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How is glucose broken down in glycolysis and the link reaction, and what are the products of each stage?

Describe glycolysis and the link reaction, including the products and the role of substrate-level phosphorylation

A focused answer to the H2 Biology Energy and Equilibrium outcome on the first stages of respiration. Glycolysis in the cytoplasm, the net yield of ATP, NADH and pyruvate, and the link reaction producing acetyl coenzyme A, with the role of substrate-level phosphorylation.

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  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe glycolysis (in the cytoplasm) and the link reaction (in the mitochondrial matrix), to state the products of each, and to understand substrate-level phosphorylation. These are the first two stages of aerobic respiration and feed the Krebs cycle that follows.

The answer

Glycolysis

Glycolysis occurs in the cytoplasm and does not require oxygen.

  1. Phosphorylation. Glucose (six carbons) is phosphorylated using two ATP, which activates it and makes it less stable.
  2. Splitting. The phosphorylated six-carbon sugar is split into two three-carbon molecules of triose phosphate.
  3. Oxidation. Each triose phosphate is oxidised to pyruvate. Hydrogen is removed and accepted by NAD to form reduced NAD (NADH), and ATP is made directly by substrate-level phosphorylation.

Net products per glucose: two pyruvate, a net gain of two ATP (four made minus two used), and two reduced NAD.

The link reaction

Each pyruvate enters the mitochondrial matrix, where it is:

  • decarboxylated (a carbon dioxide is removed), and
  • dehydrogenated (hydrogen removed and accepted by NAD to form reduced NAD).

The two-carbon acetyl group joins coenzyme A to form acetyl coenzyme A, which feeds the Krebs cycle. Per glucose (two pyruvate), the link reaction yields two acetyl coenzyme A, two reduced NAD and two carbon dioxide.

Examples in context

Example 1. Anaerobic respiration begins the same way. Glycolysis runs whether or not oxygen is present, which is why even anaerobic organisms and hard-working muscles can make some ATP quickly from glucose. The fate of the pyruvate afterwards is what differs.

Example 2. Why reduced NAD matters. The two ATP from glycolysis are a small fraction of the total yield. The real payoff comes when the reduced NAD made here delivers its hydrogen to the electron transport chain, linking these early stages to oxidative phosphorylation.

Try this

Q1. State where in the cell glycolysis takes place. [1 mark]

  • Cue. In the cytoplasm.

Q2. State the net yield of ATP, reduced NAD and pyruvate from glycolysis of one glucose molecule. [2 marks]

  • Cue. A net 2 ATP, 2 reduced NAD and 2 pyruvate.

Q3. Name the two-carbon molecule formed in the link reaction that enters the Krebs cycle. [1 mark]

  • Cue. Acetyl coenzyme A (the acetyl group joined to coenzyme A).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDescribe the process of glycolysis, including where it occurs and the net products from one molecule of glucose.
Show worked answer →

The answer should give the location, the phosphorylation step, the splitting, and the net yield.

Glycolysis occurs in the cytoplasm and does not require oxygen. First, glucose (a six-carbon sugar) is phosphorylated using two molecules of ATP, which activates it and lowers its stability. This produces a phosphorylated six-carbon sugar.

The six-carbon sugar is then split into two three-carbon molecules (triose phosphate). Each triose phosphate is oxidised and converted to pyruvate. During this, hydrogen is removed and accepted by the coenzyme NAD to form reduced NAD (NADH), and ATP is produced directly by substrate-level phosphorylation.

The net products per glucose are two molecules of pyruvate, a net gain of two ATP (four made, two used), and two reduced NAD.

Markers reward the cytoplasmic location, the initial use of two ATP, the splitting into two triose phosphate, the production of pyruvate, and the correct net yield of 2 ATP, 2 NADH and 2 pyruvate.

Original4 marksDescribe the link reaction and explain why it is important in aerobic respiration.
Show worked answer →

The answer should describe the conversion and its significance.

In the link reaction, each pyruvate produced by glycolysis is transported into the mitochondrial matrix. There it is decarboxylated (a carbon dioxide is removed) and dehydrogenated (hydrogen is removed and accepted by NAD to form reduced NAD). The resulting two-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A.

The link reaction is important because it connects glycolysis to the Krebs cycle: acetyl coenzyme A feeds the two-carbon acetyl group into the Krebs cycle. It also produces reduced NAD, which carries hydrogen to the electron transport chain for ATP synthesis, and it releases carbon dioxide as a waste product.

Markers reward the location in the matrix, decarboxylation and dehydrogenation, the formation of acetyl coenzyme A, and the role of linking glycolysis to the Krebs cycle.

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