12.2 Link reaction and Krebs cycle

Question 1

What happens to the pyruvate molecules produced in the cytoplasm during glycolysis

Answer 1

They are actively transported into the matrix of the mitrochondria

Question 2

What is the link reaction

Answer 2

1. Pyruvate is oxidised to acetate. In this reaction, the 3-carbon pyruvate loses a carbon dioxide molecule and two hydrogens. These hydrogens are later accepted by NAD to produce rNAD which produces ATP
2. The 2-carbon acetate combines with coenzyme-A to produce acetylcoenzyme-A

Question 3

Where does the link reaction and krebs cycle occur

Answer 3

In the matrix of the mitochondria

Question 4

What is the krebs cycle

Answer 4

1. The 2-carbon acetylcoenzyme-A combines with a 4-carbon molecule producing a 6-carbon molecule
2. In a series of reactions this 6-carbon molecule loses carbon dioxide and hydrogen to give a 4-carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation
3. The 4-carbon molecule can now combine with a new molecule of acetylcoenzyme-A to begin the cycle again

Question 5

For each molecule of pyruvate what does the link reaction and krebs cycle produce

Answer 5

1. Reduced co-enzymes such as NAD and FAD
2. One molecule of ATP (SLP)
3. Three molecules of carbon dioxide

Question 6

What is the yield from one glucose molecule

Answer 6

1. Reduced co-enzymes such as NAD and FAD
2. TWO molecules of ATP
3. Six molecules of carbon dioxide

Question 7

What are coenzymes

Answer 7

Coenzymes are molecules that some enzymes require in order to function

They carry hydrogen atoms from one molecule to another

Question 8

What are examples of coenzymes

Answer 8

FAD - krebs cycle
NAD - throughout respiration
NADP - Photosynthesis

Question 9

Why is NAD the most important carrier in respiration

Answer 9

It works with dehydrogenase enzymes that catalyse the removal of hydrogen atoms from substanes and transfer them to other molecules involved in oxidative phopshorylation

Question 10

Why is the krebs cycle significant

Answer 10

1. It breaks down macromolecules into smaller ones - pyruvate into carbon dioxide
2. It produces hydrogen atoms that are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation. This leads to ATP production
3. It regenerates the 4-carbon molecule that combines with acetylcoenyme A, which would otherwise accumulate
4. It is a source of intermediate compounds used by cells in the manufacture of other important substanes such as fatty acids, amino acids, and chlorophyll