The Link Reaction and the Kreb’s Cycle

Question 1

What has to happen to the pyruvate molecules produced in glycolysis for them to enter the Krebs cycle?

Answer 1

They must be oxidised

Question 2

In eukaryotic cells where do the Krebs cycle and the link reaction take place?

Answer 2

Inside the mitochondria

Question 3

(The link reaction) How do the pyruvate molecules produced in glycolysis get into the matrix of the mitochondria?

Answer 3

They are actively transported in

Question 4

(The link reaction) What happens to the the pyruvate molecules once they are actively transported into the matrix of the mitochondria?

Answer 4

The pyruvate is oxidised to acetate

Question 5

(The link reaction) How is pyruvate oxidised to acetate?

Answer 5

The pyruvate loses a carbon dioxide molecule and two hydrogens

Question 6

(The link reaction) What happens to the hydrogens removed from pyruvate?

Answer 6

They re accepted by NAD to form reduced NAD

Question 7

(The link reaction) Which enzymes remove carbon dioxide?

Answer 7

Decarboxylases

Question 8

(The link reaction) Which enzymes remove hydrogen?

Answer 8

Dehydrogenases

Question 9

(The link reaction) How many carbons does acetate have?

Answer 9

2

Question 10

(The link reaction) What happens to the acetate?

Answer 10

It combines with coenzyme A to form acetyl coenzyme A

Question 11

How can the link reaction be summarized in an equation?

Answer 11

Pyruvate + NAD + CoA => acetyl CoA + reduced NAD + CO2

Question 12

What reactions does the Krebs cycle involve?

Answer 12

Oxidation-reduction

Question 13

Where does the Krebs cycle take place?

Answer 13

The matrix of the mitochondria

Question 14

(Krebs cycle) What happens to (2-carbon) acetyl coenzyme A?

Answer 14

It combines with a 4-carbon molecule to form a 6-carbon molecule

Question 15

(Krebs cycle) What happens to the 6-carbon molecule formed?

Answer 15

It loses a carbon dioxide molecule and a hydrogen to give a 4-carbon molecule and a single molecule of ATP

Question 16

(Krebs cycle) How is the single molecule of ATP formed?

Answer 16

Substrate-level phosphorylation

Question 17

(Krebs cycle) What happens to the 4-carbon molecule at the end of the Krebs cycle?

Answer 17

It can combine with a new molecule of acetylcoenzyme A to begin the cycle again

Question 18

For each molecule of pyruvate what do the link reaction and the Krebs cycle produce?

Answer 18

• reduced coenzymes
• one molecule of ATP
• three molecules of carbon dioxide

Question 19

What reduced coenzymes are produced as a result of the Krebs cycle and the link reaction?

Answer 19

NAD and FAD

Question 20

Why are reduced NAD and FAD an important product of the Krebs cycle and link reaction?

Answer 20

They have the potential to provide energy to produce ATP molecules by oxidative phosphorylation

Question 21

How many pyruvate molecules are produced in glycolysis?

Answer 21

2

Question 22

Per glucose molecule (entered into glycolysis) what are the overall products?

Answer 22

2 ATP and 6 molecules of carbon dioxide

Question 23

What are coenzymes?

Answer 23

Molecules that some enzymes require in order to function

Question 24

What are the important coenzymes in respiration?

Answer 24

NAD and FAD

Question 25

Why is NAD important in respiration?

Answer 25

It works with dehydrogenase enzymes which catalyse the removal of hydrogen atoms from substrates and transfer them to other molecules involved in oxidative phosphorylation

Question 26

What is the significance of the Krebs cycle?

Answer 26

• it breaks down macromolecules into smaller ones
• it produces hydrogen atoms which are carried by NAD to the electron transfer chain
• it regenerates the 4 carbon molecule that combines with acetylcoenzyme A
• it is a source of intermediate compounds

Question 27

Give an example of the Krebs cycle breaking down macromolecules?

Answer 27

Pyruvate is broken down into carbon dioxide

Question 28

Why is it important that the Krebs cycle produces hydrogen atoms?

Answer 28

The hydrogen atoms are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation. Leading to the production of ATP which provides metabolic energy for the cell

Question 29

Why is it important that the Krebs cycle regenerates the 4 carbon molecule that combines with acetyl coenzyme A?

Answer 29

It would otherwise accumulate

Question 30

Why is it important that the Krebs cycle is a source of intermediate compounds?

Answer 30

The intermediate compounds are used in the manufacture of other important substances such as fatty acids, amino acids and chlorophyll

Question 31

How many carbons does pyruvate have?

Answer 31

3

Question 32

Name the 2-carbon molecule that pyruvate in converted to during the link reaction

Answer 32

Acetate

Question 33

In which part of the cell does the Krebs cycle take place?

Answer 33

The matrix of the mitochondria

Question 34

Is ATP produced in the Krebs cycle?

Answer 34

Yes

Question 35

Is ATP needed in the Krebs cycle?

Answer 35

No

Question 36

Is NAD reduced in the Krebs cycle?

Answer 36

Yes

Question 37

Is NADP oxidised in the Krebs cycle?

Answer 37

No

Question 38

Is carbon dioxide produced in the Krebs cycle?

Answer 38

Yes

Question 39

Is carbon dioxide needed in the Krebs cycle?

Answer 39

No