Krebs Cycle Definition- Cellular Respiration
What Is the Krebs Cycle?
The Krebs cycle—also called the citric acid cycle or TCA cycle—is the second stage of cellular respiration. It takes place in the mitochondrial matrix and generates energy by oxidizing acetyl-CoA.
Here's the brutal truth: without this cycle, your cells produce almost nothing. It's the metabolic hub where carbohydrates, fats, and proteins converge to release stored energy.
Where the Krebs Cycle Happens
The entire cycle operates inside the mitochondrial matrix—the innermost compartment of mitochondria. This location is critical because the enzymes needed for the cycle float freely in this space.
The outer mitochondrial membrane keeps the whole operation contained. The inner membrane, with its cristae folds, is where the electron transport chain later harvests the energy products from this cycle.
The 8 Steps of the Krebs Cycle
The cycle consists of 8 enzymatic reactions. Each step transforms substrates into products that feed into the next reaction.
Step 1: Acetyl-CoA Entry
Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons). The enzyme citrate synthase catalyzes this irreversible condensation.
Step 2: Citrate Formation
Citrate is rearranged into isocitrate by aconitase. This step involves a dehydration followed by rehydration—molecular rearrangement without carbon loss.
Step 3: First Oxidation
Isocitrate dehydrogenase catalyzes the first oxidative decarboxylation. CO₂ is released, and NAD⁺ is reduced to NADH. The product is α-ketoglutarate.
Step 4: Second Oxidation
α-ketoglutarate dehydrogenase complex removes another CO₂ and reduces NAD⁺ to NADH. Succinyl-CoA is formed.
Step 5: Substrate-Level Phosphorylation
Succinyl-CoA synthetase converts succinyl-CoA to succinate. This step directly generates 1 GTP (or ATP) through substrate-level phosphorylation.
Step 6: Succinate to Fumarate
Succinate dehydrogenase oxidizes succinate to fumarate. This enzyme is unique—it's part of both the Krebs cycle and the electron transport chain. FADH₂ is produced.
Step 7: Fumarate to Malate
Fumarase adds water across the double bond in fumarate, producing malate.
Step 8: Regeneration of Oxaloacetate
Malate dehydrogenase oxidizes malate back to oxaloacetate. NAD⁺ is reduced to NADH. The cycle is complete and ready to start again.
What the Krebs Cycle Produces
Per one acetyl-CoA molecule entering the cycle:
- 2 molecules of CO₂ (released as waste)
- 3 NADH molecules
- 1 FADH₂ molecule
- 1 GTP (or ATP)
These energy carriers feed into the electron transport chain, where the real ATP production happens. The Krebs cycle itself yields only 2 CO₂ and 1 GTP per acetyl-CoA.
Krebs Cycle vs. Other Stages of Cellular Respiration
Cellular respiration has three main stages. Here's how they compare:
| Stage | Location | Input | Output | ATP Yield |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose | 2 Pyruvate, 2 ATP, 2 NADH | 2 ATP |
| Krebs Cycle | Mitochondrial matrix | 2 Acetyl-CoA (from 2 pyruvate) | 2 ATP, 6 NADH, 2 FADH₂, 6 CO₂ | 2 ATP |
| Electron Transport Chain | Inner mitochondrial membrane | NADH, FADH₂ | H₂O, ATP | ~32-34 ATP |
The Krebs cycle produces minimal ATP directly. Its real value is generating high-energy electron carriers (NADH and FADH₂) that power ATP synthesis later.
Why the Krebs Cycle Matters
The cycle isn't just about energy. It provides metabolic intermediates for biosynthesis:
- α-ketoglutarate → amino acid synthesis
- Succinyl-CoA → heme synthesis
- Oxaloacetate → gluconeogenesis
Cancer cells hijack this aspect of the Krebs cycle to fuel rapid growth. When oxygen is scarce, some cells switch to aerobic glycolysis (the Warburg effect) instead of oxidative metabolism.
Getting Started: Memorizing the Cycle
If you're studying this for an exam, focus on these core facts:
- Citrate synthase starts the cycle—acetyl-CoA + oxaloacetate → citrate
- Two CO₂ molecules are released per acetyl-CoA
- Three NADH and one FADH₂ are produced per turn
- One GTP is made via substrate-level phosphorylation
- The cycle requires oxygen indirectly—NAD⁺ must be regenerated for the cycle to continue
Draw the cycle diagram once. Label each intermediate and enzyme. Repeat until you can sketch it from memory without checking notes.
Common Mistakes Students Make
The Krebs cycle does not produce CO₂ directly from carbon chains. CO₂ is released during the two decarboxylation steps (isocitrate dehydrogenase and α-ketoglutarate dehydrogenase).
Another error: confusing substrate-level phosphorylation with oxidative phosphorylation. Only one ATP (as GTP) comes directly from the cycle. The rest comes from the electron transport chain.
The Bottom Line
The Krebs cycle is the metabolic engine that converts acetyl-CoA into electron carriers. It feeds the electron transport chain, which actually generates most of the ATP. Without this cycle, aerobic respiration collapses. 🧬