5.7.4: The link reaction and the Krebs cycle.

Question 1

How is pyruvate produced during glycolysis transported across the outer and inner mitochondrial membranes?

Answer 1

via a specific pyruvate-H+ symport.

Question 2

What is a symport?

Answer 2

A transport protein that transports two ions or m molecules in the same direction, and into the matrix.

Question 3

What happens once pyruvate is in the matrix?

Answer 3

1. Pyruvate is converted into a two carbon acetyl group during the link reaction.
2. The acetyl group is oxidised during the Krebs cycle.

Question 4

where does the link reaction take place?

Answer 4

In the mitochondrial matrix.

Question 5

Pyruvate is decarboxylated and dehydrogenated in the link reaction, what is this catalysed by?

Answer 5

A large multi-enzyme complex, pyruvate dehydrogenase, which catalyses the sequence of reactions occurring during the link reaction.

Question 6

How much ATP is produced in the link reaction?

Answer 6

No ATP is produced during the link reaction.

Question 7

What are the stages of the link reaction?

Answer 7

1. The carboxyl group is removed and is the origin of some of the carbon dioxide produced.
2. This decarboxylation of pyruvate together with dehydrogenation produces an acetyl group.
3. The acetyl group combines with coenzyme A (CoA) to become acetyl CoA.
4. The coenzyme NAD becomes reduced

Question 8

which equation summarises the link reaction for two molecules of pyruvate?

Answer 8

2pyruvate+2NAD+2CoA –> 2CO2 + 2NADH+2acetylCoA

Question 9

What is the role of coenzyme A?

Answer 9

It accepts the acetyl group and, in the form of acetyl CoA, carries the acetyl group on to the Krebs cycle.

Question 10

What is the Krebs cycle?

Answer 10

A series of enzyme-controlled reactions that oxudise the acetate from the link reaction ro 2 molecules of carbon dioxide, while conserving energy by reducing the coenzymes NAD and FAD.

Question 11

What do the reduced coenzymes from the Krebs cycle do?

Answer 11

They carry the hydrogen atoms to the electron transport chain on the cristae, where they will be involved in the production of many more ATP molecules.

Question 12

There are six steps to the Krebs cycle

Step 1:

Answer 12

The acetyl group released from the acetyl CoA combines with a four-carbon compound, oxaloacetate, to form a 6 carbon compound, citrate.

Question 13

There are six steps to the Krebs cycle
Step 1: The acetyl group released from the acetyl CoA combines with a four-carbon compound, oxaloacetate, to form a 6 carbon compound, citrate.

Step 2:

Answer 13

Citrate is decarboxylated and dehydrogenated, producing a five-carbon compound, one molecule of CO2 and one molecule of reduced NAD.

Question 14

There are six steps to the Krebs cycle
Step 2: Citrate is decarboxylated and dehydrogenated, producing a five-carbon compound, one molecule of CO2 and one molecule of reduced NAD.

Step 3:

Answer 14

This five-carbon compound is further decarboxylated and dehydrogenated, producing a four-carbon compound, one molecule of CO2 and one molecule of reduced NAD.

Question 15

There are six steps to the Krebs cycle
Step 3: This five-carbon compound is further decarboxylated and dehydrogenated, producing a four-carbon compound, one molecule of CO2 and one molecule of reduced NAD.

Step 4:

Answer 15

This four-carbon compound combines temporarily with, and is then released from, coenzyme A. At this stage, substrate level phosphorylation takes place, producing one molecule of ATP.

Question 16

There are six steps to the Krebs cycle
Step 4: This four-carbon compound combines temporarily with, and is then released from, coenzyme A. At this stage, substrate level phosphorylation takes place, producing one molecule of ATP.

Step 5:

Answer 16

This four-carbon compound is dehydrogenated, producing a different four carbon compound and a molecule of reduced FAD.

Question 17

There are six steps to the Krebs cycle
Step 5: This four-carbon compound is dehydrogenated, producing a different four carbon compound and a molecule of reduced FAD.

Step 6:

Answer 17

Rearrangement of atoms in the four-carbon molecule, catalysed by an isomerase enzyme, followed by further dehydrogenation, regenerate a molecule of oxaloacetate, so the cycle can continue.

Question 18

For every molecule of glucose, how many turns of the Krebs cycle are there?

Answer 18

2

Question 19

What are the products of the link reaction?

Answer 19

2 NADH
0 FADH
2 CO2
0ATP

Question 20

What are the products of the Krebs cycle?

Answer 20

6 NADH
2 FADH
4 CO2
2 ATP

Question 21

Why are the Krebs cycle and the link reaction aerobic?

Answer 21

Although they do not directly use oxygen, they will not take place in the absence of oxygen.

Question 22

What is completed by the end of the Krebs cycle?

Answer 22

The production of carbon dioxide.

Question 23

Other substrates besides glucose can be respired aerobically:
Fatty acids…

Answer 23

…Can be broken down to many molecules of acetate that enter the Krebs cycle via acetyl CoA.

Question 24

Other substrates besides glucose can be respired aerobically:
Glycerol…

Answer 24

…my be converted to pyruvate and enter the Krebs cycle via the link reaction.

Question 25

Other substrates besides glucose can be respired aerobically:
Amino acids…

Answer 25

…may be deaminated (NH2 removed) and the rest of the molecule can enter the Krebs cycle directly or be changed to pyruvate or acetyl CoA.