Krebs Cycle 18.3

Question 1

Where does the Krebs cycle take place?

Answer 1

In the mitochondrial matrix

Question 2

What happens to start the Krebs cycle?

Answer 2

Acetyl CoA delivers the acetyl group to the Krebs cycle by combining it with OAA (4c) which is called oxaloacetate. It forms citrate (6c)

Question 3

Describe the rest of the Krebs cycle

Answer 3

• Citrate is decarboxylated so a CO2 is removed and it is also dehydrogenated so an NAD+ becomes NADH, it is oxidised.
• The 5c compound formed undergoes decarboxylation and dehydrogenation again to form a 4c compound
• This 4c compound takes part in substrate level phosphorylation to make ATP
• It also undergoes an oxidation as an FAD becomes a reduced FAD.
• NAD+ then becomes reduced again and oxaloacetate is formed again

Question 4

What is the net production of the Krebs cycle?

Answer 4

Per glucose:

• 4CO2
• 6 NADH
• 2 ATP
• 2 FADH2

Question 5

Why do we need coenzymes?

Answer 5

We need them to transfer protons, electrons and functional groups between enzyme controlled reactions. They take part in redox reactions and without them, enzymes wouldn’t function

Question 6

What are the differences between coenzymes NAD and FAD?

Answer 6

• NAD takes part in all stages of respiration, FAD only takes part in the Krebs cycle
• NAD accepts one hydrogen, FAD accepts 2
• reduced NAD results in the synthesis of 3 ATP, FAD results in 2
• NAD is oxidised at the start of the electron transport chain and donates protons and electrons, FAD is oxidised later

Question 7

What happens to NAD when it is reduced?

Answer 7

It accepts two protons and an electron pair which can then transfer these.

Question 8

What is the structure of NAD?

Answer 8

It has 2 riboses, 2 adenines and 2 phosphates

Question 9

Why can ATP be described as a coenzyme?

Answer 9

It releases energy when hydrolysed which is then used by enzymes in order to take part in metabolic reactions.

Question 10

Why is the Krebs cycle part of aerobic respiration when it doesn’t use oxygen?

Answer 10

It produces many reduced coenzymes which are then used later to donate protons and electrons to the electron transport chain. If the Krebs cycle doesn’t occur, oxidative phosphorylation can’t occur and here oxygen is used. Also oxygen is required to allow pyruvate to move from the cytoplasm to mitochondria