Glycolysis and Anaerobic Respiration — AQA A-level Biology

SPEC 3.5.2 - ENERGY TRANSFERS

Respiration: Glycolysis, Anaerobic Respiration & the Link Reaction

ATP production, phosphorylation, reduced NAD, pyruvate, lactate and ethanol fermentation, and the mitochondrial link reaction

Respiration: The Big Picture

Respiration is the process by which cells release energy from organic molecules and use it to produce ATP. It occurs in all living cells. The four main stages are:

StageLocationO₂ required?Key products

GlycolysisCytoplasmNo — occurs in aerobic and anaerobic conditionsPyruvate, 2 ATP (net), 2 reduced NAD

Link reactionMitochondrial matrixYes (aerobic)Acetyl CoA, CO₂, reduced NAD

Krebs cycleMitochondrial matrixYes (aerobic)CO₂, reduced NAD, reduced FAD, ATP

Oxidative phosphorylationInner mitochondrial membraneYes (aerobic)ATP, water

This lesson: Stages 1–2

Lesson 5.03 covers glycolysis, anaerobic respiration and the link reaction. Lesson 5.04 covers the Krebs cycle and oxidative phosphorylation.

Synoptic Link — ATP (1.07)

ATP is resynthesised from ADP + Pᵢ during respiration. In glycolysis this occurs by substrate-level phosphorylation — direct transfer of a phosphate group from a metabolic intermediate to ADP. This is distinct from oxidative phosphorylation (ETC/ATP synthase) covered in Lesson 5.04. Note: substrate-level phosphorylation also occurs once per turn of the Krebs cycle.

Key Definitions

Definition — Glycolysis

The metabolic pathway that occurs in the cytoplasm in which glucose is converted to pyruvate, with a net gain of 2 ATP and 2 reduced NAD. It is the first stage of both aerobic and anaerobic respiration.

Definition — NAD (Nicotinamide Adenine Dinucleotide)

A hydrogen carrier molecule. Reduced to reduced NAD (NADH) during respiration by accepting hydrogen (H⁺ + electrons) from metabolic intermediates. Carries hydrogen to the electron transfer chain.

Definition — Pyruvate

A 3-carbon molecule; the end product of glycolysis. Under aerobic conditions it enters the mitochondrial matrix for the link reaction. Under anaerobic conditions it is reduced in the cytoplasm.

Definition — Substrate-level Phosphorylation

The direct synthesis of ATP from ADP by transfer of a phosphate group from a metabolic intermediate — without involving the electron transfer chain or chemiosmosis.

Glycolysis: Step by Step

Location: cytoplasm. Glucose is broken down to pyruvate in a series of enzyme-catalysed reactions.

1

Phosphorylation of glucose: Glucose 6C is phosphorylated using 2 ATP → glucose phosphate → splits into 2 × triose phosphate3C. ATP is invested at this stage.

2

Oxidation of triose phosphate: Each triose phosphate is oxidised to pyruvate3C. Hydrogen removed is accepted by NAD → 2 × reduced NAD. 4 ATP produced by substrate-level phosphorylation.

3

Net yield per glucose: 4 ATP produced − 2 ATP invested = net 2 ATP. Also: 2 reduced NAD and 2 pyruvate.

Examiner Alert — Location

Glycolysis occurs in the cytoplasm — NOT the mitochondria. This error appears in AQA examiner reports every year. Glycolysis predates the evolution of mitochondria and occurs in all living cells, including prokaryotes.

Net yield from glycolysis per glucose

2 ATP | 2 reduced NAD | 2 pyruvate (3C)

Glycolysis: Flow Diagram

Red = ATP invested. Green = ATP produced. Blue = reduced NAD produced. Pink dashed = anaerobic pathway.

Anaerobic Respiration

When oxygen is absent or insufficient, pyruvate cannot enter the mitochondria. Glycolysis must continue to supply ATP — but this requires NAD to be regenerated from reduced NAD. Anaerobic pathways achieve this.

Examiner Alert — Purpose of Anaerobic Respiration

The purpose of lactate fermentation and ethanol fermentation is NAD regeneration — NOT to produce ethanol or lactate as end goals. Without NAD regeneration, glycolysis stops because there is no NAD to accept hydrogen. No additional ATP is produced beyond the net 2 ATP from glycolysis.

Animals & some bacteria — Lactate fermentation

Pyruvate + reduced NAD → Lactate + NAD

Pyruvate is reduced to lactate by lactate dehydrogenase

NAD is regenerated → glycolysis continues

Lactate accumulates → muscle fatigue; removed by blood to liver where it is resynthesised to glucose (Cori cycle — not required by spec)

Yeast & plants — Ethanol fermentation

Pyruvate → Ethanal + CO₂

Ethanal + reduced NAD → Ethanol + NAD

Pyruvate is decarboxylated → ethanal + CO₂ (by pyruvate decarboxylase)

Ethanal is reduced → ethanol , regenerating NAD (by alcohol dehydrogenase)

Ethanol is toxic — yeast eventually killed by accumulation

Anaerobic Pathways: Comparison

FeatureLactate fermentationEthanol fermentation

OrganismsAnimals, some bacteriaYeast, plants

SubstratePyruvatePyruvate

End productsLactateEthanol + CO₂

CO₂ released?NoYes (decarboxylation step)

NAD regenerated?YesYes

ATP yield2 ATP (from glycolysis only)2 ATP (from glycolysis only)

Reversible?Yes — lactate → pyruvate (aerobic recovery)No — ethanol cannot be converted back to pyruvate in animals

Equation summary

Glucose → 2 Lactate (animals, no O₂)

Glucose → 2 Ethanol + 2 CO₂ (yeast, no O₂)

Synoptic Link — Biotechnology

Ethanol fermentation by yeast is the basis of brewing and bread-making. In bread, CO₂ from fermentation causes dough to rise; ethanol evaporates on baking. In brewing, ethanol accumulates. Both processes are exploited industrially by controlling O₂ availability.

Aerobic Respiration: Pyruvate Enters the Mitochondria

When oxygen is available, pyruvate from glycolysis is transported into the mitochondrial matrix to continue aerobic respiration.

1

Pyruvate is produced in the cytoplasm by glycolysis

2

Pyruvate is transported across the inner mitochondrial membrane by active transport into the mitochondrial matrix

3

Pyruvate enters the mitochondrial matrix where the link reaction occurs

Why active transport?

The inner mitochondrial membrane is selectively impermeable. Pyruvate is a charged molecule and requires carrier proteins to cross. This active transport requires ATP — a minor energy cost. The AQA spec specifically states pyruvate enters the matrix by active transport.

Synoptic Link — Cell Membranes (2.05)

Active transport across the inner mitochondrial membrane requires ATP and carrier proteins — exactly as for active transport across the cell-surface membrane. The mitochondrion has two membranes: pyruvate crosses both to reach the matrix where the link reaction occurs.

The Link Reaction

Definition — Link Reaction

The conversion of pyruvate to acetyl coenzyme A in the mitochondrial matrix, releasing CO₂ and producing reduced NAD. It links glycolysis to the Krebs cycle.

Per pyruvate molecule (×2 per glucose):

1

Decarboxylation: Pyruvate 3C → acetate 2C + CO₂ released. One carbon atom is removed as carbon dioxide.

2

Oxidation: Acetate is oxidised → hydrogen removed → accepted by NAD → reduced NAD produced.

3

Combination with coenzyme A: Acetate 2C + coenzyme A → acetyl coenzyme A2C. Acetyl CoA enters the Krebs cycle.

Per glucose (2 pyruvates → 2 link reactions)

2 CO₂ released | 2 reduced NAD | 2 acetyl CoA formed

No ATP is produced directly in the link reaction.

Link Reaction: Diagram

Examiner Alert — CO₂ location

CO₂ is produced in the link reaction AND the Krebs cycle. It is NOT produced in glycolysis (except in ethanol fermentation) or in the electron transfer chain. Water is produced at the end of the ETC. Students regularly misplace CO₂ production.

Running Total: Glycolysis + Link Reaction

Before entering the Krebs cycle, the products from one glucose molecule are:

StageLocationATPReduced NADCO₂Other

GlycolysisCytoplasm+2 (net)+202 pyruvate

Link reaction (×2)Matrix0+2+22 acetyl CoA

Running total—2 ATP4 reduced NAD2 CO₂2 acetyl CoA → Krebs

Why reduced NAD matters

Reduced NAD carries hydrogen (H⁺ + electrons) to the electron transfer chain in Lesson 5.04, where it drives the production of most ATP via oxidative phosphorylation. Each reduced NAD from glycolysis and the link reaction is a "hydrogen delivery vehicle" for the ETC.

Comparison: aerobic vs anaerobic ATP yield

Anaerobic respiration: 2 ATP per glucose (glycolysis only). Aerobic respiration: ~30–32 ATP per glucose (glycolysis + link + Krebs + oxidative phosphorylation). The vast difference is why aerobic respiration is so important for sustained activity.

DEEP Framework: Extended Answer

"Explain why anaerobic respiration produces much less ATP per glucose than aerobic respiration."

DEEP Answer Framework

DDefine: Anaerobic respiration produces only 2 ATP per glucose (from glycolysis only). Aerobic respiration produces ~30–32 ATP per glucose.

EExplain mechanism: In anaerobic respiration, pyruvate is reduced to lactate (or ethanol + CO₂) in the cytoplasm. This regenerates NAD to keep glycolysis running but produces no additional ATP. Pyruvate cannot enter the mitochondria, so the link reaction, Krebs cycle and oxidative phosphorylation cannot occur. In aerobic respiration, pyruvate enters the mitochondrial matrix → link reaction → acetyl CoA → Krebs cycle → reduced NAD and reduced FAD → electron transfer chain → chemiosmosis → large quantities of ATP via oxidative phosphorylation.

EEvidence/example: Sprinters use anaerobic respiration for short bursts — ATP from glycolysis only, with lactate accumulation. Marathon runners rely on aerobic respiration for sustained ATP production.

PPractical/quantitative: Anaerobic respiration yields 2 ATP per glucose; aerobic yields approximately 30–32. The ~15-fold difference reflects the oxidative phosphorylation stage, which accounts for ~90% of total ATP yield.

Synoptic Links

Link to Photosynthesis (5.01–5.02)

Triose phosphate appears in both the Calvin cycle (Lesson 5.02) and glycolysis. The TP produced in photosynthesis can feed directly into glycolysis when the plant respires. Reduced NAD in respiration performs the same hydrogen-carrying function as reduced NADP in photosynthesis — both deliver hydrogen to an electron transfer chain.

Link to Cell Membranes & Transport (2.05)

Pyruvate enters the mitochondrial matrix by active transport across the inner mitochondrial membrane. This requires ATP and carrier proteins — the same principles as active transport at the cell-surface membrane. The specificity of the inner membrane is critical for maintaining the proton gradient used in oxidative phosphorylation.

Link to Krebs Cycle (5.04)

Acetyl CoA produced in the link reaction is the substrate for the Krebs cycle. The 2-carbon acetyl group combines with a 4-carbon oxaloacetate to form 6-carbon citrate — beginning a series of oxidations that produce more reduced NAD, reduced FAD and ATP. Nothing enters the Krebs cycle without first passing through the link reaction.

Practice Question

During a 100 m sprint, a runner's muscle cells switch from aerobic to anaerobic respiration.

(a) State where in the cell glycolysis occurs and give the net products per glucose molecule. [3 marks]

(b) Explain why lactate is produced in the muscle cells during the sprint. [3 marks]

(c) After the sprint, the runner continues to breathe heavily. Explain how this helps remove the lactate that has accumulated. [3 marks]

(a) Location: cytoplasm. Net products per glucose: 2 ATP (net), 2 reduced NAD, 2 pyruvate. (3 marks: cytoplasm; 2 ATP; 2 pyruvate / 2 reduced NAD)

(b) During the sprint, oxygen demand exceeds supply — conditions are anaerobic. Pyruvate cannot enter the mitochondria. Reduced NAD accumulates and must be reoxidised to NAD so that glycolysis can continue to supply ATP. Pyruvate is reduced to lactate by lactate dehydrogenase, which reoxidises reduced NAD to NAD. (3 marks: insufficient O₂; NAD must be regenerated; pyruvate reduced to lactate)

(c) Continued heavy breathing increases O₂ delivery to tissues. Lactate is transported in the blood to the liver. In the liver, with O₂ available, lactate is oxidised back to pyruvate, which then enters aerobic respiration (link reaction → Krebs cycle). This is the oxygen debt / excess post-exercise oxygen consumption. (3 marks: lactate transported to liver; O₂ available; lactate → pyruvate → aerobic respiration)

Lesson Summary

Glycolysis, Anaerobic Respiration & Link Reaction — Key Points

Glycolysis location: Cytoplasm — occurs in ALL cells, aerobic and anaerobic

Glycolysis: Glucose → 2 pyruvate + 2 ATP (net) + 2 reduced NAD

Phosphorylation: −2 ATP invested, +4 ATP produced = net 2 ATP (substrate-level)

Anaerobic (animals): Pyruvate → lactate; regenerates NAD; no additional ATP

Anaerobic (yeast/plants): Pyruvate → ethanal + CO₂ → ethanol; regenerates NAD; no additional ATP

Purpose of anaerobic pathways: NAD regeneration (NOT ATP production)

Aerobic — pyruvate transport: Active transport across inner mitochondrial membrane → matrix

Link reaction location: Mitochondrial matrix

Link reaction: Pyruvate (3C) → acetyl CoA (2C) + CO₂ + reduced NAD (per pyruvate)

Per glucose (link ×2): 2 CO₂, 2 reduced NAD, 2 acetyl CoA; no ATP directly

Never Write

"Glycolysis occurs in the mitochondria" → it occurs in the cytoplasm

"Anaerobic respiration produces extra ATP" → anaerobic respiration regenerates NAD; only 2 ATP (from glycolysis)

"CO₂ is produced in glycolysis" → CO₂ is produced in the link reaction and Krebs cycle; the decarboxylation step in ethanol fermentation occurs after glycolysis, not within it

"Water is produced in the Krebs cycle" → water produced at the end of the ETC