Adp

Figure 6.7 ATP Energy is released when high-energy bonds are broken; an input of energy is required to convert ADP back to ATP.

Substrate-level phosphorylation uses the chemical energy released in an exergonic reaction to add Pi to ADP; oxidative phosphorylation harvests the energy of proton motive force to do the same thing. Recall from chapter 3 that proton motive force is the form of energy that results from the electrochemical gradient established as protons are expelled from the cell. The electron transport chain that generates this type of energy will be discussed later in this chapter. Photosynthetic organisms can generate ATP using the process of photophosphorylation, utilizing radiant energy of the sun to drive the formation of a proton motive force. The mechanisms they use to do this will be discussed later. ■ proton motive force, p. 55

The Role of the Chemical Energy Source

The compound broken down by a cell to release energy is called the energy source. As a group, prokaryotes show remarkable diversity in the variety of energy sources they can use. Many use organic compounds such as glucose. Others use inorganic compounds including hydrogen sulfide and ammonia. Harvesting energy from a compound involves a series of coupled oxidation-reduction reactions.

Oxidation-Reduction Reactions In oxidation-reduction reactions, or redox reactions, one or more electrons are transferred from one substance to another (figure 6.8). The compound that loses electrons becomes oxidized; the compound that gains those electrons becomes reduced. ■ electrons, p. 18

When electrons are removed from a compound, protons often follow. In other words, an electron-proton pair, or hydrogen atom, is often removed. Thus, the removal of a hydrogen atom is an oxidation; correspondingly, the addition of a hydrogen atom is a reduction. An oxidation reaction in which an electron and an accompanying proton are removed is called a dehy-drogenation. A reduction reaction in which an electron and an accompanying proton are added is called a hydrogenation.

When electrons are removed from the energy source, or electron donor, during catabolism, they are temporarily transferred to a specific molecule that serves as an electron carrier. That carrier can also be viewed as a hydrogen carrier if a proton accompanies the electron. Protons (H+), however, unlike electrons, do not require carriers when in an aqueous solution. Because of this, the whereabouts of protons in biological reactions are often ignored.

The Role of Electron Carriers

Just as cells use ATP as a carrier of free energy, they use designated molecules as carriers of electrons. Cells have several different types of electron carriers, and each of these serves a different function.

Three different types of electron carriers directly participate in reactions that oxidize the energy source (table 6.2). They are NAD+ (nicotinamide adenine dinucleotide), FAD (flavin adenine dinucleotide), and NADP+ (NAD phosphate). The reduced forms of these carriers are NADH, FADH2, and NADPH, respectively. These electron carriers can also be considered as hydrogen carriers because along with electrons, they carry protons. NAD+ and NADP+ can each carry a hydride ion, which consists of two electrons and one proton; FADH2 carries two electrons and two protons.

Reduced electron carriers represent reducing power because their bonds contain a form of usable energy. The reducing power of NADH and FADH2 is used to generate the proton motive force, which drives the synthesis of ATP in the process of oxidative phosphorylation. Ultimately the electrons are transferred to a compound such as O2 that functions as a terminal electron acceptor. The reducing power of NADPH has an entirely different fate; it is used in biosynthetic reactions when a reduction is required. Note, however, that many microbial cells have a membrane-associated enzyme that is able to use proton motive force to reduce NADP+. This allows them to convert reducing power in the form of NADH to NADPH.

Precursor Metabolites

Precursor metabolites are metabolic intermediates produced at specific steps in catabolic pathways that can be siphoned off and used in anabolic pathways. In anabolism, they serve as building blocks used to make the subunits of macromolecules

Loss of electron (oxidation)

Compound A

+

(oxidized)

(reduced)

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