The central metabolic pathways of glycolysis, the link reaction, the Krebs cycle (also known as the citric acid cycle or TCA cycle), and the electron transport chain (ETC) are interconnected processes that play a crucial role in cellular respiration. Here's an overview of each pathway and their connections:
1. Glycolysis
- Location: Cytoplasm
- Overview: Glycolysis is the initial pathway of glucose metabolism that breaks down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons). This process occurs in ten enzyme-catalyzed steps and can operate both aerobically and anaerobically.
- Products:
- 2 pyruvate molecules
- 2 ATP (net gain)
- 2 NADH
2. Link Reaction (Pyruvate Decarboxylation)
- Location: Mitochondrial matrix
- Overview: The link reaction occurs after glycolysis when pyruvate is transported into the mitochondria. Each pyruvate undergoes decarboxylation, where one carbon is removed and released as CO2, and the remaining two-carbon molecule (acetyl-CoA) is generated.
- Products:
- 2 acetyl-CoA (one from each pyruvate)
- 2 CO2 (one from each pyruvate)
- 2 NADH (one from each pyruvate)
3. Krebs Cycle (Citric Acid Cycle)
- Location: Mitochondrial matrix
- Overview: The Krebs cycle begins when acetyl-CoA enters the cycle and combines with oxaloacetate to form citrate. This cycle consists of a series of enzymatic reactions that oxidize the acetyl group to CO2, regenerating oxaloacetate in the process.
- Products per Acetyl-CoA:
- 1 ATP (or GTP)
- 3 NADH
- 1 FADH2
- 2 CO2
Since each glucose molecule yields two acetyl-CoA, the total yield per glucose for the Krebs cycle will be:
- 2 ATP
- 6 NADH
- 2 FADH2
- 4 CO2
4. Electron Transport Chain (ETC)
- Location: Inner mitochondrial membrane
- Overview: The electron transport chain is the final stage of oxidative phosphorylation. NADH and FADH2 generated from glycolysis, the link reaction, and the Krebs cycle donate electrons to protein complexes in the inner mitochondrial membrane. As electrons pass through these complexes, protons are pumped from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient (proton motive force).
- Final Electron Acceptor: Oxygen, which combines with electrons and protons to form water.
- Products:
- Approximately 28-34 ATP via ATP synthase, driven by chemiosmosis.
Summary
In summary, the pathways work together in cellular respiration:
- Glycolysis converts glucose into pyruvate, generating a small amount of ATP and NADH.
- The link reaction converts pyruvate into acetyl-CoA, yielding more NADH and releasing CO2.
- The Krebs cycle further oxidizes acetyl-CoA, producing ATP, NADH, FADH2, and CO2.
- The electron transport chain utilizes NADH and FADH2 to generate the majority of ATP through oxidative phosphorylation. Thus, the entire process enables cells to efficiently extract energy from glucose.