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An electron transport chain (ETC [1]) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H + ions) across a membrane.
The mitochondrial NADH is then oxidized in turn by the electron transport chain, which pumps protons across a membrane and generates ATP through oxidative phosphorylation. [60] These shuttle systems also have the same transport function in chloroplasts. [61]
The chain of redox reactions driving the flow of electrons through the electron transport chain, from electron donors such as NADH to electron acceptors such as oxygen and hydrogen (protons), is an exergonic process – it releases energy, whereas the synthesis of ATP is an endergonic process, which requires an input of energy.
NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two low-energy waste products, H 2 O and CO 2, are created during this cycle. [9] [10]
The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation (electron transport) pathway. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable chemical energy in the form of ATP.
The mitochondrial shuttles are biochemical transport systems used to transport reducing agents across the inner mitochondrial membrane. NADH as well as NAD+ cannot cross the membrane, but it can reduce another molecule like FAD and [QH 2] that can cross the membrane, so that its electrons can reach the electron transport chain.
NADH and FADH 2 undergo oxidation in the electron transport chain by transferring an electrons to regenerate NAD + and FAD. Protons are pulled into the intermembrane space by the energy of the electrons going through the electron transport chain. Four electrons are finally accepted by oxygen in the matrix to complete the electron transport chain.
The net effect of the malate–aspartate shuttle is purely redox: NADH in the cytosol is oxidized to NAD +, and NAD + in the matrix is reduced to NADH. The NAD + in the cytosol can then be reduced again by another round of glycolysis, and the NADH in the matrix can be used to pass electrons to the electron transport chain so ATP can be synthesized.