Q.Explain the reactions taking place in mitochondrial inner membrane.
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The Big Picture: Why Do We Breathe?
You already know that food gives you energy. But how does a glucose molecule turn into something your cells can actually use? The answer involves a chain — literally — of protein complexes embedded in the inner membrane of your mitochondria. This is the Electron Transport Chain (ETC).
Think of it as a carefully arranged series of relay runners. Each runner passes a "hot potato" (an electron) to the next, and with each pass, a little energy is released. That energy is used to pump protons (H⁺) across the membrane, creating a gradient — like water stored behind a dam. When the protons flow back through a turbine (the enzyme ATP synthase), they spin it to make ATP, the cell's energy currency.
The final runner in this relay is oxygen. It catches the electrons, combines with protons, and forms water. That's why you breathe: to supply the oxygen that keeps the chain running.
The Precise Statement
The Electron Transport Chain is a sequence of four multi-protein complexes (Complex I, II, III, and IV) and two mobile carriers (ubiquinone and cytochrome c) embedded in the inner mitochondrial membrane. These complexes transfer electrons from reduced coenzymes (NADH and FADH₂) to molecular oxygen, the final electron acceptor. The energy released during electron transfer is used to pump protons from the mitochondrial matrix into the intermembrane space, establishing an electrochemical gradient that drives ATP synthesis via chemiosmosis.
NADH+21O2+H+→NAD++H2O+energy (for 2.5 ATP)
FADH2+21O2→FAD+H2O+energy (for 1.5 ATP)
How It Works, Step by Step
1. The Donors: NADH and FADH₂
These are the reduced coenzymes produced during glycolysis, the Krebs cycle, and fatty acid oxidation. They carry high-energy electrons. NADH delivers its electrons to Complex I; FADH₂ delivers to Complex II (which feeds into the chain at a later point via ubiquinone).
2. The Complexes and Mobile Carriers
| Component | What it does | Key fact |
|---|---|---|
| Complex I (NADH dehydrogenase) | Accepts electrons from NADH, transfers them to ubiquinone (Q), and pumps 4 H⁺ across the membrane. | The entry point for NADH. |
| Complex II (Succinate dehydrogenase) | Accepts electrons from FADH₂ (from succinate in the Krebs cycle), transfers them to Q. | Does not pump protons. |
| Ubiquinone (Q) | A small, lipid-soluble molecule that shuttles electrons from Complex I/II to Complex III. | Mobile carrier. |
| Complex III (Cytochrome bc₁) | Accepts electrons from Q, transfers them to cytochrome c, and pumps 4 H⁺. | Uses the Q cycle to move electrons. |
| Cytochrome c | A small, water-soluble protein that carries one electron at a time from Complex III to Complex IV. | Mobile carrier. |
| Complex IV (Cytochrome c oxidase) | Accepts electrons from cytochrome c, transfers them to O₂, and pumps 2 H⁺. | The final step: O₂ + 4e⁻ + 4H⁺ → 2H₂O. |
A common mistake is to think that Complex II directly pumps protons. It does not. FADH₂ therefore yields fewer ATP molecules than NADH because its electrons enter the chain at a lower energy level.
3. The Proton Gradient and ATP Synthesis
As electrons move through Complexes I, III, and IV, protons are pumped from the matrix into the intermembrane space. This creates a high concentration of H⁺ outside and a low concentration inside — a proton motive force. The only way protons can flow back into the matrix is through ATP synthase, a molecular turbine. As protons pass through it, the synthase rotates and catalyzes the formation of ATP from ADP and inorganic phosphate. …
The inner mitochondrial membrane houses the electron transport chain (Complexes I-IV) and ATP synthase (Complex V), which together carry out oxidative phosphorylation. …
Step 1. The inner mitochondrial membrane is folded into cristae bearing the four electron-transport complexes and the ATP synthase. Complex I (NADH dehydrogenase) accepts electrons and protons from matrix NADH and passes them to ubiquinone (UQ).
Step 2. Complex II (succinate dehydrogenase) accepts electrons directly from succinate as it is oxidised to fumarate in the Krebs cycle, also passing them to ubiquinone.
Step 3. Complex III (cytochrome bc1 complex) oxidises reduced ubiquinone (ubiquinol) and passes electrons, via an iron-sulphur centre, to the mobile carrier cytochrome c.
Step 4. Complex IV (cytochrome c oxidase) accepts electrons from cytochrome c and finally reduces molecular oxygen to water -- the terminal oxidation step of respiration. …
Walk through the four electron-transport complexes in order and connect their electron flow to …
- Describing only the electron flow (Complexes I-IV) without connecting it to the proton gradient and ATP synthase, missing the actual point of oxidative phosphorylation. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is responsible for the transport of electrons in electron transport system?(a) Pyruvic acid(b) Fucoxanthin(c) Cytochrome(d) Acetic acid
›Reveal solutionSolution
Cytochromes are the electron-carrier proteins of the mitochondrial electron transport system (ETS).
During aerobic respiration, NADH and FADH2 generated in glycolysis, the link reaction, and the Krebs cycle are oxidised in the inner mitochondrial membrane by the electron transport system. This system is made up of a series of electron carriers, chiefly the cytochromes (haem-containing proteins such as cytochrome b, c1, c, a, a3) along with other complexes, which pass electrons from one to the next in a stepwise, energy-releasing manner, ultimately reducing molecular oxygen to water while pumping protons that drive ATP synthesis (oxidative phosphorylation, chemiosmosis).
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- CBSE 2026Set ANNUAL1 markQ.Write the full form of ETS.
›Reveal solutionSolution
ETS is short for Electron Transport System, the chain of carrier complexes on the inner mitochondrial membrane responsible for oxidative phosphorylation.
During aerobic respiration, the reduced coenzymes NADH and FADH2 generated in glycolysis and the Krebs cycle carry high-energy electrons to a series of membrane-embedded protein complexes located on the cristae of the mitochondrion. These complexes — Complex I through Complex IV, along with mobile carriers like ubiquinone and cytochrome c — together constitute the Electron Transport System (ETS). As electrons pass along this chain, protons are pumped acro …
- CBSE 2026Set ANNUAL1 markMCQQ.The Ultimate electron acceptor in an Aerobic organism during respiration is :(a) Cytochrome(b) Oxygen(c) Hydrogen(d) Pyruvic acid
›Reveal solutionSolution
Oxygen is the terminal electron acceptor of the mitochondrial electron transport chain in aerobic respiration.
During aerobic respiration, electrons released from NADH and FADH2 (generated in glycolysis, the link reaction, and the Krebs cycle) are passed sequentially through a series of electron carriers/complexes (including cytochromes) embedded in the inner mitochondrial membrane — this is the electron transport chain (ETC). As electrons move through the chain, protons are pumped across the membrane, creating a gradient used by ATP synthase to make ATP (oxidative phosphorylation). At the very end of the chain, the electrons (along with H+ ions) are accepted by molecular oxygen (O2), which is reduced to form water (H2O). Because oxygen is the la …
- CBSE 2025Set ANNUAL1 markQ.Which is final electron acceptor in aerobic respiration ?
›Reveal solutionSolution
Oxygen (O2) is the final electron acceptor of the electron transport chain in aerobic respiration.
During aerobic respiration, NADH and FADH2 (generated in glycolysis, the link reaction, and the Krebs cycle) donate electrons to a series of carriers (Complexes I-IV) in the electron transport chain (ETC) located on the inner mitochondrial membrane. As electrons pass down this chain, energy is released and used to pump protons, ultimately driving ATP synthesis by chemiosmosis (oxidative phosphorylation). At the very end of the chain, the electrons (together with protons) are accepted by …
- CBSE 2025Set botany-sz1 markMCQQ.Which of the following is a mobile electron carrier ?(a) Cytochrome 'a'(b) Cytochrome 'a3'(c) Cytochrome 'c'(d) Cytochrome 'b'
›Reveal solutionSolution
Cytochrome c is the mobile electron carrier of the electron transport chain, since unlike the other cytochromes it is not fixed within a membrane complex.
The electron transport chain (ETC), located on the inner mitochondrial membrane, consists of a series of large, membrane-embedded protein complexes: NADH dehydrogenase (Complex I), succinate dehydrogenase (Complex II), the cytochrome bc1 complex containing cytochromes b and c1 (Complex III), and cytochrome c oxidase containing cytochromes a and a3 (Complex IV).
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- CBSE 2025Set botany-hz21 markMCQQ.Electron transport system is located in mitochondrial:(a) Outer membrane(b) Inner membrane(c) Inter membrane space(d) Matrix
›Reveal solutionSolution
The electron transport chain (Complexes I-IV and ATP synthase) is located in the inner mitochondrial membrane.
The mitochondrion has two membranes: a smooth outer membrane and a highly folded inner membrane (folds called cristae). The inner membrane houses the protein complexes of the electron transport system (Complex I: NADH dehydrogenase, Complex II: succinate dehydrogenase, Complex III: cytochrome bc1, Complex IV: cytochrome c oxidase) along with ATP synthase (Complex V). Electrons from NADH and FADH2 (produced in glycolysis, pyruvate oxidation, and the Krebs cycle occurring in the mitochondrial matrix) are passed along these complexes in the inner membrane, pumping protons into the intermembrane space and creating a proton gradient that drives ATP synthesis via chemiosmosis (oxi …
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