Glycolysis and the Link Reaction Revision Notes | A Level AQA Biology

1. Why Respiration Starts With Glucose Breakdown

Every cell in your body needs ATP — the universal energy currency — to drive active transport, muscle contraction, protein synthesis, and hundreds of other processes. Yet ATP cannot be stored in large quantities. The cell must make it continuously, on demand, from the chemical energy locked inside glucose.

Respiration is the process that releases this energy and uses it to synthesise ATP. It does not happen in one step; it is a sequence of carefully controlled reactions, each releasing a manageable amount of energy. Glycolysis is where that sequence begins — and it happens before oxygen is even involved.

Here is the key insight to hold onto throughout this lesson: glycolysis is the first stage of both aerobic and anaerobic respiration. Whether or not oxygen is available, every respiring cell starts by splitting glucose in the cytoplasm. What happens to the products afterwards depends on oxygen availability — but the starting point is always the same.

2. Glycolysis — Breaking Glucose in the Cytoplasm

Glycolysis takes place entirely in the cytoplasm. This is significant: the cytoplasm has no membrane-bound compartments, no electron transport chains, and no mitochondria involved at this stage. Glycolysis is therefore described as an anaerobic process — it does not require oxygen to proceed.

Picture glucose as a six-carbon chain. Glycolysis has two broad phases: an energy investment phase, where ATP is used to activate the glucose molecule, and an energy payoff phase, where more ATP and reduced NAD are generated. The net result is a gain of ATP — the cell gets back more than it put in.

2a. Stage 1 — Phosphorylation of Glucose

Glucose on its own is chemically stable and relatively unreactive. To make it reactive enough to be broken down, the cell first phosphorylates it — that is, it attaches phosphate groups to the glucose molecule using ATP. Two ATP molecules are consumed in this stage, converting glucose into glucose phosphate and then into a hexose bisphosphate (a six-carbon molecule with two phosphate groups attached).

The phosphorylation serves two purposes. First, it destabilises the glucose molecule, making subsequent reactions thermodynamically favourable. Second, the phosphorylated glucose cannot pass back through the cell membrane, so it is effectively trapped inside the cell and committed to the glycolytic pathway.

2b. Stage 2 — Production of Triose Phosphate

The phosphorylated six-carbon molecule is then split into two three-carbon molecules, each carrying one phosphate group. These three-carbon molecules are called triose phosphate (TP). Because one glucose produces two triose phosphate molecules, every subsequent step in glycolysis happens twice per original glucose molecule.

2c. Stage 3 — Oxidation of Triose Phosphate to Pyruvate

Each triose phosphate molecule is then oxidised — hydrogen is removed from it. This oxidation is coupled to the reduction of NAD: the hydrogen atoms (electrons and protons) are accepted by NAD, converting it to reduced NAD. This is a critical point because reduced NAD carries those electrons to the electron transport chain later, where the majority of ATP is generated in aerobic respiration.

As triose phosphate is oxidised, the phosphate groups are used to phosphorylate ADP, producing ATP directly. This direct production of ATP from substrate-level phosphorylation yields four ATP molecules in total (two per triose phosphate). Since two ATP were invested in the phosphorylation stage, the net gain from glycolysis is two ATP molecules per glucose.

The end product of glycolysis is pyruvate — a three-carbon molecule. Two pyruvate molecules are produced per glucose. Glycolysis has therefore converted one six-carbon glucose into two three-carbon pyruvate molecules, with a net gain of 2 ATP and 2 reduced NAD.

Worked example: Tracing the ATP and Reduced NAD Yield Through Glycolysis

Describe and account for the net ATP yield and the reduced NAD yield from the complete glycolysis of one glucose molecule.

1. 2 ATP used to phosphorylate glucose → glucose phosphate → hexose bisphosphate — ATP is consumed here to activate glucose. This is the energy investment phase. Without this step, glucose is too stable to react.
2. 1 hexose bisphosphate → 2 triose phosphate molecules — Each triose phosphate carries one phosphate group. All subsequent steps occur twice — once for each triose phosphate.
3. Each triose phosphate → pyruvate produces 2 ATP 2 triose phosphate × 2 ATP = 4 ATP produced — ATP is made by substrate-level phosphorylation — the phosphate groups from triose phosphate are transferred directly to ADP.
4. Net ATP = 4 ATP produced − 2 ATP invested = 2 ATP — The net gain is 2 ATP per glucose. Examiners often ask for the net figure, not the gross figure of 4 ATP.
5. Each triose phosphate oxidised → 1 reduced NAD 2 triose phosphate × 1 reduced NAD = 2 reduced NAD — NAD accepts hydrogen removed during the oxidation of triose phosphate. Reduced NAD will later donate these electrons to the electron transport chain.

Answer: Net yield from glycolysis per glucose: 2 ATP (net) and 2 reduced NAD

3. What Happens to Pyruvate Next — The Aerobic Route

At the end of glycolysis, the cell has two pyruvate molecules and a small but real supply of ATP and reduced NAD. If oxygen is available, the cell can extract far more energy from pyruvate by continuing into aerobic respiration. If oxygen is absent, the cell takes a different route — anaerobic respiration — to regenerate NAD so that glycolysis can continue. This lesson focuses on the aerobic route.

For aerobic respiration to continue, pyruvate must move from the cytoplasm into the mitochondrial matrix. This movement is not passive — pyruvate crosses the inner mitochondrial membrane by active transport, which requires ATP. This is an important detail: the movement of pyruvate into the mitochondrion is an energy-requiring process, not simple diffusion.

4. The Link Reaction — Connecting Glycolysis to the Krebs Cycle

Once inside the mitochondrial matrix, pyruvate undergoes the link reaction. The name is apt: this reaction links glycolysis (in the cytoplasm) to the Krebs cycle (also in the matrix). The link reaction converts the three-carbon pyruvate into a two-carbon compound called acetate, which then combines with coenzyme A to form acetyl coenzyme A — the molecule that enters the Krebs cycle.

The link reaction involves two key events happening simultaneously. First, pyruvate is decarboxylated — a carboxyl group is removed as carbon dioxide. This reduces the carbon skeleton from three carbons to two carbons (acetate). Second, the remaining two-carbon compound is oxidised — hydrogen is removed and accepted by NAD, producing reduced NAD.

The two-carbon acetate then combines with coenzyme A to produce acetyl coenzyme A (acetyl CoA). Coenzyme A acts as a carrier molecule, holding the acetate group and delivering it to the Krebs cycle. Because two pyruvate molecules are produced per glucose, the link reaction occurs twice per glucose molecule, yielding 2 acetyl CoA, 2 reduced NAD, and 2 carbon dioxide molecules in total.

Worked example: Tracing the Link Reaction Step by Step

Describe what happens to one pyruvate molecule during the link reaction, accounting for all carbon atoms and identifying all products.

1. Pyruvate: a 3-carbon molecule (CH₃COCOO⁻) in the mitochondrial matrix — Pyruvate has arrived from the cytoplasm via active transport. It is now in the correct location for the link reaction to proceed.
2. Pyruvate (3C) → acetate (2C) + CO₂ — One carbon is lost as carbon dioxide. This is why CO₂ is released during aerobic respiration even before the Krebs cycle. The carbon skeleton is now two carbons long.
3. Acetate (2C) is oxidised → reduced NAD is produced NAD + 2H → reduced NAD — Hydrogen atoms are removed from acetate and accepted by NAD. This oxidation step produces one reduced NAD per pyruvate. Reduced NAD will carry electrons to the electron transport chain.
4. Acetate (2C) + coenzyme A → acetyl coenzyme A (acetyl CoA) — Coenzyme A acts as a carrier, binding to the acetate group. The resulting acetyl CoA is the substrate that enters the Krebs cycle.
5. 1 glucose → 2 pyruvate → 2 × link reaction Total per glucose: 2 acetyl CoA + 2 reduced NAD + 2 CO₂ — Because glycolysis produces two pyruvate per glucose, the link reaction runs twice. Always state totals per glucose in exam answers unless the question specifies per pyruvate.

Answer: Per glucose: 2 acetyl CoA, 2 reduced NAD, and 2 CO₂ are produced in the link reaction.

5. Putting It Together — The Full Picture from Glucose to Acetyl CoA

Now that both stages are clear, it helps to see them as a continuous sequence. Glucose enters glycolysis in the cytoplasm and is converted to pyruvate. Pyruvate crosses into the mitochondrial matrix by active transport. In the matrix, the link reaction converts each pyruvate into acetyl CoA, releasing CO₂ and producing reduced NAD along the way.

Notice that the link reaction produces no ATP directly. Its contribution is to generate reduced NAD and to produce acetyl CoA, which feeds into the Krebs cycle. The ATP payoff from the link reaction comes indirectly — via the electron transport chain, where the reduced NAD donates its electrons to drive ATP synthesis.

6. Exam Technique — Scoring Full Marks on Glycolysis and the Link Reaction

Questions on glycolysis and the link reaction appear regularly in AQA A Level Biology papers, often as 'describe', 'explain', or 'compare' questions worth 4–6 marks. The marks are almost always awarded for specific, precise statements — vague answers about 'energy being released' will not score.

When a question asks you to 'describe glycolysis', structure your answer around the three stages: phosphorylation of glucose using ATP, production of triose phosphate, and oxidation of triose phosphate to pyruvate with production of ATP and reduced NAD. Always state the net ATP gain (2 ATP) rather than the gross figure (4 ATP), and always specify that glycolysis occurs in the cytoplasm.

For the link reaction, the two processes to name are decarboxylation (pyruvate → acetate + CO₂) and oxidation (acetate is oxidised; NAD accepts hydrogen to form reduced NAD). Then state that acetate combines with coenzyme A to form acetyl coenzyme A. Examiners award marks for naming acetyl coenzyme A in full — writing 'acetyl CoA' is acceptable shorthand, but make sure you have defined it at least once in your answer if the question is extended.

Glycolysis and the link reaction together form the essential foundation of aerobic respiration: they convert glucose into acetyl CoA, generate a small but immediate supply of ATP and reduced NAD, and feed the Krebs cycle with its key substrate — satisfying the core AQA specification requirements for this topic.