Respiration – Energy Yield in Aerobic vs Anaerobic Conditions
Learning objective
Explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions (Cambridge 9700 – 12.2).
1. Overview of the two types of respiration
- Aerobic respiration: glucose is completely oxidised to CO₂ and H₂O; O₂ is the final electron acceptor.
- Anaerobic respiration (fermentation): glucose is only partially oxidised; the final electron acceptor is an organic molecule (pyruvate or acetaldehyde) and O₂ is not used.
2. Location and purpose of each stage (eukaryotes)
| Stage | Cellular location | Purpose (one‑sentence statement) |
|---|
| Glycolysis | Cytoplasm | Breaks glucose into 2 pyruvate, generates 2 ATP (substrate‑level) and 2 NADH. |
| Link reaction (pyruvate → acetyl‑CoA) | Mitochondrial matrix | Decarboxylates pyruvate, produces 1 NADH per pyruvate and links glycolysis to the Krebs cycle. |
| Krebs (citric‑acid) cycle | Mitochondrial matrix | Oxidises acetyl‑CoA to CO₂, yielding NADH, FADH₂, GTP (≈ATP) and CO₂. |
| Electron‑transport chain (ETC) | Inner mitochondrial membrane | Transfers electrons from NADH/FADH₂ to O₂, pumping protons to create a proton‑motive force. |
| Oxidative phosphorylation (ATP synthase) | Inner mitochondrial membrane (F₀F₁‑ATP synthase) | Uses the proton gradient to synthesise ATP from ADP + Pᵢ. |
3. Key enzymes and cofactors required by the syllabus
Glycolysis (cytoplasm)
- Hexokinase, Phosphofructokinase‑1 (PFK‑1), Pyruvate kinase – regulatory enzymes.
- Glyceraldehyde‑3‑phosphate dehydrogenase (produces NADH).
- Cofactors: NAD⁺/NADH, ADP/ATP, inorganic phosphate (Pᵢ).
Link reaction (mitochondrial matrix)
- Pyruvate dehydrogenase complex (PDH).
- Cofactors: NAD⁺, Coenzyme‑A, thiamine‑pyrophosphate (TPP), lipoic acid, FAD, and a flavin‑containing subunit.
Krebs cycle (mitochondrial matrix)
- Citrate synthase
- Aconitase
- Isocitrate dehydrogenase
- α‑Ketoglutarate dehydrogenase complex
- Succinyl‑CoA synthetase (produces GTP)
- Succinate dehydrogenase (also Complex II of the ETC)
- Fumarase
- Malate dehydrogenase
- Cofactors: NAD⁺, FAD, GDP (→ GTP), CoA‑SH, Mg²⁺.
Electron‑transport chain (inner mitochondrial membrane)
- Complex I – NADH‑ubiquinone oxidoreductase (FMN, Fe‑S clusters).
- Complex II – Succinate‑ubiquinone oxidoreductase (FAD, Fe‑S clusters).
- Complex III – Cytochrome bc₁ complex (cytochrome b, cytochrome c₁, Rieske Fe‑S).
- Complex IV – Cytochrome c oxidase (cytochromes a, a₃, Cu⁺/Cu²⁺).
- Ubiquinone (Q) and cytochrome c act as mobile carriers.
ATP synthase (Complex V)
- F₀ subunit – proton channel in the membrane.
- F₁ subunit – catalytic site for ADP + Pᵢ → ATP.
4. Anaerobic pathways (fermentation)
Lactic‑acid fermentation (muscle cells, many bacteria)
- Reaction: 2 pyruvate + 2 NADH → 2 lactate + 2 NAD⁺
- Enzyme: Lactate dehydrogenase (LDH).
- Purpose: Regenerates NAD⁺ so glycolysis can continue to produce its 2 ATP.
Alcoholic fermentation (yeast, some bacteria)
- Step 1 (decarboxylation): Pyruvate → acetaldehyde + CO₂ Enzyme: Pyruvate decarboxylase.
- Step 2 (reduction): Acetaldehyde + NADH → ethanol + NAD⁺ Enzyme: Alcohol dehydrogenase.
- Again, the sole aim is NAD⁺ regeneration.
5. Why oxygen gives a much larger energy yield
- Electrons from NADH (E°′ ≈ –0.32 V) or FADH₂ (E°′ ≈ +0.03 V) are passed to the O₂/H₂O couple (E°′ ≈ +0.82 V). The large positive ΔE°′ (≈ 1.14 V for NADH) makes each electron transfer highly exergonic.
- Each NADH can drive the pumping of ~4 protons across the inner membrane; each FADH₂ pumps ~2 protons.
- The resulting proton‑motive force powers ATP synthase, allowing ≈3 ATP per NADH and ≈2 ATP per FADH₂ (exact values depend on the NADH shuttle used).
- In the absence of O₂ the ETC stops because there is no terminal electron acceptor; consequently oxidative phosphorylation cannot occur and only the 2 ATP from glycolysis are obtained.
6. Approximate ATP yield per molecule of glucose
| Process | Reduced carriers formed per glucose | ATP from substrate‑level phosphorylation | ATP from oxidative phosphorylation | Net ATP (approx.) |
|---|
| Aerobic respiration | 10 NADH + 2 FADH₂ (including 2 cytosolic NADH) | 2 ATP (glycolysis) + 2 GTP (Krebs) ≈ 2 ATP | ~2.5 ATP per NADH, ~1.5 ATP per FADH₂ → 25–28 ATP | ≈30–32 ATP. The range reflects the NADH shuttle: malate‑aspartate (≈3 ATP/NADH) vs glycerol‑phosphate (≈2 ATP/NADH). |
| Anaerobic fermentation | 2 NADH (from glycolysis only) | 2 ATP (glycolysis) | 0 (no ETC, no oxidative phosphorylation) | 2 ATP |
7. Respiration Quotient (RQ) and its link to energy yield
- Complete aerobic oxidation of glucose:
\$\text{C}6\text{H}{12}\text{O}6 + 6\text{O}2 \;\longrightarrow\; 6\text{CO}2 + 6\text{H}2\text{O}\$
RQ = CO₂ produced / O₂ consumed = 6/6 ≈ 1.0.
- Alcoholic fermentation:
\$\text{C}6\text{H}{12}\text{O}6 \;\longrightarrow\; 2\text{C}2\text{H}5\text{OH} + 2\text{CO}2 + 2\text{ATP}\$
No O₂ is consumed, so RQ > 1.0. The higher RQ reflects that less chemical energy is extracted per mole of glucose.
8. Summary – why aerobic respiration yields far more ATP
- Complete oxidation of glucose to CO₂ and H₂O releases the maximum amount of chemical energy.
- More reduced carriers (NADH, FADH₂) are generated in the link reaction and Krebs cycle.
- Oxygen’s high redox potential makes electron transfer highly exergonic, allowing the pumping of many protons.
- Oxidative phosphorylation converts the proton gradient into several ATP per NADH/FADH₂, whereas fermentation relies solely on the 2 ATP made directly in glycolysis.
- Fermentation’s purpose is only to regenerate NAD⁺, which limits the total ATP yield.
9. Suggested diagram (flow‑chart)
A side‑by‑side flow‑chart that shows:
- Glycolysis → pyruvate (cytoplasm).
- Aerobic route: pyruvate → acetyl‑CoA → Krebs cycle → NADH/FADH₂ → ETC → O₂ (inner membrane) → ATP synthase.
- Anaerobic route: pyruvate → lactate (or acetaldehyde → ethanol) – NAD⁺ regeneration – no ETC.
- Locations (cytoplasm, matrix, inner membrane) and where ATP is produced (substrate‑level vs oxidative phosphorylation).
10. Key equations (LaTeX)
Overall aerobic respiration:
\$\text{C}6\text{H}{12}\text{O}6 + 6\text{O}2 \;\longrightarrow\; 6\text{CO}2 + 6\text{H}2\text{O} + \text{energy}\$
Lactic‑acid fermentation:
\$\text{C}6\text{H}{12}\text{O}6 \;\longrightarrow\; 2\text{CH}3\text{CH(OH)COOH} + 2\text{ATP}\$
Alcoholic fermentation:
\$\text{C}6\text{H}{12}\text{O}6 \;\longrightarrow\; 2\text{C}2\text{H}5\text{OH} + 2\text{CO}2 + 2\text{ATP}\$
11. Practical investigation ideas (linked to assessment objectives)
- Respirometer with yeast (AO1 – knowledge; AO2 – data handling). Measure the volume of CO₂ produced during alcoholic fermentation at different temperatures; relate the rate to enzyme activity.
- Muscle‑cell fatigue test (AO1/AO2). Use a hand‑grip dynamometer while a subject performs repeated contractions; record the decline in force and the accompanying increase in lactate (via a lactate test strip).
- Effect of an ETC inhibitor (e.g., cyanide) on isolated rat liver mitochondria (AO2/AO3). Compare oxygen consumption and ATP production with and without the inhibitor to demonstrate the role of O₂ as the terminal electron acceptor.
- Comparison of ATP yield using a colour‑imetric assay for ADP/ATP in cells grown under aerobic vs anaerobic conditions (AO2/AO3). Quantify the difference in total ATP and discuss it in terms of oxidative phosphorylation.
Key take‑away
Oxygen acts as a high‑energy terminal electron acceptor, enabling a long electron‑transport chain and a large proton gradient. This drives oxidative phosphorylation, producing many more ATP molecules than the limited substrate‑level phosphorylation possible in anaerobic fermentation.