Chemiosmotic Coupling

Rotation of bacterial flagella

► FIGURE 8-1 Overview of the generation and utilization of a proton-motive force.

A transmembrane proton concentration gradient and a voltage gradient, collectively called the proton-motive force, are generated during photosynthesis and the aerobic oxidation of carbon compounds in mitochondria and aerobic bacteria. In chemiosmotic coupling, a proton-motive force powers an energy-requiring process such as ATP synthesis (A), transport of metabolites across the membrane against their concentration gradient (B) or rotation of bacterial flagella (C).

▲ FIGURE 8-2 Membrane orientation and the direction of proton movement during chemiosmotically coupled ATP synthesis in bacteria, mitochondria, and chloroplasts. The membrane surface facing a shaded area is a cytosolic face; the surface facing an unshaded area is an exoplasmic face. Note that the cytosolic face of the bacterial plasma membrane, the matrix face of the inner mitochondrial membrane, and the stromal face of the thylakoid membrane are all equivalent. During electron transport, protons are always pumped from the cytosolic face to the exoplasmic face, creating a proton concentration gradient (exoplasmic face > cytosolic face) and an electric potential (negative cytosolic face and positive exoplasmic face) across the membrane. During the coupled synthesis of ATP; protons flow in the reverse direction (down their electrochemical gradient) through ATP synthase (F0F-| complex), which protrudes from the cytosolic face in all cases.

Eukaryotic plasma membrane

Endocytosis of bacterium capable of oxidative phosphorylation

Bacterial plasma membrane

Bacterial plasma membrane becomes inner membrane of mitochondrion

Bacterial plasma membrane

Bacterial plasma membrane becomes inner membrane of mitochondrion

Mitochondrial matrix

Endocytosis of bacterium capable of photosynthesis

Eukaryotic plasma membrane

Endocytosis of bacterium capable of oxidative phosphorylation

Endocytosis of bacterium capable of photosynthesis

Bacterial plasma membrane becomes inner membrane of chloroplast

Bacterial plasma membrane

Bacterial plasma membrane becomes inner membrane of chloroplast

Inner membrane buds off thylakoid vesicles

Bacterial plasma membrane

Inner membrane buds off thylakoid vesicles

Thylakoid membrane

▲ FIGURE 8-3 Evolutionary origin of mitochondria and chloroplasts according to endosymbiont hypothesis.

Membrane surfaces facing a shaded area are cytosolic faces; surfaces facing an unshaded area are exoplasmic faces. Endocytosis of a bacterium by an ancestral eukaryotic cell would generate an organelle with two membranes, the outer membrane derived from the eukaryotic plasma membrane and the inner one from the bacterial membrane. The Ft subunit of

ATP synthase, localized to the cytosolic face of the bacterial membrane, would then face the matrix of the evolving mitochondrion (left) or chloroplast (right). Budding of vesicles from the inner chloroplast membrane, such as occurs during development of chloroplasts in contemporary plants, would generate the thylakoid vesicles with the Ft subunit remaining on the cytosolic face, facing the chloroplast stroma.

hypothesis, the inner mitochondrial membrane would be derived from the bacterial plasma membrane with the globular Fj domain still on its cytosolic face pointing toward the matrix space of the mitochondrion. Similarly, the globular Fj domain would be on the cytosolic face of the thylakoid membrane facing the stromal space of the chloroplast.

In addition to powering ATP synthesis, the protonmotive force can supply energy for the transport of small molecules across a membrane against a concentration gradient (see Figure 8-1). For example, a H+/sugar symport protein catalyzes the uptake of lactose by certain bacteria, and proton-driven antiporters catalyze the accumulation of ions and sucrose by plant vacuoles (Chapter 7). The proton-motive force also powers the rotation of bacterial flagella. (The beating of eukaryotic cilia, however, is powered by ATP hydrolysis.) Conversely, hydrolysis of ATP by V-class ATP-powered proton pumps, which are similar in structure to F-class pumps (see Figure 7-6), provides the energy for transporting protons against a concentration gradient. Chemiosmotic coupling thus illustrates an important principle introduced in our discussion of active transport in Chapter 7: the membrane potential, the concentration gradients of protons (and other ions) across a membrane, and the phosphoanhydride bonds in ATP are equivalent and interconvertible forms of chemical potential energy.

In this brief overview, we've seen that oxygen and carbohydrates are produced during photosynthesis, whereas they are consumed during aerobic oxidation. In both processes, the flow of electrons creates a H+ electrochemical gradient, or proton-motive force, that can power ATP synthesis. As we examine these two processes at the molecular level, focusing first on aerobic oxidation and then on photosynthesis, the striking parallels between them will become evident.

bulk of the ATP produced during the conversion of glucose to CO2. In this section, we discuss the biochemical pathways that oxidize glucose and fatty acids to CO2 and H2O; the fate of the released electrons is described in the next section.

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