Bacteria, including the strict anaerobes, maintain a proton gradient across their plasma membrane that is harnessed to drive many other processes. It can be used to drive a flagellar motor, for example (Figure 14–35). This gradient is harnessed to pump Na+ out of the bacterium via a Na+–H+ antiporter that takes the place of the Na+-K+ pump of eukaryotic cells. The gradient is also used for the active inward transport of nutrients, such as most amino acids and many sugars: each nutrient is dragged into the cell along with one or more protons through a specific symporter (Figure 14–36; see also Chapter 11). In animal cells, by contrast, most inward transport across the plasma membrane is driven by the Na+ gradient (high Na+ outside, low Na+ inside) that is established by the Na+-K+ pump (see Figure 11–15).
Some unusual bacteria have adapted to live in a very alkaline environment and yet must maintain their cytoplasm at a physiological pH. For these cells, any attempt to generate an electrochemical H+ gradient would be opposed by a large H+ concentration gradient in the wrong direction (H+ higher inside than outside). Presumably for this reason, some of these bacteria substitute Na+ for H+ in all of their chemiosmotic mechanisms. The respiratory chain pumps Na+ out of the cell, the transport systems and flagellar motor are driven by an inward flux of Na+, and a Na+-driven ATP synthase synthesizes ATP. The existence of such bacteria demonstrates a critical point: the principle of chemiosmosis is more fundamental than the proton-motive force on which it is normally based.
As we discuss next, an ATP synthase coupled to chemiosmotic processes is also a central feature of plants, where it plays critical roles in both mitochondria and chloroplasts.
Figure 14–36 The importance of H+-driven transport in bacteria.
a proton-motive force generated across the plasma membrane pumps nutrients into the cell and expels Na+. (a) In an aerobic bacterium, a respiratory chain fed by the oxidation of substrates produces an electrochemical proton gradient across the plasma membrane. this gradient is then harnessed to make atp, as well as to transport nutrients (proline, succinate, lactose, and lysine) into the cell and to pump Na+ out of the cell.
(B) When the same bacterium grows under anaerobic conditions, it derives its atp from glycolysis. as indicated, the atp synthase in the plasma membrane then hydrolyzes some of this atp to establish an electrochemical proton gradient that drives the same transport processes that depend on respiratory chain proton-pumping in (a).
Figure 14–35 The rotation of the bacterial flagellum driven by H+ flow.
the flagellum is attached to a series of protein rings (pink), which are embedded in the outer and inner membranes and rotate with the flagellum. the rotation is driven by a flow of protons through an outer ring of proteins (the stator) by mechanisms that may resemble those used by the atp synthase. however, the flow of protons in the flagellar motor is always toward the cytosol, both during clockwise and counterclockwise rotation, whereas in atp synthase this flow reverses with the direction of rotation (Movie 14.8).