The electron transport chain (ETC) is a series of protein complexes and other molecules embedded in the inner mitochondrial membrane (in eukaryotes) or the plasma membrane (in prokaryotes) that facilitate the production of ATP through oxidative phosphorylation. Here’s a detailed look at the process:
Components of the Electron Transport Chain
- Complex I (NADH: Ubiquinone Oxidoreductase):
- Accepts electrons from NADH, which are transferred to flavin mononucleotide (FMN) and then to a series of iron-sulfur (Fe-S) clusters.
- The electrons are finally transferred to ubiquinone (coenzyme Q), reducing it to ubiquinol (QH2).
- This process is coupled with the translocation of four protons (H⁺) from the mitochondrial matrix to the intermembrane space.
- Complex II (Succinate: Ubiquinone Oxidoreductase):
- Accepts electrons from succinate via FADH2, which are then passed through Fe-S clusters to ubiquinone, reducing it to ubiquinol.
- Unlike Complex I, no protons are pumped across the membrane at this stage.
- Ubiquinone (Coenzyme Q):
- A lipid-soluble electron carrier that transfers electrons from Complexes I and II to Complex III.
- Ubiquinone is reduced to ubiquinol and then diffuses within the membrane to deliver electrons to Complex III.
- Complex III (Cytochrome bc1 Complex):
- Transfers electrons from ubiquinol to cytochrome c via the Q-cycle mechanism.
- Contains cytochromes b and c1 and an Fe-S protein (Rieske center).
- Pumps four protons into the intermembrane space for each pair of electrons transferred.
- Cytochrome c:
- A small heme protein that shuttles electrons from Complex III to Complex IV.
- It is located in the intermembrane space and interacts with the membrane-bound complexes.
- Complex IV (Cytochrome c Oxidase):
- Accepts electrons from cytochrome c and transfers them to oxygen, the final electron acceptor, forming water.
- Contains cytochromes a and a3, and copper centers.
- Pumps two protons across the membrane for each pair of electrons.
Proton Gradient and ATP Synthesis
The movement of electrons through the ETC is coupled with the pumping of protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient known as the proton motive force (PMF). This gradient represents stored energy.
ATP Synthase
- Structure: ATP synthase is a large enzyme complex composed of two main units, F₀ and F₁. F₀ is embedded in the membrane and forms a proton channel, while F₁ protrudes into the matrix and synthesizes ATP.
- Function: Protons flow back into the matrix through the F₀ unit, driven by the PMF. This flow causes rotation of the F₀ unit, which induces conformational changes in the F₁ unit, catalyzing the conversion of ADP and inorganic phosphate (Pi) to ATP.
Summary of the Process
- Electron donors: NADH and FADH2, produced in the citric acid cycle, donate electrons to the ETC.
- Electron transport: Electrons are passed through complexes I-IV via a series of redox reactions.
- Proton pumping: Complexes I, III, and IV pump protons into the intermembrane space, creating a PMF.
- ATP production: Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi.
- Oxygen: The final electron acceptor is oxygen, which combines with protons to form water, completing the electron transport process.
Overall Reaction
The overall reaction of the ETC can be summarized as follows:
[ \text{NADH} + \text{H}^+ + \frac{1}{2} \text{O}_2 \rightarrow \text{NAD}^+ + \text{H}_2\text{O} ]
[ \text{FADH}_2 + \frac{1}{2} \text{O}_2 \rightarrow \text{FAD} + \text{H}_2\text{O} ]
This process is highly efficient and generates the majority of ATP in aerobic organisms, highlighting its crucial role in cellular respiration and energy production.
Eneregitcs of 1 glucose molecule:
The complete oxidation of one molecule of glucose (C₆H₁₂O₆) through cellular respiration involves a series of metabolic pathways: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation via the electron transport chain. Here is a detailed breakdown of the energetics of one glucose molecule:
1. Glycolysis
- Location: Cytoplasm
- Process: Glucose (6-carbon molecule) is converted into two molecules of pyruvate (3-carbon molecules).
- Net ATP gain: 2 ATP (4 ATP produced, 2 ATP consumed)
- NADH produced: 2 NADH
2. Pyruvate Decarboxylation (Link Reaction)
- Location: Mitochondrial matrix
- Process: Each pyruvate is converted into acetyl-CoA, releasing one molecule of CO₂ and reducing NAD⁺ to NADH.
- NADH produced: 2 NADH (1 per pyruvate)
3. Citric Acid Cycle (Krebs Cycle)
- Location: Mitochondrial matrix
- Process: Each acetyl-CoA (2-carbon molecule) enters the cycle and is completely oxidized to CO₂, producing ATP (or GTP), NADH, and FADH₂.
- For each acetyl-CoA:
- ATP (or GTP) produced: 1 ATP (or GTP)
- NADH produced: 3 NADH
- FADH₂ produced: 1 FADH₂
- Since 2 acetyl-CoA molecules are produced per glucose:
- ATP (or GTP) produced: 2 ATP (or GTP)
- NADH produced: 6 NADH
- FADH₂ produced: 2 FADH₂
4. Electron Transport Chain and Oxidative Phosphorylation
- Location: Inner mitochondrial membrane
- Process: NADH and FADH₂ donate electrons to the ETC, driving the production of ATP through the proton gradient established across the inner mitochondrial membrane.
ATP Yield from Electron Carriers
- Each NADH results in approximately 2.5 ATP molecules.
- Each FADH₂ results in approximately 1.5 ATP molecules.
Total ATP Calculation
- Glycolysis:
- 2 ATP (direct gain)
- 2 NADH → 2 × 2.5 = 5 ATP
- Pyruvate Decarboxylation:
- 2 NADH → 2 × 2.5 = 5 ATP
- Citric Acid Cycle:
- 2 ATP (direct gain)
- 6 NADH → 6 × 2.5 = 15 ATP
- 2 FADH₂ → 2 × 1.5 = 3 ATP
Total ATP Yield from One Glucose Molecule
- Glycolysis: 2 ATP + 5 ATP (from 2 NADH) = 7 ATP
- Pyruvate Decarboxylation: 5 ATP (from 2 NADH)
- Citric Acid Cycle: 2 ATP + 15 ATP (from 6 NADH) + 3 ATP (from 2 FADH₂) = 20 ATP
Total ATP Yield: 7 + 5 + 20 = 32 ATP
Additional Considerations
- The exact ATP yield can vary slightly depending on the cell type and the efficiency of the transport systems that shuttle ATP, ADP, and Pi across the mitochondrial membranes.
- In some cells, the transport of NADH produced in glycolysis into the mitochondria may involve the glycerol phosphate shuttle (yielding approximately 1.5 ATP per NADH) or the malate-aspartate shuttle (yielding approximately 2.5 ATP per NADH), which can affect the total ATP yield slightly.
Overall, the complete oxidation of one molecule of glucose results in a net gain of approximately 30-32 ATP molecules, highlighting the efficiency of cellular respiration in producing energy.