Glycolysis oxidizes glucose to pyruvate, yielding some ATP and NADH and 6 precursor metabolites. The pentose phosphate pathway initiates the breakdown of glucose; its greatest significance is its contribution of 2 precursor metabolites and NADPH for biosynthesis. The transition step and the TCA cycle, repeated twice, complete the oxidation of glucose, yielding some ATP, a great deal of reducing power, and 3 precursor metabolites.

■ What is the product of the transition step?

■ Explain why the TCA cycle ultimately results in a greater ATP gain than glycolysis.

■ Which compound contains more free energy—pyruvate or oxaloacetate? Why?

6.4 Respiration

Respiration uses the NADH and FADH2 generated in gycoly-sis, the transition step, and the TCA cycle to synthesize ATP. The process, called oxidative phosphorylation, occurs through a combination of two mechanisms—the electron transport chain, which generates proton motive force, and an enzyme called ATP synthase, which harvests the proton motive force to drive the synthesis of ATP. In 1961, the British scientist Peter Mitchell originally proposed the chemiosmotic theory that describes the remarkable mechanism by which ATP synthesis is linked to electron transport, but his hypothesis was widely dismissed. Only through years of self-funded research was he finally able to convince others of its validity, and he was awarded the Nobel Prize in 1978.

The Electron Transport Chain—Generating Proton Motive Force

The electron transport chain is the term used to collectively describe a group of membrane-embedded electron carriers that pass electrons sequentially from one to another. In prokaryotes, the electron transport chain is located in the cytoplasmic membrane, whereas in eukaryotic cells it is in the inner membrane of mitochondria (see figure 3.55). Because of the asymmetrical arrangement of the electron carriers, the sequential oxidation/ reduction reactions result in the ejection of protons to the outside of the cell or, in the case of mitochondria, to the space between the inner and outer membranes. This expulsion of protons creates a proton gradient, or electrochemical gradient, across the membrane. Energy represented in this gradient, proton motive force, can be harvested by cells and used to fuel the synthesis of ATP. Recall from chapter 3 that prokaryotes can also use proton motive force as a source of energy to transport substances into or out of the cell, and to power the rotation of flagella.

Four types of electron carriers participate in the electron transport chain:

■ Flavoproteins are proteins to which an organic molecule called a flavin is attached. FAD is an example of a flavin.

■ Iron-sulfur proteins are proteins that contain iron and sulfur molecules arranged in a cluster.

■ Quinones are lipid soluble molecules that move freely in the membrane and can therefore transfer electrons between different enzyme structures in the membrane. Several types of quinones exist, one of the most common being ubiquinone (meaning ubiquitous quinone).

■ Cytochromes are protein molecules that contain heme, a chemical structure that holds an iron atom in the center. Several different cytochromes exist, each distinguished with a letter after the term, for example, cytochrome c.

Because of the order of the carriers in the electron transport chain, energy is gradually released as the electrons are passed from one carrier to another, much like a ball falling down a flight of stairs (figure 6.17). Energy release is coupled to the ejection of protons to establish a proton gradient.

General Mechanisms of Proton Ejection

An important characteristic of the electron carriers is that some accept only hydrogen atoms (proton-electron pairs), whereas others accept only electrons. The spatial arrangement of these two types of carriers in the membrane causes protons to be shuttled from the inside of the membrane to the outside. This occurs because a hydrogen carrier that receives electrons from an electron carrier must pick up protons; because of the hydrogen carrier's relative location in the membrane, those protons come from inside the cell (or matrix of the mitochondrion). Conversely, when a hydrogen carrier passes electrons to a carrier that accepts electrons but not protons, free protons are released to the outside of the cell (or intermembrane space of the mitochondrion). The net effect of

Electrons from energy source

High energy

Energy for synthesis of

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