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When oxygen is unavailable or in short supply, cells must use anaerobic pathways to continue producing ATP. This lesson covers the two main types of anaerobic respiration (in animals and in yeast/plants) and the use of different respiratory substrates. These topics are required for the Edexcel A-Level Biology (9BI0) specification.
In the absence of oxygen:
The solution: anaerobic pathways that regenerate NAD⁺ from reduced NAD without using the ETC, allowing glycolysis to continue producing a small amount of ATP.
Exam Tip: The purpose of anaerobic respiration is NOT primarily to produce ATP — it is to regenerate NAD⁺ so that glycolysis can continue to produce ATP by substrate-level phosphorylation.
In mammalian muscle cells during vigorous exercise, oxygen supply cannot meet demand. The anaerobic pathway used is:
Pyruvate + Reduced NAD → Lactate + NAD⁺
| Feature | Detail |
|---|---|
| Enzyme | Lactate dehydrogenase |
| Input | Pyruvate (3C) + reduced NAD |
| Output | Lactate (3C) + NAD⁺ |
| Reversible? | Yes — lactate can be converted back to pyruvate when oxygen is available |
| CO₂ produced? | No — lactate has the same number of carbon atoms as pyruvate |
Glucose → 2 Lactate + 2 ATP (net)
Exam Tip: Lactate is NOT a "waste product" that is simply excreted. It is transported to the liver in the blood, where it is converted back to pyruvate and then either oxidised aerobically or converted to glucose via gluconeogenesis.
In yeast and some plant cells under anaerobic conditions, a different pathway is used:
Pyruvate (3C) → Ethanal (acetaldehyde, 2C) + CO₂
This reaction is catalysed by pyruvate decarboxylase and requires the coenzyme thiamine pyrophosphate (TPP).
Ethanal + Reduced NAD → Ethanol (2C) + NAD⁺
This reaction is catalysed by alcohol dehydrogenase (also called ethanol dehydrogenase).
| Feature | Detail |
|---|---|
| Organisms | Yeast, some plant tissues |
| Products | Ethanol (2C) + CO₂ |
| NAD⁺ regenerated? | Yes |
| Reversible? | No — ethanol is toxic and cannot be converted back to pyruvate |
| ATP yield | 2 ATP (net) per glucose — from glycolysis only |
Glucose → 2 Ethanol + 2 CO₂ + 2 ATP (net)
| Application | Detail |
|---|---|
| Brewing | Yeast ferments sugars in grain/fruit to produce ethanol and CO₂ |
| Baking | CO₂ from fermentation causes dough to rise; ethanol evaporates during baking |
| Biofuel production | Yeast ferments plant sugars to produce bioethanol |
| Feature | Aerobic | Anaerobic (lactate) | Anaerobic (alcoholic) |
|---|---|---|---|
| Oxygen required? | Yes | No | No |
| Stages used | Glycolysis + link + Krebs + oxidative phosphorylation | Glycolysis + lactate fermentation | Glycolysis + alcoholic fermentation |
| Products | CO₂ + H₂O | Lactate | Ethanol + CO₂ |
| ATP yield per glucose | ~32 | 2 (net) | 2 (net) |
| Complete oxidation? | Yes | No | No |
| Location | Cytoplasm + mitochondria | Cytoplasm only | Cytoplasm only |
Exam Tip: Anaerobic respiration produces far less ATP per glucose because the energy-rich products (lactate or ethanol) still contain a large amount of chemical energy that has not been released. Most of the ATP in aerobic respiration comes from oxidative phosphorylation, which requires oxygen.
Cells do not only respire glucose. Respiratory substrates are any organic molecules that can be oxidised to release energy for ATP synthesis.
| Substrate | How it enters respiration |
|---|---|
| Glucose | Directly enters glycolysis |
| Glycogen (animals) | Hydrolysed to glucose by glycogen phosphorylase |
| Starch (plants) | Hydrolysed to glucose by amylase |
| Fructose, galactose | Converted to glucose 6-phosphate or fructose 6-phosphate and enter glycolysis |
| Sucrose | Hydrolysed to glucose + fructose by sucrase/invertase |
Lipids are first hydrolysed by lipases into glycerol and fatty acids.
| Component | Pathway |
|---|---|
| Glycerol | Converted to triose phosphate (G3P) and enters glycolysis at the midpoint |
| Fatty acids | Undergo β-oxidation in the mitochondrial matrix, producing acetyl CoA (enters Krebs cycle), reduced NAD and reduced FAD |
Lipids yield significantly more ATP per gram than carbohydrates because:
Amino acids are used as respiratory substrates only when carbohydrate and lipid supplies are depleted (e.g. during prolonged starvation).
| Step | Process |
|---|---|
| Deamination | The amino group (–NH₂) is removed in the liver, producing ammonia (converted to urea for excretion) |
| Carbon skeleton | The remaining carbon chain enters the Krebs cycle at various points (e.g. as pyruvate, acetyl CoA, or Krebs cycle intermediates) |
The respiratory quotient (RQ) indicates which substrate is being respired:
RQ = CO₂ produced / O₂ consumed
| Substrate | RQ | Explanation |
|---|---|---|
| Carbohydrate | 1.0 | Equal amounts of CO₂ produced and O₂ consumed |
| Lipid | ~0.7 | More O₂ needed to oxidise C–H bonds; less CO₂ per O₂ |
| Protein | ~0.8 | Intermediate value |
| Anaerobic respiration | >1.0 | CO₂ produced without O₂ consumption (e.g. alcoholic fermentation) |
| Mixed diet | ~0.85 | Typical for humans at rest |
For glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
RQ = 6CO₂ / 6O₂ = 1.0
For a typical fat (tripalmitin): 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O
RQ = 102 / 145 = 0.703
Exam Tip: You may be asked to calculate RQ from experimental data (e.g. using a respirometer). An RQ greater than 1 suggests anaerobic respiration is occurring alongside aerobic. You may also need to deduce the substrate being respired from a given RQ value.
| Term | Definition |
|---|---|
| Anaerobic respiration | Respiration without oxygen, producing lactate or ethanol + CO₂ |
| Lactate fermentation | Anaerobic pathway in animals: pyruvate is reduced to lactate, regenerating NAD⁺ |
| Alcoholic fermentation | Anaerobic pathway in yeast/plants: pyruvate is decarboxylated and reduced to ethanol, regenerating NAD⁺ |
| Oxygen debt | The extra oxygen consumed after exercise to metabolise accumulated lactate |
| Respiratory substrate | Any organic molecule that can be oxidised in respiration to release energy |
| Respiratory quotient (RQ) | The ratio of CO₂ produced to O₂ consumed during respiration |
| β-oxidation | The breakdown of fatty acids to acetyl CoA in the mitochondrial matrix |
This material sits in Edexcel 9BI0 Topic 5 (On the Wild Side — Photosynthesis, Energy and Ecosystems) and represents the fallback pathway when O2 is unavailable, plus the substrate flexibility of cellular respiration. Anaerobic respiration in animals (lactate) and yeast (ethanol + CO2) regenerates NAD+ so that glycolysis alone can continue producing ATP by substrate-level phosphorylation. Synoptic links run backwards to lesson 4 (glycolysis) — the only ATP-producing pathway available in anaerobic conditions — and lesson 6 (oxidative phosphorylation) — the stage that anaerobic respiration skips, hence the dramatic ATP-yield drop from ~30–32 to just 2 ATP per glucose. Further links: Topic 7 (exercise physiology) for oxygen debt and the Cori cycle; Topic 1 (biological molecules) for lipids as alternative respiratory substrates with higher per-gram energy density via eta-oxidation; clinical relevance in diabetic ketoacidosis when glucose access fails. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks): Compare the per-glucose ATP yields of aerobic and anaerobic respiration, and explain the role of NAD+ regeneration in fermentation.
(a) Calculate the net ATP yield per glucose under (i) fully aerobic and (ii) anaerobic conditions, and state where each ATP comes from. (3)
(b) Explain why glycolysis can continue under anaerobic conditions while the link reaction and Krebs cycle cannot. (3)
(c) Contrast the lactate-fermentation pathway in animals with the ethanol-fermentation pathway in yeast, including reversibility and biological consequence. (2)
Solution with mark scheme:
(a) M1 (AO2) — Aerobic: glycolysis (2 net) + Krebs (2) + oxidative phosphorylation (~28) = ~30–32 ATP per glucose.
M1 (AO2) — Anaerobic: glycolysis only — 2 ATP net per glucose by substrate-level phosphorylation (4 produced, 2 consumed in the activation phase).
A1 (AO1) — All anaerobic ATP originates in the payoff phase of glycolysis (phosphoglycerate kinase and pyruvate kinase steps); fermentation itself produces no ATP.
(b) M1 (AO1) — Glycolysis requires NAD+ at the glyceraldehyde-3-phosphate dehydrogenase (G3PDH) step, which oxidises G3P and reduces NAD+ to reduced NAD.
M1 (AO2) — Without an ETC running, reduced NAD cannot be re-oxidised on the inner mitochondrial membrane, so the cellular NAD+ pool would be exhausted within seconds, halting G3PDH and therefore glycolysis.
A1 (AO2) — Fermentation regenerates NAD+ by transferring its electrons to pyruvate (animals) or acetaldehyde (yeast), allowing G3PDH to keep running. The link reaction and Krebs cycle also require NAD+ and FAD, but they are matrix processes and have no fermentation route to regenerate their oxidised coenzymes — they stall.
(c) M1 (AO1) — Animals: pyruvate + reduced NAD ightarrow lactate + NAD+, catalysed by lactate dehydrogenase (LDH); reversible — lactate can be reconverted to pyruvate when O2 returns (Cori cycle).
A1 (AO1) — Yeast: pyruvate ightarrow acetaldehyde + CO2 via pyruvate decarboxylase, then acetaldehyde + reduced NAD ightarrow ethanol + NAD+ via alcohol dehydrogenase (ADH); irreversible — ethanol cannot be reconverted, and is lethal to yeast at high concentration (the cells eventually die in their own product). (Total: 8 marks; M5 A3.)
Question (6 marks): A respirometer is used to measure gas exchange in germinating peas. Over a 30-minute period, the apparatus records O2 uptake of 4.50 cm3 and CO2 production of 3.15 cm3. Calculate the respiratory quotient (RQ) and use it to deduce which respiratory substrate is being predominantly used. Explain your reasoning, referring to the H : C ratio of the substrate.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO2.1 | Calculating RQ = CO2 produced / O2 consumed = 3.15 / 4.50 = 0.70 |
| 2 | AO1.1 | Identifying RQ ~0.7 as characteristic of lipid (fat / fatty-acid) respiration |
| 3 | AO2.2 | Stating that lipids are H-rich and O-poor (long C–H hydrocarbon chains; few oxygens in the molecule) |
| 4 | AO3.1 | Explaining that more O2 is consumed per CO2 produced because the H atoms must also be oxidised to H2O — this lowers the CO2 : O2 ratio below 1.0 |
| 5 | AO3.2 | Recognising that germinating oily seeds (e.g. sunflower, castor) typically mobilise stored lipid via eta-oxidation before carbohydrate stores; pea seedlings would more usually show RQ ~1.0 unless the lipid endosperm dominates |
| 6 | AO3.3 | Concluding that the data are consistent with predominant fat oxidation; if RQ rose toward 1.0 over time, this would indicate a switch to carbohydrate; an RQ above 1.0 would indicate anaerobic respiration alongside aerobic, since CO2 would be produced without O2 consumption |
Total: 6 marks (UMS-band-anchored at A; AO1 = 1, AO2 = 2, AO3 = 3). This question structure mirrors Edexcel's preference for combining a numerical calculation with substrate-deduction reasoning that requires understanding the chemical basis of RQ values (H : C ratio of the substrate determines the O2 demand).
Lesson 4 (glycolysis) — the only ATP-producing pathway available anaerobically. Glycolysis is cytoplasmic and does not require O2. Its absolute requirement is NAD+ at the G3PDH step; fermentation exists solely to regenerate this. The 2 ATP net come from substrate-level phosphorylation at phosphoglycerate kinase and pyruvate kinase.
Lesson 6 (oxidative phosphorylation) — what anaerobic respiration skips. Oxidative phosphorylation contributes ~28 of the ~30–32 ATP per glucose. Skipping it explains the ~15-fold yield drop to just 2 ATP. The skipped reduced NAD and reduced FAD have their bond energy left unharvested — re-oxidised by pyruvate or acetaldehyde with no further ATP capture.
Topic 7 (exercise physiology) — oxygen debt and the Cori cycle. During sprinting, LDH converts pyruvate to lactate, regenerating NAD+ and sustaining high glycolytic flux. Lactate is exported to liver in the bloodstream, oxidised back to pyruvate, then converted to glucose via gluconeogenesis (~6 ATP cost per glucose), and exported back to muscle — the Cori cycle, named for Carl and Gerty Cori. "Oxygen debt" is the post-exercise O2 uptake repaying this cost plus re-oxygenation of myoglobin / haemoglobin.
Topic 1 (lipids) — fatty-acid eta-oxidation. Triglycerides are hydrolysed to glycerol + 3 fatty acids. Glycerol enters glycolysis at the triose-phosphate stage; fatty acids undergo eta-oxidation in the matrix, sequentially yielding acetyl-CoA, reduced NAD and reduced FAD. Palmitate (16C) yields ~106 ATP — explaining lipid's ~9 kcal/g vs ~4 kcal/g for carbohydrate.
Topic 1 (proteins) — amino-acid catabolism as a last resort. Amino acids are deaminated in liver (–NH2 removed, converted to urea); carbon skeletons enter respiration as pyruvate, acetyl-CoA or Krebs intermediates. Used only after carbohydrate and most fat reserves are exhausted, explaining the muscle wasting of advanced starvation.
Clinical relevance — diabetic ketoacidosis (DKA). When insulin signalling fails, cells respond as if starving and ramp up fat oxidation. eta-oxidation outpaces Krebs capacity, so acetyl-CoA is diverted to ketone bodies (acetoacetate, eta-hydroxybutyrate, acetone). Blood pH falls; Kussmaul respiration and fruity acetone breath develop — a clinical demonstration that substrate-switching can be pathological.
| AO | Typical share on anaerobic / substrate questions | Earned by |
|---|---|---|
| AO1 (knowledge) | 30–40% | Naming LDH and ADH; stating animal vs yeast products (lactate vs ethanol + CO2); stating the 2-ATP net yield; defining RQ; naming the Cori cycle; identifying eta-oxidation as the fatty-acid pathway |
| AO2 (application) | 35–45% | Calculating RQ from gas-exchange data; deducing substrate from RQ; comparing aerobic vs anaerobic ATP yields; reasoning about why glycolysis can continue while Krebs cannot; predicting effects of O2 depletion on a working muscle |
| AO3 (analysis / evaluation) | 20–30% | Distinguishing the purpose of fermentation (NAD+ regeneration) from the incidental ATP yield; evaluating reversibility (lactate yes, ethanol no); reasoning about why lipid RQ is ~0.7 from the H : C ratio; evaluating whether an RQ above 1.0 indicates anaerobic respiration |
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