7.3 Biological Electron Transport Chains
2 min read•july 25, 2024
Electron carriers and the mitochondrial electron transport chain are crucial for cellular energy production. These molecules and protein complexes work together to transfer electrons, creating a proton gradient that powers ATP synthesis.
ATP synthesis, driven by the electron transport chain, is the primary source of cellular energy. This process is not only vital in mitochondria but also plays a role in other cellular processes like photosynthesis, bacterial energy production, and various metabolic pathways.
Electron Carriers and Mitochondrial Electron Transport Chain
Structure and function of electron carriers
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- Electron carriers accept and donate electrons in redox reactions transferring electrons between components of electron transport chains
- Nicotinamide adenine dinucleotide (NAD+/) consists of two nucleotides joined by phosphate groups carrying two electrons and one proton
- Flavin adenine dinucleotide (FAD/) contains riboflavin attached to an ADP molecule carrying two electrons and two
- Coenzyme Q (ubiquinone) lipid-soluble benzoquinone with long isoprenoid tail shuttles electrons between complexes in mitochondrial membrane
- Cytochromes heme-containing proteins transfer single electrons through changes in iron oxidation state
Organization of mitochondrial electron transport
- houses electron transport chain components
- (NADH dehydrogenase) accepts electrons from NADH transferring to coenzyme Q
- (Succinate dehydrogenase) accepts electrons from FADH2 transferring to coenzyme Q
- (Cytochrome bc1 complex) accepts electrons from reduced coenzyme Q transferring to
- (Cytochrome c oxidase) accepts electrons from cytochrome c transferring to molecular oxygen forming water
- Electrons move from complexes with lower to higher reduction potentials releasing energy used to pump protons into intermembrane space
ATP Synthesis and Other Cellular Processes
Coupling of electron transport to ATP synthesis
- Chemiosmosis drives ATP synthesis using electrochemical gradient
- Proton gradient forms as Complexes I, III, and IV pump protons into intermembrane space creating electrochemical gradient across inner mitochondrial membrane
- (Complex V) consists of F0 and F1 subunits catalyzing ATP synthesis using energy from proton flow
- ATP synthesis mechanism:
- Protons flow through ATP synthase from intermembrane space to matrix
- Proton flow energy drives F0 subunit rotation
- F0 rotation causes F1 conformational changes leading to ATP synthesis
- ATP yield approximately 2.5 ATP molecules per NADH and 1.5 ATP molecules per FADH2
Electron transport chains in cellular processes
- Photosynthesis light-dependent reactions involve electron transport chains in thylakoid membranes generating proton gradient for ATP synthesis (chloroplasts)
- Bacteria and archaea use diverse electron transport chains for various energy sources with alternative terminal electron acceptors (nitrate, sulfate)
- Plasma membrane electron transport present in various cell types functions in redox homeostasis and cell signaling (red blood cells)
- Endoplasmic reticulum electron transport involved in protein folding and lipid metabolism including cytochrome P450 systems for detoxification and hormone synthesis (liver cells)
- Nitrogen fixation electron transport chains in nitrogen-fixing bacteria provide energy for N2 reduction (Rhizobium)
Key Terms to Review (20)
Aerobic respiration: Aerobic respiration is a metabolic process in which organisms convert glucose and oxygen into energy, carbon dioxide, and water. This process is crucial for the production of ATP, the energy currency of cells, and involves a series of chemical reactions that take place primarily in the mitochondria, utilizing oxygen as the final electron acceptor in the electron transport chain.
Anaerobic organisms: Anaerobic organisms are life forms that can survive and thrive in environments devoid of oxygen. They rely on anaerobic processes to generate energy, which often involves fermentation or anaerobic respiration instead of the oxygen-dependent pathways seen in aerobic organisms. These organisms play crucial roles in various ecological systems and are essential for processes like decomposition and nutrient cycling.
ATP Synthase: ATP synthase is a complex enzyme found in the membranes of cells that catalyzes the production of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi). This enzyme plays a critical role in energy metabolism, linking electron transport chains to the synthesis of ATP during cellular respiration and photosynthesis.
Chemi-osmosis: Chemi-osmosis is the process by which ATP is produced in cells through the movement of protons (H ext{+}) across a membrane, driven by an electrochemical gradient. This mechanism is crucial in cellular respiration and photosynthesis, as it helps convert the energy from electrons into usable ATP, powering various biological functions.
Complex I: Complex I, also known as NADH:ubiquinone oxidoreductase, is a crucial enzyme in the mitochondrial electron transport chain that facilitates the transfer of electrons from NADH to ubiquinone while pumping protons across the inner mitochondrial membrane. This process contributes to the establishment of a proton gradient, which is essential for ATP production during cellular respiration. Its role is vital in energy metabolism and the regulation of redox balance within the cell.
Complex II: Complex II, also known as succinate dehydrogenase, is an essential component of the electron transport chain located in the inner mitochondrial membrane. It plays a dual role in cellular respiration by participating in both the citric acid cycle and the electron transport chain, facilitating the transfer of electrons from succinate to ubiquinone while also contributing to the generation of ATP through oxidative phosphorylation.
Complex III: Complex III, also known as the cytochrome bc1 complex, is a crucial component of the electron transport chain located in the inner mitochondrial membrane. It plays a vital role in cellular respiration by facilitating the transfer of electrons from coenzyme Q (ubiquinone) to cytochrome c, while simultaneously pumping protons (H+) across the mitochondrial membrane, contributing to the proton gradient necessary for ATP synthesis. This complex is essential for both energy production and maintaining metabolic balance within cells.
Complex IV: Complex IV, also known as cytochrome c oxidase, is the final enzyme in the electron transport chain of mitochondria that catalyzes the reduction of oxygen to water, utilizing electrons transferred from cytochrome c. This process is crucial for cellular respiration, as it helps establish the proton gradient used to produce ATP through oxidative phosphorylation, connecting the concept of energy metabolism to electron transport.
Coupling efficiency: Coupling efficiency is a measure of how effectively the energy produced from electron transport is used to drive ATP synthesis in biological systems. This term highlights the relationship between the amount of energy generated through electron transport chains and the amount of that energy that is converted into usable chemical energy in the form of ATP. A high coupling efficiency indicates that most of the energy is harnessed for ATP production, while a low efficiency suggests significant energy loss, often as heat.
Cytochrome c: Cytochrome c is a small heme-containing protein that plays a crucial role in the electron transport chain within mitochondria, facilitating the transfer of electrons between complex III and complex IV. It is vital for cellular respiration, as it helps in the generation of ATP by shuttling electrons, which ultimately contributes to the production of energy in aerobic organisms.
Electron transfer: Electron transfer refers to the movement of electrons from one molecule or atom to another, playing a crucial role in various biochemical processes, particularly in energy production. This process is integral to oxidation-reduction reactions, where one substance is oxidized (loses electrons) and another is reduced (gains electrons), facilitating energy release and utilization within biological systems.
Fadh2: FADH2 is a reduced form of flavin adenine dinucleotide, an important coenzyme that plays a critical role in cellular respiration and metabolism by acting as an electron carrier. It is produced during metabolic processes like the Krebs cycle and fatty acid oxidation, transferring electrons to the electron transport chain to help generate ATP, the energy currency of the cell.
Inner mitochondrial membrane: The inner mitochondrial membrane is a selectively permeable membrane that separates the mitochondrial matrix from the intermembrane space, playing a crucial role in cellular respiration and energy production. This membrane is highly folded into structures called cristae, which increase the surface area for the electron transport chain, allowing for efficient ATP synthesis through oxidative phosphorylation.
Membrane potential: Membrane potential refers to the electrical charge difference across a cell's plasma membrane, created by the unequal distribution of ions inside and outside the cell. This potential is crucial for various cellular processes, including the transmission of electrical signals in nerve and muscle cells, as well as the transport of molecules across membranes. It plays a fundamental role in energy production in cells and communication between them.
NADH: NADH, or Nicotinamide adenine dinucleotide (reduced form), is a crucial coenzyme found in all living cells that plays a vital role in cellular metabolism. It acts as an electron carrier in various metabolic processes, facilitating the transfer of electrons in redox reactions, and is essential for energy production in both aerobic and anaerobic conditions. As a key player in biological electron transport chains, NADH helps convert energy stored in nutrients into usable cellular energy.
Oligomycin: Oligomycin is a potent inhibitor of ATP synthase, the enzyme responsible for synthesizing ATP during oxidative phosphorylation in the mitochondria. By blocking the proton channel of ATP synthase, oligomycin effectively prevents protons from flowing back into the mitochondrial matrix, halting ATP production and disrupting the electron transport chain. This disruption highlights the critical role of ATP synthase in cellular energy metabolism.
Oxidative Phosphorylation: Oxidative phosphorylation is a metabolic process that takes place in the mitochondria, where ATP is produced as electrons are transferred through a series of proteins in the electron transport chain, ultimately combining with oxygen to form water. This process is crucial for energy production in cells and connects various aspects of metabolism, cellular respiration, and bioenergetics.
Protons: Protons are positively charged subatomic particles found within the nucleus of an atom. They play a crucial role in defining the atomic number of an element, which determines its identity and properties. The number of protons in an atom also influences its behavior in chemical reactions and interactions, particularly in biological systems where they are vital for processes like electron transport and energy production.
Redox potential: Redox potential, also known as reduction potential, is a measure of the tendency of a chemical species to acquire electrons and be reduced. It is crucial in understanding how electron transport chains operate, as it determines the direction and flow of electrons during cellular respiration and photosynthesis. Higher redox potentials indicate a greater ability to accept electrons, which plays a key role in energy production in biological systems.
Rotenone: Rotenone is a natural compound derived from the roots of certain plants, particularly from the Derris and Lonchocarpus genera, and is known for its ability to inhibit cellular respiration by blocking the electron transport chain. This compound functions primarily as a pesticide but is also significant in studying the biochemical pathways involved in energy production and mitochondrial function.