[12.2] link reaction and krebs cycle

Question 1

where does the link reaction occur?

Answer 1

• in the matrix of the mitochondria
• pyruvate molecules produced in the cytoplasm during glycolysis are actively transported here

Question 2

describe the link reaction

Answer 2

• pyruvate is oxidised to acetate
• the 3-carbon pyruvate loses a carbon dioxide molecule and two hydrogens
• these hydrogens are accepted by NAD to form reduced NAD, which is later used to produce ATP
• 2-carbon acetate combines with coenzyme (CoA) to produce a compound called acetylcoenzyme A

Question 3

what is the overall equation for the link reaction?

Answer 3

pyruvate + NAD + CoA -> acetylcoenzyme A + NADH + carbon dioxide

Question 4

where does the krebs cycle take place?

Answer 4

in the matrix of the mitochondria

Question 5

summarise the events of the krebs cycle

Answer 5

• 2-carbon acetylcoenzyme A from the link reaction combines with oxaloacetate (4C) to produce a 6-carbon molecule
• 6C molecule loses a carbon dioxide and a hydrogen, which is accepted by an NAD+ to form NADH
• 5C molecule loses a carbon dioxide and a hydrogen, which is accepted by a NAD+ to form NADH
• 4C molecule is formed with a single molecule of ATP (ADP + Pi ->) during substrate-level phosphorylation
• 4C molecule is converted into another 4C molecule via FAD²⁺ -> FADH
• 4C molecule is converted back into oxaloacetate (4C) via NAD+ -> NADH so the whole process starts again

Question 6

what are the products of the link reaction and the krebs cycle from one molecule of pyruvate?

Answer 6

• reduced coenzymes eg. NAD, FAD
• one molecule of ATP
• three molecules of carbon dioxide

Question 7

what do NAD and FAD have the potential to do?

Answer 7

provide energy to produce ATP molecules by oxidative phosphorylation

Question 8

how does the krebs cycle play an important role in the cells of organisms?

Answer 8

• breaks down macromolecules into smaller ones eg. pyruvate -> carbon dioxide
• produces hydrogen atoms that are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation
  > this leads to the production of ATP that provides metabolic energy for the cell
• regenerates the 4C molecules that combines with acetyl CoA, which would otherwise accumulate
• source of intermediate compounds used by cells in the manufacture of other important substances eg. fatty aids, amino acids, and chlorophyll

Question 9

what is substrate-level phosphorylation?

Answer 9

direct transfer of phosphate from a respiratory intermediate to ADP to produce ATP