Fundamentals of Molecular Biology. Part 1: Cell Molecular Biology - A. N. Ogurtsov 2011

Oxidation of Glucose and Fatty Acids
Chemiosmotic Coupling

At first glance, Photosynthesis and aerobic oxidation have nothing in common. However, it turns out that Cell/35.html">Mitochondria, METABOLISM/14.html">Chloroplasts, and bacterial Plasma Membranes all share a single underlying mechanism for synthesizing ATP from ADP and P_i, known as chemiosmosis (or chemiosmotic coupling).

The proton-motive force is defined as the combination of the proton concentration gradient (formed during chemiosmosis) and the electrical potential difference across the inner and outer sides of a biomembrane (Membrane Potential) (Figure 143).

Named by analogy with the electromotive force (EMF) in electrodynamics, the proton-motive force serves as the energy source driving ATP synthesis (Figure 143, bottom). It is generated by the stepwise translocation of high-energy electrons via specialized membrane-bound electron carriers, utilizing The energy released by these electrons at each step of their passage along the Electron Transport Chain.

Chemiosmosis can only take place within an isolated cellular compartment whose membrane is impermeable to protons (H⁺).

In mitochondria and non-photosynthetic bacterial Cells, such electrons are transferred from NADH (generated through the metabolism of sugars, Fatty acids, and other substances) to O₂, the terminal electron acceptor.

In the thylakoid membranes of chloroplasts, the energy of absorbed photons is transferred to electrons "extracted" from Water molecules (with the release of O₂) and used to drive these electrons along a chain of carriers, notably NADP⁺. Ultimately, these electrons are passed to CO₂ to drive carbohydrate synthesis.

All these systems feature analogous carriers that couple electron transport to the pumping of protons across the membrane—always directed from the Cytosol of the isolated compartment into the exoplasmic space—thereby generating the proton-motive force (Figure 143, top). Consequently, the cytosolic layer of the membrane is always negatively charged relative to the exoplasm. Such vectorality ensures the proper physiological Functions of Biomembranes.

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Figure 143 - Schematic representation of the generation and utilization of the proton-motive force. Through chemiosmotic coupling, the proton-motive force provides energy for processes such as: 1 - ATP synthesis; 2 - metabolite Transport Across the membrane; 3 - bacterial flagellar rotation

Furthermore, mitochondria (Figure 144), chloroplasts (Figure 145), and Bacteria (Figure 146) all utilize the exact same type of membrane protein for ATP synthesis: ATP synthase (the F₀F₁ complex).

In all cases, ATP synthase is oriented such that the catalytic F₁ domain faces the cytosolic surface of the membrane; consequently, ATP is invariably synthesized within the cytosolic region of the isolated compartment.

Figure 144 - Schematic layout of chemiosmotic coupling components in a mitochondrion

The proton flux through ATP synthase is directed from the exoplasmic space into the cytosolic region of the compartment.

Figure 145 - Schematic layout of chemiosmotic coupling components in a chloroplast

This flux is driven by the combined action of both the proton concentration gradient ([H⁺]_exoplasm > [H⁺]_Cytoplasm) and the membrane electrical potential (the potential of the exoplasmic membrane surface (+) being higher than that of the cytosolic membrane surface (-)).