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Detailed look at how mitochondria extract maximum energy from pyruvate using oxygen.
Imagine your body is a high-tech power plant. While basic digestion gets the turbines spinning, the Krebs Cycle and Chemiosmosis are the massive reactors that generate 90% of your energy. How does a single molecule of glucose turn into the literal spark of life?
Before the main event, we must prep the fuel. In the mitochondrial matrix, pyruvate (a 3-carbon molecule) undergoes a transformation. One carbon is removed as , and the remaining 2-carbon fragment is attached to Coenzyme A, forming Acetyl-CoA. During this process, is reduced to . Think of Acetyl-CoA as the 'VIP pass' that allows carbon to enter the Citric Acid Cycle. Without this step, the energy stored in the bonds of pyruvate remains locked away. This transition links glycolysis in the cytoplasm to the aerobic processes inside the mitochondria.
Quick Check
What is the primary 2-carbon molecule that enters the Citric Acid Cycle?
Answer
Acetyl-CoA
The Citric Acid Cycle (or Krebs Cycle) is an eight-step metabolic engine. It starts when Acetyl-CoA () joins with Oxaloacetate () to form Citrate (). As the cycle turns, two carbons are released as . The real goal, however, isn't the carbon—it's the electrons. By the end of one turn, the cell has produced: - molecules of - molecule of - molecule of (or GTP)
These electron carriers ( and ) are like batteries loaded with high-energy potential, headed straight for the inner membrane.
Let's track the carbons for a single glucose molecule. 1. One glucose produces pyruvates via glycolysis. 2. Each pyruvate loses carbon during oxidation ( total). 3. Each Acetyl-CoA loses carbons in the Krebs cycle ( total). 4. Total produced: . This matches the original carbons in glucose ().
Quick Check
Why is the Krebs Cycle considered a 'cycle'?
Answer
Because it begins and ends with the same molecule, Oxaloacetate, which is regenerated at the final step.
Now, the loaded 'batteries' ( and ) deposit their electrons into the Electron Transport Chain (ETC), a series of proteins embedded in the inner mitochondrial membrane. As electrons pass from one protein to the next, they release energy. The cell uses this energy to pump protons () from the matrix into the intermembrane space. This creates a massive concentration gradient—a 'proton-motive force.' Oxygen sits at the end of the chain as the final electron acceptor, combining with electrons and to form water ().
Imagine a hydroelectric dam. 1. The ions are the water behind the dam (intermembrane space). 2. The ETC proteins are the pumps that move the water uphill using electron energy. 3. The concentration gradient is measured by differences: the intermembrane space becomes more acidic () than the matrix.
The final step is Chemiosmosis. Those crowded ions want to move back into the matrix (down their gradient), but they can only pass through a special protein called ATP Synthase. As flows through, it physically spins a rotor in the protein. This mechanical energy is used to phosphorylate into . This process, known as oxidative phosphorylation, generates roughly to per glucose molecule, far eclipsing the made in glycolysis.
Cyanide is a deadly poison because it binds to the final protein in the ETC (Cytochrome c Oxidase). 1. If the ETC is blocked, electrons stop moving. 2. If electrons stop moving, is no longer pumped. 3. Without a gradient, ATP Synthase stops spinning. 4. The cell runs out of energy almost instantly, leading to death.
Where exactly is the proton gradient established?
Which molecule 'catches' the electrons at the very end of the Electron Transport Chain?
ATP Synthase uses the movement of electrons to directly create ATP.
Review Tomorrow
In 24 hours, try to sketch the mitochondria and draw the path of an electron from NADH to Oxygen. Can you explain how that path leads to ATP?
Practice Activity
Research 'Mitochondrial Diseases.' How would a defect in the ETC affect a person's ability to perform intense exercise?