18.2 The Link Reaction And The Krebs Cycle

Question 1

Where does the link reaction take place?

Answer 1

Matrix of the mitochondria

Question 2

Describe and explain the link reaction

Answer 2

Pyruvate is actively transported into the matrix of the mitochondria.
Here, the link reaction converts pyruvate to acetyl coenzyme A (acetyl CoA)

Pyruvate is decarboxylated, so one carbon atom is removed from from pyruvate in the form of carbon dioxide.
NAD is reduced - is collects hydrogen from pyruvate, changing pyruvate into acetate (2C).

Acetate is combined with coenzyme A to form acetyl coenzyme A (CoA).

The link reaction happens twice for each glucose molecule
NO ATP produced in this reaction

Question 3

What are the products from two link reactions?
Where do these products go?

Answer 3

2 acetyl CoA - to the Krebs cycle
2 CO2 - released as a waste product
2 reduced NAD (NADH) - to oxidative phosphorylation

Question 4

Where does the Krebs cycle take place?

Answer 4

In the matrix of the mitochondria

Question 5

Describe and explain the Krebs cycle

Answer 5

The cycle happens once every pyruvate molecule, so it goes around twice for every glucose molecule

1) Formation of citrate
Acetyl CoA (2C) from the link reaction combines with oxaloacetate (4c) to form citrate (6C).
This is catalysed by citrate synthase.
Co enzyme A goes back to the link reaction to be used again

2) Formation of a 5-carbon compound
The 6C citrate molecule is converted to a 5C molecule.
Decarboxylation occurs, where CO2 is removed.
Dehydrogenation also occurs.
The hydrogen is used to produce NADH from NAD.

3) Regeneration of oxoloacetate
The 5C molecule is then converted to a 4C molecule.
Decarboxylation and dehydrogenation occur, producing one molecule of reduced FAD, and two reduced NAD.
ATP is produced by the direct transfer of phosphate group from an intermediate compound to ADP.
When a phosphate group is directly transferred from one molecule to another its called substrate-level phosphorylation

Question 6

What are the products of the Krebs cycle?

Answer 6

1 coenzyme A - reused in the next link reaction
Oxaloacetate - regenerated for use in the next Krebs cycle
2 carbon dioxide - released as waste product
1 ATP - used for energy
3 NADH - to oxidative phosphorylation
1 FADH - oxidative phosphorylation