.
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Protons Are Unusually Easy to Move
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 chainto another, following a special channel through the protein.

Figure 14-20
How protons behave in water. (A) Protons move very rapidly along a chain of hydrogen-bonded water molecules. In this diagram, proton jumps are indicated by blue arrows, and hydronium ions are indicated by green shading. As discussed in Chapter 2, naked (more...)
Protons are also special with respect to electron transport. Whenever a moleculeis 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, while the proton is passed to the water. Therefore, in a membrane in which electrons are being passed along an electron-transport chain, pumping protons from one side of the membrane to another can be relatively simple. The electron carrier merely needs to be arranged in the membrane in a way that causes it to pick up a proton from one side of the membrane when it accepts an electron, and to release the proton on the other side of the membrane as the electron is passed to the next carrier molecule in the chain
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.
Go to:
Protons Are Unusually Easy to Move
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 chainto another, following a special channel through the protein.

Figure 14-20
How protons behave in water. (A) Protons move very rapidly along a chain of hydrogen-bonded water molecules. In this diagram, proton jumps are indicated by blue arrows, and hydronium ions are indicated by green shading. As discussed in Chapter 2, naked (more...)
Protons are also special with respect to electron transport. Whenever a moleculeis 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, while the proton is passed to the water. Therefore, in a membrane in which electrons are being passed along an electron-transport chain, pumping protons from one side of the membrane to another can be relatively simple. The electron carrier merely needs to be arranged in the membrane in a way that causes it to pick up a proton from one side of the membrane when it accepts an electron, and to release the proton on the other side of the membrane as the electron is passed to the next carrier molecule in the chain
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