18.3 - The Krebs Cycle

Question 1

Where does the Krebs Cycle occur and what processes does it involve?

Answer 1

The Krebs cycle takes place in the mitochondrial matrix, each complete cycles gives a breakdown of an acetyl group. Involves substrate level phosphorylation, decarboxylation and dehydrogenation

They hydrogen atoms from dehydrogenation are picked up by NAD and FAD coenzymes (now reduced) – they can be used in the final stage of cellular respiration (oxidative phosphorylation)

Question 2

What are the stages of the Krebs Cycle?

Answer 2

1. 2 carbon acetyl reacts with 4 carbon oxaloacetate to form a 6-carbon citrate
2. Citrate molecules undergoes decarboxylation and dehydrogenation – produced 1 NAD and 1 carbon dioxide.
   
   1. A 5-carbon compound is formed
3. The 5-carbon compound undergoes dehydrogenation and decarboxylation (oxidative decarboxylation) to form a 4-carbon molecule
4. ADP is phosphorylated to from an ATP molecule. FAD is reduced to FADH₂. And NAD⁺ is reduced to NADH
5. This regenerates oxaloacetate

The products of the citrate cycle: 4 NADH, 1 FADH₂, 3 CO_2, 1 ATP

Question 3

Briefly explain the importance of coenzymes

Answer 3

• Coenzymes required to transfer protons, electrons and functional groups
• Since there are many redox reactions, without coenzymes, respiratory enzymes cannot function as electrons and protons will not be transferred

Question 4

What are the differences between NAD and FAD?

Answer 4

• NAD is involved in all aspects of respiration; FAD only accepts hydrogen in the Krebs cycle
• NAD accepts one hydrogen and FAD accepts 2
• Reduced NAD is oxidised at the beginning of the ETC to release protons and electrons
  
  • FAD is oxidised further along the chain
• NADH results in the synthesis of 3ATP whereas FADH results in 2ATP in the ETC