Stage 3 Krebs Cycle

Question 1

what is an overview of the Krebs cycle?

Answer 1

an 8-step cyclical process whereby the product in step 8 is the reactant in step 1, this highlights the significance of reaction coupling

Question 2

what role does the enzyme citrate synthase play in controlling cellular respiration?

Answer 2

citrate synthase is activated by ADP. Its activity is not inhibited by ATP. The cell does not want to create too much ATP if it is not needed

Question 3

what role does the molecule citrate play in controlling cellular respiration?

Answer 3

it does negative feedback (a case where outputs from a system are subsequently fed back into it) by slowing down or increasing the rate of cellular respiration. For example, If citrate builds up, this is a sign that glycolysis can slow down because the citric acid cycle is backed up and doesn’t need more fuel.

Question 4

what is significant about many of the enzymes involved in these reactions?

Answer 4

Many of the enzymes are found as enzyme complexes and require a variety of cofactors to function indicating their role in the process

Question 5

what significant products are released from both steps 3 and 4.

Answer 5

NADH, H+, CO2

Question 6

even though oxygen is not directly involved in the Krebs cycle, why must it be used for the process to continue

Answer 6

Oxygen is needed to produce NADH and FADH2. If oxygen is not present, these carries will not be able to give their H+ ions

Question 7

how is the ATP derived from the GTP produced in step 5?

Answer 7

The phosphate from GTP is transferred to ADP resulting in 1 ATP. The GDP is recycled to undergo the reaction again

Question 8

What new intermediate energy carries is produced in step 6 that we are now seeing for the first time. what will the destination be of this carrier

Answer 8

FADH2. The destination will be in the electron transport chain

Question 9

An additional NADH is produced in step 8 of the Krebs Cycle. How many NADH is produced in one turn of the cycle?

Answer 9

3 NADH

Question 10

what is the fate of the carbons that enter the Krebs cycle as Acetyl CoA?

Answer 10

they are found as carbon dioxide created by the Krebs Cycle

Question 11

what are the three key functions that the Krebs cycle serves?

Answer 11

1. Provide regeneration of oxaloacetate
2. Production of ATP via substrate-level phosphorylation
3. Transfer of protons and electrons to the coenzymes NAD+ and FAD

Question 12

where must pyruvate move in the cell after glycolysis?

Answer 12

To the mitochondrial matrix

Question 13

how does your body rectify the problem that pyruvate is a 3-carbon molecule but only a 2-carbon molecule can enter the Krebs cycle?

Answer 13

Pyruvate oxidation removed carbon dioxide with the enzyme pyruvate decarboxylate to become Acetyl CoA

Question 14

in one turn of the Krebs cycle, what are the main products that are produced?

Answer 14

3 NADH are produced in steps 3,4, 8

1 FADH2 is produced in step 6

2 CO2 are produced in steps 3,4

1 ATP is produced in step 5

Question 15

given that two pyruvate molecules are produced in glycolysis, what is the overall energy yield in the Kreds Cycle?

Answer 15

6 NADH are produced in steps 3,4,6

2 FADH2 are produced in step 6

4 CO2 are produced in steps 3,4

2 ATP are produced in step 5

Question 16

what is the primary purpose of the Krebs Cycle

Answer 16

To produce the energy molecules ATP to the cell and NADH and FADH2 to the electron transport chain

Question 17

what is the total energy yield for the first three stages of cellular respiration?

Answer 17

10 NADH 2 from Glycolysis, 2 from Pyruvate Oxidation, 6 from the Krebs Cycle

2 FADH from the Krebs

6 CO2 2 form Pyruvate Oxidation and 4 from the Krebs Cycle

4 ATP 2 from Glycolysis, 2 from the Krebs

Question 18

where are the pyruvate molecules at the end of glycolysis?

Answer 18

in the cytoplasm

Question 19

where do the pyruvate molecules go in order for aerobic respiration to occur?

Answer 19

to the mitochondria

Question 20

what is the overall reaction of the Krebs Cycle after one turn?

Answer 20

Acetyl CoA + 3 NAD+ + FAD+ ADP + Pi + 2H2O —> CoA-SH + 3NADH + FADH2 + 3H+ + ATP + 2CO2

Question 21

when do proteins enter the Krebs cycle?

Answer 21

at times of starvation, amino acids can be used for energy. Their amino groups are removed and their remaining keto-acid enters the Krebs Cycle at various points.

Question 22

when do lipids enter the Krebs Cycle?

Answer 22

beta-oxidation of fatty acids results in the formation of Acetyl Coa and Succinyl-CoA which can be fed into the Krebs cycles when glucose levels are low.