Every movement you make, every thought you think, every protein your body builds — all of it runs on ATP (adenosine triphosphate). Cellular respiration is how your cells make ATP from glucose. It’s not breathing — breathing just delivers the oxygen that respiration needs. The actual energy extraction happens inside your cells, mostly inside the mitochondria, through a series of carefully controlled chemical reactions.
Understanding cellular respiration deeply means understanding three stages (glycolysis, Krebs cycle, electron transport chain), how they connect, why aerobic respiration yields so much more ATP than anaerobic, and what happens in different organisms when oxygen runs out.
Key Terms & Definitions
ATP (Adenosine Triphosphate): The universal energy currency of cells. Energy released by breaking the bond between the second and third phosphate group (~30.5 kJ/mol) powers cellular work.
Glycolysis: The first stage of cellular respiration, occurring in the cytoplasm. Splits one glucose (6C) into two pyruvate (3C) molecules. Produces 2 net ATP and 2 NADH.
Pyruvate: The 3-carbon product of glycolysis. The branch point — it goes into the mitochondria (aerobic) or undergoes fermentation (anaerobic).
Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix. Each pyruvate (as acetyl-CoA) is fully oxidised to CO₂. Generates NADH, FADH₂, and 1 ATP per turn (2 turns per glucose).
Electron Transport Chain (ETC): Located on the inner mitochondrial membrane. Uses electrons from NADH and FADH₂ to pump H⁺ across the membrane, creating a gradient that drives ATP synthesis. This is where most ATP is made.
Chemiosmosis: The flow of H⁺ (protons) through ATP synthase down a concentration gradient, powering ATP synthesis. This is Peter Mitchell’s chemiosmotic theory (Nobel Prize 1978).
NAD⁺/NADH: Electron carrier. NAD⁺ accepts electrons (gets reduced to NADH); NADH donates electrons to the ETC (gets oxidised back to NAD⁺). Essential for glycolysis to continue.
FAD/FADH₂: Another electron carrier, works similarly to NAD⁺/NADH. Used in the Krebs cycle.
Fermentation: Anaerobic process that regenerates NAD⁺ (allowing glycolysis to continue) without producing more ATP directly. Two types: alcoholic (in yeast) and lactic acid (in muscle cells).
Stage 1: Glycolysis — The Universal First Step
Glycolysis occurs in the cytoplasm — it requires no organelles. This means every living cell, even prokaryotes with no mitochondria, can perform glycolysis. It is the most ancient energy-extraction pathway.
Net reaction of glycolysis:
Steps in brief:
- Energy investment phase (Steps 1–5): 2 ATP are consumed to phosphorylate glucose and split it into two G3P (glyceraldehyde-3-phosphate) molecules.
- Energy payoff phase (Steps 6–10): Each G3P generates 2 ATP and 1 NADH. Since there are 2 G3P: 4 ATP and 2 NADH.
- Net yield: 4 ATP produced − 2 ATP consumed = 2 net ATP per glucose.
Glycolysis is not very efficient — it extracts only ~2% of the energy in glucose. But it works without oxygen, can run very fast, and provides pyruvate + NADH for the subsequent aerobic stages. Its speed is critical during explosive exercise when oxygen delivery lags demand.
Stage 2: Pyruvate Oxidation and the Krebs Cycle
Pyruvate Oxidation (Pyruvate Decarboxylation)
Before entering the Krebs cycle, pyruvate is transported into the mitochondrial matrix and converted to acetyl-CoA by the pyruvate dehydrogenase complex:
Two carbons enter the Krebs cycle; one CO₂ is released. This is the first CO₂ you exhale from glucose metabolism.
The Krebs Cycle (Per Turn = Per Pyruvate)
Each acetyl-CoA (2C) joins with oxaloacetate (4C) to form citrate (6C). As the cycle turns:
- 2 carbons leave as CO₂ (completing the oxidation of the original glucose carbons)
- 3 NADH are produced
- 1 FADH₂ is produced
- 1 ATP (or GTP) is produced
The 4C oxaloacetate is regenerated, ready for the next turn.
Per glucose (2 turns of the Krebs cycle):
- 6 CO₂ released (2 from pyruvate oxidation + 4 from two turns)
- 6 NADH produced
- 2 FADH₂ produced
- 2 ATP produced
At this point, the carbon skeleton of glucose is completely oxidised to CO₂. But most of the energy is still “stored” in the electron carriers NADH and FADH₂ — not yet in ATP.
Stage 3: The Electron Transport Chain and Oxidative Phosphorylation
This is where the payoff happens. NADH and FADH₂ donate their electrons to the electron transport chain (ETC), embedded in the inner mitochondrial membrane.
Electron flow: NADH → Complex I → CoQ → Complex III → Cytochrome c → Complex IV → O₂ (final electron acceptor)
FADH₂ → Complex II → CoQ → (same path)
At Complexes I, III, and IV: the energy released as electrons flow “downhill” is used to pump H⁺ ions from the matrix into the intermembrane space, creating a proton-motive force (electrochemical gradient).
ATP Synthase (Complex V): H⁺ ions flow back into the matrix through ATP synthase — this flow powers the synthesis of ATP from ADP + Pᵢ. This is chemiosmosis.
Yield: Each NADH yields ~2.5 ATP; each FADH₂ yields ~1.5 ATP. (Older textbooks say 3 and 2 respectively; newer NCERT values are 2.5 and 1.5.)
Total from oxidative phosphorylation:
- 10 NADH × 2.5 = 25 ATP
- 2 FADH₂ × 1.5 = 3 ATP
- 28 ATP from ETC
Grand total: 4 (from glycolysis + Krebs) + 28 (from ETC) = ~32 ATP per glucose (newer values) or 36–38 ATP (older NCERT textbooks still use 36).
NEET and CBSE Class 11 still largely use 36 ATP as the total. Use 36 ATP in board and NEET answers unless the question specifically asks for the “revised” value. JEE may use 30–32 ATP (the biologically accurate value). Check the context of your question.
Anaerobic Respiration and Fermentation
When oxygen is unavailable, the ETC cannot function (no final electron acceptor). NADH accumulates, and NAD⁺ runs out. Without NAD⁺, glycolysis cannot continue (it needs NAD⁺ to accept electrons from G3P oxidation).
Fermentation solves this by using pyruvate itself (or a derivative) as an electron acceptor to re-oxidise NADH back to NAD⁺:
Alcoholic Fermentation (Yeast)
Industrial applications: beer, wine, bread (CO₂ from fermentation makes dough rise).
Lactic Acid Fermentation (Muscle cells, bacteria)
No CO₂ released. Lactic acid accumulates in muscles during intense exercise → muscle fatigue and burning sensation. During recovery (rest), lactic acid is transported to the liver where it’s converted back to glucose (Cori cycle).
Applications: yoghurt production, cheese, sauerkraut.
Comparison: Aerobic vs Anaerobic
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required | Yes | No |
| Location | Cytoplasm + Mitochondria | Cytoplasm only |
| ATP yield | ~36–38 ATP | 2 ATP |
| End products | CO₂ + H₂O | Lactic acid OR Ethanol + CO₂ |
| Completeness | Complete oxidation | Partial oxidation |
| Rate | Slower (sustained) | Faster (burst) |
Exam-Specific Tips
CBSE Class 10: Focus on the overall equation for aerobic and anaerobic respiration. Know that respiration ≠ breathing. Know fermentation with yeast (ethanol + CO₂) and muscle cells (lactic acid). These are the testable items.
CBSE Class 11/12 and NEET: Know all three stages of aerobic respiration with locations (glycolysis = cytoplasm, Krebs cycle = mitochondrial matrix, ETC = inner mitochondrial membrane). Know the ATP yield at each stage. Know the role of NADH and FADH₂. Chemiosmosis and ATP synthase are important conceptual questions.
Common Mistakes to Avoid
Mistake 1 — Confusing respiration and breathing: Breathing is the mechanical process of getting air in and out. Respiration is the chemical process in cells. Breathing serves respiration.
Mistake 2 — Glycolysis in mitochondria: Glycolysis occurs in the CYTOPLASM, not in the mitochondria. Many students assume all respiration happens in the mitochondria. Only stages after pyruvate (Krebs cycle and ETC) are in the mitochondria.
Mistake 3 — Fermentation produces ATP: Fermentation itself (the pyruvate→lactate or pyruvate→ethanol step) does NOT produce ATP. Its sole purpose is to regenerate NAD⁺ so glycolysis can continue producing the 2 ATP. The 2 ATP from anaerobic respiration all come from glycolysis.
Mistake 4 — All NADH yields the same ATP: NADH from glycolysis (cytoplasmic NADH) yields ~1.5–2 ATP each, not 2.5, because its electrons must be shuttled into the mitochondria at an energy cost. NADH from the Krebs cycle (already in the matrix) yields ~2.5 ATP. This distinction appears in some JEE questions.
Mistake 5 — Oxygen is “used up” directly in glycolysis or Krebs: Oxygen’s role is specifically at the END of the ETC — it is the final electron acceptor at Complex IV, being reduced to water. It plays no direct role in glycolysis or the Krebs cycle.
Practice Questions
Q1: Where does glycolysis occur, and why can it happen in the absence of oxygen?
Glycolysis occurs in the cytoplasm. It doesn’t require oxygen because it doesn’t involve the electron transport chain (which needs oxygen as the final electron acceptor). Glycolysis uses substrate-level phosphorylation (direct ATP synthesis without an electron gradient) and regenerates NAD⁺ through fermentation if needed. This is why even organisms with no mitochondria (bacteria) can perform glycolysis.
Q2: Why does the Krebs cycle stop if there is no oxygen?
The Krebs cycle itself doesn’t directly use oxygen. However, the NADH and FADH₂ it produces must be re-oxidised to NAD⁺ and FAD by the electron transport chain. Without oxygen as the final electron acceptor, the ETC cannot function, NADH and FADH₂ accumulate, and NAD⁺ and FAD run out. Since the Krebs cycle requires NAD⁺ and FAD to accept electrons, it stops when these are depleted.
Q3: Why is the ATP yield from aerobic respiration so much higher than from anaerobic?
Aerobic respiration fully oxidises glucose to CO₂ and H₂O — extracting virtually all the chemical energy through the electron transport chain and chemiosmosis. The ETC converts the energy stored in NADH and FADH₂ into ~28 more ATP via the proton gradient.
Anaerobic respiration only completes glycolysis (2 ATP). Pyruvate — which still contains most of the original energy — is converted to lactic acid or ethanol (both still energy-rich), and that energy is wasted. No ETC, no chemiosmosis, no extra ATP.
Q4: A marathon runner uses aerobic respiration; a 100-metre sprinter initially uses anaerobic respiration. Why?
A marathon runner maintains a steady pace — oxygen delivery to muscles keeps up with demand. Aerobic respiration (36 ATP/glucose) provides sustained, efficient energy over 2+ hours.
A 100-metre sprinter needs maximum power in 10 seconds. Even though the lungs and heart work hard, oxygen simply can’t reach muscles fast enough at maximum intensity. Muscles switch to anaerobic respiration (glycolysis only) — which is much faster per second even though it produces only 2 ATP per glucose. Speed of ATP production matters more than efficiency during a sprint. The lactic acid accumulation is why sprinters can feel burning in their muscles afterward.
FAQs
Q: Do plants also perform cellular respiration? Yes — all living organisms, including plants, perform cellular respiration 24 hours a day. Plants also photosynthesise during the day (producing glucose and oxygen). During the day, the net gas exchange appears reversed (taking in CO₂, releasing O₂) because photosynthesis rate exceeds respiration rate. But at night, plants only respire — taking in O₂ and releasing CO₂.
Q: Is ATP the only energy currency in cells? ATP is the most important and universal. GTP (guanosine triphosphate) is used in some reactions (the Krebs cycle produces GTP in some organisms). NADPH (similar to NADH but with a phosphate group) is the reducing power used in anabolic (biosynthetic) reactions like photosynthesis and fatty acid synthesis. But ATP is by far the most prevalent energy carrier for cellular work.
Q: Why do we breathe faster during exercise? Muscles need more ATP → more aerobic respiration → more O₂ consumed and more CO₂ produced. Rising CO₂ (detected as falling blood pH) is the primary signal that stimulates the respiratory centres in the brain to increase breathing rate and depth. The goal is to deliver more O₂ to muscles and remove CO₂ faster.
Q: What happens to lactic acid after exercise? During recovery (rest), lactic acid diffuses from muscle cells into the bloodstream and is taken up by the liver. The liver converts lactic acid back to glucose via gluconeogenesis (the Cori cycle). The glucose can then be stored as glycogen or re-used. This process requires oxygen — which is why we continue to breathe heavily for a few minutes after intense exercise (“oxygen debt” or EPOC — excess post-exercise oxygen consumption).