Where does glycolysis occur, compared with the link reaction and Krebs cycle?

Glycolysis occurs in the cytoplasm; the link reaction and Krebs cycle occur in the mitochondrial matrix.

What happens to pyruvate in the link reaction?

It is decarboxylated (loses CO₂) and dehydrogenated (reduces NAD), and the acetyl group joins coenzyme A to form acetyl coenzyme A.

What is decarboxylation?

Removal of a carbon atom from a molecule as carbon dioxide.

What is dehydrogenation?

Removal of hydrogen from a molecule, which is accepted by NAD or FAD.

How many carbon atoms are in pyruvate and in the acetyl group?

Pyruvate has three; the acetyl group has two.

What combines with the acetyl group in the Krebs cycle?

A four-carbon compound, forming a six-carbon compound.

What is made in one turn of the Krebs cycle?

2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP.

How is ATP made in the Krebs cycle?

By substrate-level phosphorylation.

How many CO₂ are released per glucose in the link reaction and Krebs cycle?

6 (2 in the link reaction and 4 in the Krebs cycle).

How many reduced NAD are made per glucose in the link reaction and Krebs cycle?

8 (2 in the link reaction and 6 in the Krebs cycle).

Why does the Krebs cycle stop without oxygen?

The electron transport chain cannot reoxidise reduced NAD and FAD, so no NAD or FAD is available to accept hydrogen.

What is regenerated at the end of the Krebs cycle?

The four-carbon compound, so the cycle can continue.

Link reaction and Krebs cycleEdexcel International A Level Biology: Flashcards

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Where does the link reaction occur?

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Where does the link reaction occur?: In the matrix of the mitochondrion.
Where does glycolysis occur, compared with the link reaction and Krebs cycle?: Glycolysis occurs in the cytoplasm; the link reaction and Krebs cycle occur in the mitochondrial matrix.
What happens to pyruvate in the link reaction?: It is decarboxylated (loses CO₂) and dehydrogenated (reduces NAD), and the acetyl group joins coenzyme A to form acetyl coenzyme A.
What is decarboxylation?: Removal of a carbon atom from a molecule as carbon dioxide.
What is dehydrogenation?: Removal of hydrogen from a molecule, which is accepted by NAD or FAD.
How many carbon atoms are in pyruvate and in the acetyl group?: Pyruvate has three; the acetyl group has two.
What combines with the acetyl group in the Krebs cycle?: A four-carbon compound, forming a six-carbon compound.
What is made in one turn of the Krebs cycle?: 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP.
How is ATP made in the Krebs cycle?: By substrate-level phosphorylation.
How many CO₂ are released per glucose in the link reaction and Krebs cycle?: 6 (2 in the link reaction and 4 in the Krebs cycle).
How many reduced NAD are made per glucose in the link reaction and Krebs cycle?: 8 (2 in the link reaction and 6 in the Krebs cycle).
Why does the Krebs cycle stop without oxygen?: The electron transport chain cannot reoxidise reduced NAD and FAD, so no NAD or FAD is available to accept hydrogen.
What is regenerated at the end of the Krebs cycle?: The four-carbon compound, so the cycle can continue.

Exam questions on Link reaction and Krebs cycle

In a muscle cell, glycolysis in the cytoplasm produces pyruvate. This pyruvate is transported into the mitochondria, where it is converted to a two-carbon compound that can enter the Krebs cycle.
Explain why the conversion of pyruvate to the two-carbon compound is described as oxidative decarboxylation.2 marks
A researcher studies the Krebs cycle in isolated liver mitochondria supplied with the product of the link reaction. For one molecule of glucose, the Krebs cycle turns twice, once for each pyruvate formed.
The Krebs cycle does not use oxygen directly, but it stops when oxygen is absent. Explain why.2 marks
A suspension of isolated mitochondria is supplied with 0.0020 mol of pyruvate and plenty of oxygen. Assume that all of the pyruvate is completely oxidised by the link reaction and the Krebs cycle, and that one mole of any gas occupies 24 000 cm³ at room temperature and pressure.
Calculate the volume of carbon dioxide released from the 0.0020 mol of pyruvate.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).