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The transfer of H + through a proton pump generates an electrochemical gradient of protons, called a proton motive force . The Proton Motive Force. - It drives the conversion of ADP to ATP through ATP synthase. - This process is known as the chemiosmotic theory. Figure 14.5.

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The transfer of H+ through a proton pump generates an electrochemical gradient of protons, called a proton motive force.

The Proton Motive Force

- It drives the conversion of ADP to ATP through ATP synthase.

- This process is known as the chemiosmotic theory.

Figure 14.5

When protons are pumped across the membrane, energy is stored in two different forms:

- The electrical potential (Dy) arises from the separation of charge between the cytoplasm and solution outside the cell membrane.

- The pH difference (DpH) is the log ratio of external to internal chemical concentration of H+.

The relationship between the two components of the proton potential Dp is given by:

Dp = Dy – 60DpH

The Proton Motive Force

Besides ATP synthesis, Dp drives many cell processes including: rotation of flagella, uptake of nutrients, and efflux of toxic drugs.

Dp Drives Many Cell Functions

Figure 14.9

ETS proteins such as cytochromes associate electron transfer with small energy transitions, which are mediated by cofactors.

Energy transitions typically involve these kinds of molecular structures:

- Metal ions, such as iron or copper,held in place with amino acid residues

- Conjugated double bonds and heteroaromatic rings, such as the nicotinamide ring of NAD+/NADH

The Respiratory ETS

Animation: A bacterial electron transfer system


Click box to launch animation

The F1Fo ATP synthase is a highly conserved protein complex, made of two parts:

The F1Fo ATP Synthase

- Fo: Embedded in the membrane

- Pumps protons

- F1: Protrudes in the cytoplasm

- Generates ATP

Figure 14.17

Animation: ATP Synthase Mechanism

The F1Fo ATP Synthase

Click box to launch animation

Oxidized forms of nitrogen

- Nitrate is successively reduced as follows:

NO3– → NO2– → NO → 1/2 N2O → 1/2 N2

nitric oxide

nitrous oxide

nitrogen gas



- In general, any given species can carry out only

one or two transformations in the series.

Oxidized forms of sulfur

- Sulfate is successively reduced by many bacteria as follows:

SO42– → SO32– → 1/2 S2O32– → S0 → H2S


hydrogen sulfide




Anaerobic environments, such as the bottom of a lake, offer a series of different electron acceptors.

- As each successive TEA is used up, its reduced form appears; the next best electron acceptor is then used, generally by a different microbe species.

Figure 14.20

Lithotrophy is the acquisition of energy by oxidation of inorganic electron donors.

A kind of lithotrophy of great importance in the environment is nitrogen oxidation.


1/2 O2


1/2 O2

NH4+ → NH2OH → HNO2 → HNO3



nitric acid


nitrous acid


Surprisingly, ammonium can also yield energy under anaerobic conditions through oxidation by nitrite produced from nitrate respiration.

Hydrogenotrophy is the use of molecular hydrogen (H2) as an electron donor.


-H2 has sufficient reducing potential to donate e– to nearly all biological electron acceptors.

- Including chlorinated organic molecules, via dehalorespiration

- Which has potential for bioremediation

Figure 14.24