D. Cellular Respiration
1. Aerobic Respiration
d. The Electron Transport Chain and Chemiosmosis
Learning Objectives for this Section
Aerobic respiration (def) is the aerobic catabolism of nutrients to carbon dioxide, water, and energy, and involves an electron transport system (def) in which molecular oxygen is the final electron acceptor. Most eukaryotes and prokaryotes use aerobic respiration to obtain energy from glucose. The overall reaction is:
Note that glucose (C6H12O6 ) is oxidized to produce carbon dioxide (CO2) and oxygen (O2) is reduced to produce water (H2O).
Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis. We will now look at the electron transport chain and chemiosmosis.
The Electron Transport Chain and Chemiosmosis
During various steps in glycolysis and the citric acid cycle, the oxidation of certain intermediate precursor molecules causes the reduction of NAD+ to NADH + H+ and FAD to FADH2. NADH and FADH2 then transfer protons and electrons to the electron transport chain to produce additional ATPs from oxidative phosphorylation (def).
As mentioned in the previous section on energy, during the process of aerobic respiration, coupled oxidation-reduction reactions and electron carriers are often part of what is called an electron transport chain (def), a series of electron carriers that eventually transfers electrons from NADH and FADH2 to oxygen. The diffusible electron carriers NADH and FADH2 carry hydrogen atoms (protons and electrons) from substrates in exergonic catabolic pathways such as glycolysis and the citric acid cycle to other electron carriers that are embedded in membranes. These membrane-associated electron carriers include flavoproteins, iron-sulfur proteins, quinones, and cytochromes. The last electron carrier in the electron transport chain transfers the electrons to the teminal electron acceptor, oxygen.
The chemiosmotic theory (def) explains the functioning of electron transport chains. According to this theory, the tranfer of electrons down an electron transport system through a series of oxidation-reduction reactions (def) releases energy (see Fig. 1). This energy allows certain carriers in the chain to transport hydrogen ions (H+ or protons) across a membrane.
Depending on the type of cell, the electron transport chain may be found in the cytoplasmic membrane or the inner membrane of mitochondria.
As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen ions creates an electrochemical gradient or potential difference (voltage) across the membrane. (The fluid on the side of the membrane where the protons accumulate acquires a positive charge; the fluid on the opposite side of the membrane is left with a negative charge.) The energized state of the membrane as a result of this charge separation is called proton motive force (def) or PMF.
This proton motive force provides the energy necessary for enzymes called ATP synthases (see Fig. 5), also located in the membranes mentioned above, to catalyze the synthesis of ATP from ADP and phosphate. This generation of ATP occurs as the protons cross the membrane through the ATP synthase complexes and re-enter either the bacterial cytoplasm (see Fig. 2) or the matrix of the mitochondria. As the protons move down the concentration gradient through the ATP synthase, the energy released causes the rotor and rod of the ATP synthase to rotate. The mechanical energy from this rotation is converted into chemical energy as phosphate is added to ADP tform ATP.
Proton motive force is also used to transport substances across membranes during active transport and to rotate bacterial flagella.
At the end of the electron transport chain involved in aerobic respiration, the last electron carrier in the membrane transfers 2 electrons to half an oxygen molecule (an oxygen atom) that simultaneously combines with 2 protons from the surrounding medium to produce water as an end product (see Fig. 3).
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