Summary
Highlights
The electron transport chain (ETC) occurs on the inner mitochondrial membrane, also known as the cristae. The mitochondria consist of an outer membrane, an intermembrane space, and an inner membrane (cristae), which encloses the mitochondrial matrix where the Krebs cycle takes place.
Glucose is oxidized into two pyruvate molecules during glycolysis, generating 2 NADH molecules. Pyruvate is then converted to acetyl-CoA, producing another 2 NADH. The Krebs cycle further generates 6 NADH and 2 FADH2. These NADH and FADH2 molecules act as electron shuttles, carrying hydride ions (a proton with two electrons) to the ETC.
NADH donates its electrons to Complex I, also known as NADH dehydrogenase. NADH is oxidized to NAD+, while flavin mononucleotide (FMN) within Complex I is reduced to FMNH2. These electrons are then passed to iron-sulfur clusters and subsequently to coenzyme Q, which becomes reduced to QH2. As Complex I transfers electrons, it pumps protons from the mitochondrial matrix into the intermembrane space.
FADH2 donates its electrons to Complex II, also called succinate dehydrogenase (an enzyme also involved in the Krebs cycle). FADH2 is oxidized to FAD. FMN within Complex II is reduced, and these electrons are then passed to coenzyme Q, reducing it to QH2. Unlike Complex I, Complex II does not pump protons.
Reduced coenzyme Q (QH2), an important mobile electron carrier (also known as ubiquinone or coenzyme Q10), carries electrons from Complex I and Complex II to Complex III. Complex III, also called coenzyme Q cytochrome b-c1 oxidoreductase or cytochrome bc1 complex, accepts these electrons. Cytochrome B within Complex III, containing iron, picks up the electrons, changing its oxidation state (Fe3+ to Fe2+). This transfer causes Complex III to go to a high energy level, subsequently pumping protons into the intermembrane space as it passes electrons to the next carrier.
From Complex III, electrons are passed to cytochrome C, another mobile electron carrier containing iron (and sometimes copper). Cytochrome C then transfers these electrons to Complex IV, also known as cytochrome oxidase. Complex IV features cytochromes A and A3, which also contain iron. As electrons are passed to Complex IV, it goes to a high energy state and pumps protons into the intermembrane space.
Within Complex IV, the electrons are ultimately passed to atomic oxygen (O2). This oxygen combines with two protons to form water (H2O). Oxygen serves as the final electron acceptor in the electron transport chain, marking the end of the electron cascade.
The pumping of protons at Complex I, III, and IV creates a high concentration of protons in the intermembrane space compared to the mitochondrial matrix. This 'proton-motive force' or concentration gradient is crucial for ATP synthesis, which will be discussed in subsequent videos.
The NADH generated during glycolysis cannot directly cross the mitochondrial membrane. A special mechanism for transporting these electrons into the mitochondria will be detailed in the next video.