Electron transport chain
46 flashcards covering Electron transport chain for the MCAT Biology & Biochemistry section.
The electron transport chain is a key stage in cellular respiration, occurring in the inner mitochondrial membrane. It involves a series of protein complexes that transfer electrons from molecules like NADH and FADH2 through a step-by-step process, releasing energy that pumps protons across the membrane. This creates an electrochemical gradient, which drives ATP production via oxidative phosphorylation. Essentially, it's the cell's way of converting energy from food into usable form, making it vital for understanding metabolism.
On the MCAT, the electron transport chain frequently appears in biology and biochemistry questions, often as multiple-choice items testing electron flow, proton gradients, or the effects of inhibitors like cyanide. Common traps include confusing the chain's sequence with other pathways, such as glycolysis, or overlooking the role of oxygen as the final electron acceptor. Focus on mastering the four main complexes, key electron carriers, and how disruptions impact ATP yield to handle these questions effectively.
A helpful tip: Visualize the chain with diagrams to track electron movement.
Terms (46)
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Electron Transport Chain
A sequence of protein complexes and electron carriers in the inner mitochondrial membrane that transfers electrons from NADH and FADH2 to oxygen, creating a proton gradient for ATP production during cellular respiration.
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Complex I
The first protein complex in the electron transport chain, also called NADH dehydrogenase, which oxidizes NADH and passes electrons to ubiquinone while pumping protons from the mitochondrial matrix to the intermembrane space.
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Complex II
The second protein complex, also known as succinate dehydrogenase, which accepts electrons from FADH2 derived from the Krebs cycle and passes them to ubiquinone without pumping protons.
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Complex III
The third protein complex, or cytochrome bc1 complex, that transfers electrons from ubiquinone to cytochrome c and pumps protons across the membrane, contributing to the proton gradient.
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Complex IV
The fourth protein complex, also called cytochrome c oxidase, which transfers electrons from cytochrome c to oxygen, the final electron acceptor, and pumps protons to help establish the proton gradient.
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ATP Synthase
An enzyme complex in the inner mitochondrial membrane that uses the energy from the proton gradient to synthesize ATP from ADP and inorganic phosphate through a process called chemiosmosis.
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NADH
A key electron donor in the electron transport chain, produced during glycolysis and the Krebs cycle, which donates electrons to Complex I to initiate the chain.
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FADH2
An electron donor generated in the Krebs cycle and fatty acid oxidation, which donates electrons to Complex II, entering the chain at a lower energy level than NADH.
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Ubiquinone
A mobile electron carrier, also known as coenzyme Q, that accepts electrons from Complexes I and II and transfers them to Complex III in the electron transport chain.
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Cytochrome c
A small mobile protein that carries electrons from Complex III to Complex IV, playing a crucial role in the transfer process within the electron transport chain.
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Proton Gradient
The electrochemical gradient of protons across the inner mitochondrial membrane, generated by the electron transport chain, which drives ATP synthesis by ATP synthase.
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Chemiosmosis
The process by which the proton gradient created by the electron transport chain provides the energy for ATP synthase to produce ATP as protons flow back into the mitochondrial matrix.
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Oxidative Phosphorylation
The overall process that couples the electron transport chain's electron transfer to ATP production, distinguishing it from substrate-level phosphorylation in glycolysis and the Krebs cycle.
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Inner Mitochondrial Membrane
The location of the electron transport chain complexes and ATP synthase in eukaryotic cells, where the proton gradient is established across this impermeable barrier.
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Intermembrane Space
The compartment between the inner and outer mitochondrial membranes where protons accumulate due to the electron transport chain, creating the proton gradient.
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Mitochondrial Matrix
The innermost compartment of the mitochondria where the Krebs cycle occurs and where protons are pumped out of during the electron transport chain.
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Final Electron Acceptor
Oxygen in aerobic respiration, which accepts electrons at the end of the electron transport chain in Complex IV, forming water and preventing a backup in the chain.
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Redox Reactions
The oxidation-reduction reactions in the electron transport chain where electrons are transferred between carriers, releasing energy that pumps protons.
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P/O Ratio
The ratio of ATP molecules produced per oxygen atom reduced, typically around 2.5 for NADH and 1.5 for FADH2 in the electron transport chain, reflecting efficiency of oxidative phosphorylation.
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Rotenone
An inhibitor that blocks electron flow at Complex I, preventing NADH from donating electrons and halting ATP production in the electron transport chain.
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Cyanide
A poison that inhibits Complex IV by binding to its heme groups, stopping electron transfer to oxygen and disrupting the entire electron transport chain.
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,4-Dinitrophenol
An uncoupler that dissipates the proton gradient across the inner mitochondrial membrane, allowing protons to return without producing ATP, thus reducing efficiency.
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Proton Pumping
The mechanism in Complexes I, III, and IV where electron transfer energy is used to move protons from the matrix to the intermembrane space, establishing the gradient.
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F1 Subunit
The catalytic portion of ATP synthase that protrudes into the mitochondrial matrix and synthesizes ATP using energy from proton flow through the F0 subunit.
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F0 Subunit
The membrane-spanning part of ATP synthase that forms a channel for protons, allowing their flow to drive the rotation that powers ATP synthesis.
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Electron Flow Pathway
The sequence from NADH or FADH2 through Complexes I or II, ubiquinone, Complex III, cytochrome c, and Complex IV to oxygen, with energy released at each step.
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Standard Reduction Potentials
Values that indicate the tendency of electron carriers in the chain to gain electrons, with higher potentials driving the flow from NADH to oxygen.
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Reactive Oxygen Species
Harmful byproducts like superoxide generated when electrons leak from the electron transport chain to oxygen, potentially damaging cells if not neutralized.
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Energy Conservation
The process in the electron transport chain where free energy from electron transfer is captured as a proton gradient rather than heat, maximizing ATP yield.
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Gibbs Free Energy
The energy change in redox reactions of the electron transport chain that determines how much proton pumping can occur, with negative values indicating spontaneity.
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Nernst Equation
An equation used to calculate the actual reduction potential under non-standard conditions in the electron transport chain, affecting proton gradient strength.
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Supercomplexes
Aggregates of electron transport chain complexes that enhance efficiency by allowing direct electron transfer without diffusion of mobile carriers.
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Rotational Catalysis
The mechanism in ATP synthase where proton flow causes rotation of its subunits, changing conformation to bind ADP and phosphate, then release ATP.
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Prokaryotic ETC
In bacteria, the electron transport chain occurs in the plasma membrane instead of mitochondria, with similar complexes but variations in oxygen use and efficiency.
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Antioxidant Defense
Cellular mechanisms that protect against reactive oxygen species produced by the electron transport chain, such as enzymes like superoxide dismutase.
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ATP Yield from NADH
Approximately 2.5 ATP molecules per NADH oxidized in the electron transport chain, depending on the efficiency of the proton gradient and ATP synthase.
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ATP Yield from FADH2
Approximately 1.5 ATP molecules per FADH2 oxidized, as it enters the chain at Complex II and bypasses the first proton-pumping site.
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Common Trap: Confusing ETC with Krebs
Students often mistake the electron transport chain for the Krebs cycle, but the former transfers electrons for ATP while the latter produces electron donors like NADH.
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Strategy for ETC Questions
When answering questions, trace the electron path from donors to oxygen and note how each complex contributes to the proton gradient to predict ATP output or inhibitor effects.
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Free Energy Change per Electron
The cumulative negative free energy from redox reactions in the chain powers proton translocation, with each pair of electrons yielding about 10 protons total.
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Oxygen's Role as Acceptor
As the final electron acceptor, oxygen ensures the chain continues by oxidizing the last complex, and its absence leads to fermentation in anaerobic conditions.
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Example: Electron Transfer in Complex I
In Complex I, NADH loses two electrons and a proton, reducing FMN to FMNH2, which then passes electrons to iron-sulfur clusters and finally to ubiquinone.
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Proton Stoichiometry
The number of protons pumped per electron pair: four by Complex I, four by Complex III, and two by Complex IV, totaling ten protons for NADH-derived electrons.
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Uncoupling Proteins
Proteins in the inner membrane that allow protons to bypass ATP synthase, dissipating energy as heat for thermogenesis, as seen in brown fat.
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Mitochondrial Diseases
Disorders caused by defects in electron transport chain components, leading to reduced ATP production and symptoms like muscle weakness.
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Electrochemical Gradient
The combined effect of the proton concentration gradient and membrane potential in the electron transport chain that drives ATP synthesis.