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Respiration is the metabolic process by which cells release energy from organic molecules (primarily glucose) to synthesise ATP. Glycolysis is the first stage of both aerobic and anaerobic respiration. This lesson covers the location, steps, and significance of glycolysis as required by the Edexcel A-Level Biology (9BI0) specification.
The overall equation for aerobic respiration is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP + heat)
Aerobic respiration occurs in four main stages:
| Stage | Location | Key inputs | Key outputs |
|---|---|---|---|
| 1. Glycolysis | Cytoplasm | Glucose, 2 ATP, 2 NAD⁺ | 2 pyruvate, 4 ATP (net 2), 2 reduced NAD |
| 2. Link reaction | Mitochondrial matrix | 2 pyruvate, 2 NAD⁺, 2 CoA | 2 acetyl CoA, 2 CO₂, 2 reduced NAD |
| 3. Krebs cycle | Mitochondrial matrix | 2 acetyl CoA, 6 NAD⁺, 2 FAD | 4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP |
| 4. Oxidative phosphorylation | Inner mitochondrial membrane | Reduced NAD, reduced FAD, O₂ | ~34 ATP, H₂O |
Exam Tip: Learn the precise location of each stage. Glycolysis is the only stage that occurs in the cytoplasm — all other stages occur in the mitochondria.
Glycolysis literally means "sugar splitting" (from the Greek glykos = sweet, lysis = splitting). It is a series of 10 enzyme-controlled reactions that convert one molecule of glucose (6C) into two molecules of pyruvate (3C).
Glycolysis can be divided into two main phases:
| Step | Reaction | Detail |
|---|---|---|
| 1 | Glucose → Glucose 6-phosphate | ATP is hydrolysed to add a phosphate group. Catalysed by hexokinase. |
| 2 | Glucose 6-phosphate → Fructose 6-phosphate | Isomerisation (rearrangement). |
| 3 | Fructose 6-phosphate → Fructose 1,6-bisphosphate | A second ATP is hydrolysed. Catalysed by phosphofructokinase (PFK) — the key regulatory enzyme. |
| 4 | Fructose 1,6-bisphosphate → 2 × triose phosphate (G3P) | The 6C sugar is split into two 3C molecules. |
Energy cost of Phase 1: 2 ATP molecules consumed
The phosphorylation of glucose serves two critical purposes:
Exam Tip: Examiners often ask why ATP is used at the start of glycolysis despite it being a catabolic (energy-releasing) pathway. The answer is that the initial ATP investment is necessary to activate and destabilise the glucose molecule, making subsequent reactions energetically favourable.
| Step | Reaction | Detail |
|---|---|---|
| 5 | Triose phosphate → Triose bisphosphate | Each triose phosphate is oxidised by removal of hydrogen atoms, which are transferred to NAD⁺ forming reduced NAD (NADH). An inorganic phosphate group is also added. |
| 6–10 | Triose bisphosphate → Pyruvate | Through a series of reactions, the phosphate groups are transferred to ADP to form ATP by substrate-level phosphorylation. |
Energy gain of Phase 2 (per glucose):
Per molecule of glucose:
| Product | Gross | Used | Net |
|---|---|---|---|
| ATP | 4 | 2 (invested in Phase 1) | 2 |
| Reduced NAD | 2 | 0 | 2 |
| Pyruvate | 2 | 0 | 2 |
The 2 reduced NAD molecules carry hydrogen atoms (protons and electrons) to the electron transport chain on the inner mitochondrial membrane during oxidative phosphorylation, where they will be used to generate a large amount of ATP.
In aerobic conditions, reduced NAD is re-oxidised at the electron transport chain, regenerating NAD⁺ so glycolysis can continue.
Exam Tip: If NAD⁺ is not regenerated (as happens when oxygen is absent and the electron transport chain stops), glycolysis would halt because there would be no NAD⁺ to accept hydrogen atoms in the oxidation step. This is why anaerobic pathways exist — to regenerate NAD⁺.
Glycolysis generates ATP by substrate-level phosphorylation, which is fundamentally different from oxidative phosphorylation:
| Feature | Substrate-level phosphorylation | Oxidative phosphorylation |
|---|---|---|
| Location | Cytoplasm (glycolysis), mitochondrial matrix (Krebs cycle) | Inner mitochondrial membrane |
| Mechanism | Direct transfer of a phosphate group from a substrate to ADP | Chemiosmosis (proton gradient drives ATP synthase) |
| Oxygen required? | No | Yes |
| ATP yield | Small (2 in glycolysis, 2 in Krebs) | Large (~34 per glucose) |
The rate of glycolysis is tightly controlled to match the cell's energy needs. The key regulatory enzyme is phosphofructokinase (PFK).
| Condition | Effector | Effect on PFK | Result |
|---|---|---|---|
| High ATP levels | ATP (inhibitor) | Inhibits PFK | Glycolysis slows down (cell has enough energy) |
| High AMP/ADP levels | AMP (activator) | Activates PFK | Glycolysis speeds up (cell needs more ATP) |
| High citrate levels | Citrate (inhibitor) | Inhibits PFK | Krebs cycle is saturated; slow down glucose breakdown |
This is an example of end-product inhibition and negative feedback — the products of later stages regulate the rate of earlier stages.
Glycolysis is thought to be one of the most ancient metabolic pathways because:
| Term | Definition |
|---|---|
| Glycolysis | The metabolic pathway that converts glucose into two molecules of pyruvate, producing a net gain of 2 ATP and 2 reduced NAD |
| Phosphorylation | The addition of a phosphate group to a molecule |
| Substrate-level phosphorylation | The direct transfer of a phosphate group from a phosphorylated intermediate to ADP, forming ATP |
| Phosphofructokinase (PFK) | The key regulatory enzyme of glycolysis |
| Reduced NAD (NADH) | A coenzyme carrying hydrogen atoms (protons and electrons) to the electron transport chain |
| Pyruvate | The 3-carbon end product of glycolysis |
This material sits in Edexcel 9BI0 Topic 5 (On the Wild Side — Photosynthesis, Energy and Ecosystems) and concerns glycolysis as the universal entry pathway to respiration: the cytoplasmic, oxygen-independent conversion of one 6-carbon glucose into two 3-carbon pyruvate molecules with a net yield of +2 ATP and +2 reduced NAD by substrate-level phosphorylation. Synoptic links run forwards to the link reaction and Krebs cycle (lesson 5: pyruvate enters the mitochondrial matrix via the pyruvate carrier and is decarboxylated by pyruvate dehydrogenase to acetyl-CoA), to oxidative phosphorylation (lesson 6: the reduced NAD from glycolysis must shuttle into mitochondria — at energetic cost — before its electrons can drive ATP synthesis), and backwards to the light-independent stage of photosynthesis (lessons 1–2: the glucose oxidised in glycolysis was assembled by the Calvin cycle from CO2). Synoptic links also reach Topic 1 (biological molecules) for glucose structure and the role of ATP/NAD as universal cofactors. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
A muscle cell is respiring aerobically. Glucose is being consumed at a rate that drives glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation in steady state.
(a) Track the carbon, ATP and reduced-NAD balance through glycolysis for one glucose molecule. State the precise net yield of each. (4)
(b) Identify the regulatory enzyme of glycolysis and explain how its activity is controlled when cellular ATP is high. (2)
(c) Explain why phosphorylating glucose to glucose 6-phosphate at the start of the pathway, despite being an apparent "energy waste", is essential for the cell. (2)
Solution with mark scheme:
(a) M1 (AO1) — Glucose (6C) is phosphorylated to glucose 6-phosphate using 1 ATP (hexokinase), isomerised to fructose 6-phosphate, and phosphorylated again to fructose 1,6-bisphosphate using a second ATP (phosphofructokinase) — total ATP invested = 2.
M1 (AO1) — Fructose 1,6-bisphosphate is lysed (split) into two molecules of triose phosphate (G3P, 3C each). Each triose phosphate is then oxidised to 1,3-bisphosphoglycerate, donating hydrogen to NAD+ — 2 reduced NAD are produced per glucose.
A1 (AO1) — In the energy payoff phase, each 3C intermediate transfers two phosphate groups to ADP via substrate-level phosphorylation, yielding 4 ATP total (2 per triose × 2 trioses) and finishing as 2 pyruvate (3C each).
A1 (AO2) — Net per glucose: +2 ATP (4 made − 2 invested), +2 reduced NAD, 2 pyruvate. Carbon is conserved: 6C in (glucose) = 3C × 2 (pyruvate).
(b) M1 (AO1) — The regulatory enzyme is phosphofructokinase (PFK), which catalyses the committed step (fructose 6-phosphate → fructose 1,6-bisphosphate).
A1 (AO2) — When cellular ATP is high, ATP binds allosterically to PFK, distorting its active site so that fructose 6-phosphate binds less efficiently — PFK is inhibited, slowing glycolysis. Citrate (a downstream product of the Krebs cycle) reinforces this inhibition, providing a second negative-feedback signal that respiration is meeting demand.
(c) M1 (AO2) — Phosphorylation traps glucose inside the cell because phosphorylated sugars cannot cross the plasma membrane via the GLUT transporters that admit free glucose, preserving substrate for downstream metabolism.
A1 (AO2) — Phosphorylation also destabilises the glucose ring, lowering the activation energy for subsequent isomerisation, second phosphorylation and lysis. The two ATP "spent" are returned with a net profit of two more ATP plus two reduced NAD that go on to drive ~5 ATP downstream — a small priming investment for a large later return. (Total: 8 marks; M4 A4.)
Question (6 marks): A researcher cultures human red blood cells (mature erythrocytes, which have no mitochondria) in glucose-containing buffer. The cells are observed to consume glucose, release lactate, and synthesise ATP — but no oxygen is consumed and no CO2 is released.
Use your knowledge of glycolysis to explain how erythrocytes can sustain ATP production despite lacking mitochondria, and why their net ATP yield is only 2 per glucose rather than the ~30+ achieved by cells with mitochondria.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that glycolysis occurs in the cytoplasm and does not require mitochondria |
| 2 | AO2.1 | Identifying that erythrocytes therefore retain glycolysis and produce 2 ATP per glucose by substrate-level phosphorylation |
| 3 | AO2.2 | Explaining that without mitochondria, the link reaction, Krebs cycle and oxidative phosphorylation cannot occur, removing the source of the additional ~28+ ATP |
| 4 | AO3.1 | Recognising that the reduced NAD from glycolysis cannot be re-oxidised at the electron transport chain (no ETC present) |
| 5 | AO3.2 | Explaining that pyruvate is therefore reduced to lactate by lactate dehydrogenase, regenerating NAD+ so glycolysis can continue |
| 6 | AO3.3 | Concluding that erythrocyte energetics depends entirely on glycolysis + lactate fermentation; no ATP is made by oxidative phosphorylation, fixing the yield at 2 ATP per glucose |
Total: 6 marks (UMS-band-anchored at A; AO1 = 1, AO2 = 2, AO3 = 3). This question structure mirrors Edexcel's preference for applying a core pathway to a real cell type (erythrocytes, anaerobic exercise, fermenting yeast) and for separating substrate-level from oxidative ATP yield.
Lesson 5 (link reaction + Krebs cycle) — pyruvate's fate. Pyruvate produced in the cytoplasm is transported across the inner mitochondrial membrane via the pyruvate carrier (a symporter that co-imports H+). Inside the matrix, pyruvate dehydrogenase (PDH) decarboxylates pyruvate, removing CO2 and reducing NAD+, and attaches the remaining 2C acetyl group to coenzyme A to form acetyl-CoA — the substrate for the Krebs cycle. PDH is itself regulated (inhibited by high acetyl-CoA, NADH and ATP), providing a second checkpoint downstream of PFK.
Lesson 6 (oxidative phosphorylation) — NADH must shuttle into mitochondria. Glycolytic NAD reduction happens in the cytoplasm, but the electron transport chain sits on the inner mitochondrial membrane, which reduced NAD cannot cross. Two shuttles ferry the electrons (not NADH itself) across: the glycerol-3-phosphate shuttle (used in muscle, brain) hands electrons to FAD+ on the inner membrane, yielding ~1.5 ATP per cytoplasmic NADH; the malate–aspartate shuttle (used in liver, heart, kidney) regenerates matrix NADH, yielding ~2.5 ATP. This explains why textbook total ATP yields per glucose vary between ~30 and ~32 — depending on which shuttle the tissue uses.
Lessons 1–2 (photosynthesis) — the glucose-glycolysis loop. The glucose oxidised here was originally assembled by the Calvin cycle from atmospheric CO2 using ATP and reduced NADP from the light-dependent reactions. Photosynthesis and respiration are mirror processes: photosynthesis fixes CO2 and stores energy in glucose; respiration releases CO2 and recovers that energy as ATP. The reduced NAD here parallels the reduced NADP of the light reactions — both are H-carriers built around the same nicotinamide ring.
Topic 1 (biological molecules) — glucose, ATP, NAD as universal cofactors. Glucose is a hexose monosaccharide (C6H12O6); ATP is a ribonucleotide carrying three phosphates with hydrolysis of the terminal bond releasing ~30 kJ mol−1; NAD+ accepts 2 e− + 1 H+ to become NADH. Glycolysis depends entirely on the chemistry covered in Topic 1.
Anaerobic respiration (lesson 7) — pyruvate's anaerobic fates. When O2 is absent (or absent at the ETC), pyruvate is reduced to lactate in animal tissue (lactate dehydrogenase) or to ethanol + CO2 in yeast (pyruvate decarboxylase + ethanol dehydrogenase) — both reactions exist only to regenerate NAD+ so glycolysis can continue. Without this NAD+ regeneration, glycolysis would arrest at the oxidation step within seconds.
Cancer metabolism — the Warburg effect. Many tumour cells run aerobic glycolysis at very high rates even when oxygen is plentiful, producing lactate from glucose despite having functional mitochondria. This Warburg effect is currently a research frontier (positron emission tomography exploits it: tumours light up on FDG-PET because they take up glucose analogues at much higher rates than normal tissue). The phenomenon is thought to support biosynthesis (pentose phosphate pathway, amino-acid synthesis) rather than ATP yield per glucose.
| AO | Typical share on glycolysis questions | Earned by |
|---|---|---|
| AO1 (knowledge) | 30–40% | Naming the location (cytoplasm); listing the carbon flow (glucose → G6P → F6P → F1,6BP → 2 × G3P → 2 × pyruvate); stating net yields (+2 ATP, +2 NADH, 2 pyruvate); identifying PFK as the regulatory enzyme |
| AO2 (application) | 35–45% | Tracking ATP investment vs payoff; explaining why phosphorylation traps and activates glucose; applying allosteric inhibition logic to PFK |
| AO3 (analysis / evaluation) | 20–30% | Predicting glycolytic flux changes from ATP/AMP/citrate ratios; reasoning about NAD+ regeneration in anaerobic conditions; comparing erythrocyte and skeletal-muscle yields |
Examiner-rewarded phrasing: "glycolysis occurs in the cytoplasm, not the mitochondria"; "net yield is +2 ATP (4 made − 2 invested)"; "substrate-level phosphorylation transfers phosphate directly from the substrate to ADP — no chemiosmosis"; "PFK is allosterically inhibited by ATP and citrate, providing negative feedback"; "reduced NAD must shuttle into the mitochondrion to drive oxidative phosphorylation".
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