S2) Energy Production II — TCA Cycle & ETC

Question 1

Describe the structure of the mitochondrion

Answer 1

Question 2

Pyruvate from Stage 1 (glycolysis) does not enter directly into Stage 3 (TCA cycle).

What reaction occurs?

Answer 2

The link reaction

Question 3

Where does the link reaction occur?

Answer 3

Mitochondrial matrix – pyruvate transported from cytoplasm across mitochondrial membrane

Question 4

Which enzyme catalyses the link reaction?

Answer 4

Pyruvate dehydrogenase (large multi-enzyme complex)

Question 5

Describe three features of the link reaction

Answer 5

• Reaction is sensitive to Vitamin B₁ deficiency (PDH requires factors from B₁)
• Reaction is irreversible (key regulatory step)
• Irreversible loss of CO₂

Question 6

PDH is subject to multiple regulation.

Describe its activation and inhibition in terms of phosphorylation and dephosphorylation

Answer 6

Question 7

What is the consequence of PDH deficiency?

Answer 7

Lactic acidosis

Question 8

Describe three features of the Tricarboxylic Acid Cycle

Answer 8

• Acetyl CoA converted to 2CO₂
• Oxidative
• Produces some energy & precursors for biosynthesis

Question 9

Where does the TCA cycle occur?

Answer 9

Mitochondria

Question 10

How many rounds of TCA cycle occur per glucose molecule entering from glycolysis?

Answer 10

Question 11

What is produced after the two rounds of the TCA cycle occurring per glucose molecule?

Answer 11

• 6 NADH
• 2 FADH₂
• 2 GTP

Question 12

How is the TCA cycle regulated?

Answer 12

Question 13

Appreciate the biological precusors molecules synthesised from the TCA cycle

Answer 13

Question 14

Which three events occur in oxidative phosphorylation?

Answer 14

• Electron transport and ATP synthesis
• NADH & FADH₂ re-oxidised
• Large amounts of energy (ATP) produced

Question 15

Where does the ETC occur?

Answer 15

Inner mitochondrial membrane

Question 16

What is the role of the electron carriers NADH and FAD₂H in the ETC?

Answer 16

• NADH and FAD₂H contain high energy electrons that can be transferred to O₂ through a series of carrier molecules with the release of large amounts of energy
• The energy is used to drive ATP synthesis in the final stage of catabolism (oxidative phosphorylation)

Question 17

Identify and describe the two uses of reducing power in ATP synthesis

Answer 17

• Electron transport: electrons on NADH and FAD₂H transferred through a series of carrier molecules to oxygen
• Oxidative phosphorylation: free energy released used to drive ATP synthesis

Question 18

Describe what happens in the ETC

Answer 18

• NADH and FADH₂ supply electrons from metabolic substrates
• Electrons pass through the ETC and reduce oxygen to form H₂O at Complex IV

Question 19

What do proton translocating complexes do?

Answer 19

• Complexes I, III and IV act as proton translocating complexes
• They use free energy derived from ETC to move protons from the inside to the outside of the inner mitochondrial membrane to build up a proton motive force

Question 20

Explain the concept of the proton motive force

Answer 20

• The inner mitochondrial membrane is impermeable to protons and the H⁺ concentration in the intermembrane space builds up
• A [H⁺] gradient (membrane potential) across inner mitochondrial membrane forms i.e. the proton motive force

Question 21

Oxidative phosphorylation involves electron transport coupled to ATP synthesis.

In three steps, explain how this occurs

Answer 21

⇒ Electrons are transferred from NADH and FAD₂H to molecular oxygen

⇒ Energy released is used to generate the PMF

⇒ Energy from the dissipation of the proton motive force is coupled to the synthesis of ATP from ADP

Question 22

How many proton translocating complexes are used by NADH and FADH₂ respectively?

Answer 22

Electrons in NADH have more energy than in FAD₂H:

• NADH uses 3 PTCs
• FADH₂ uses only 2 PTCs

Question 23

State the amounts of ATP synthesised from the oxidation of two moles of NADH and FADH₂ respectively

Answer 23

• Oxidation of 2 moles of NADH → synthesis of 5 moles of ATP (P/O = 2.5)
• Oxidation of 2 moles of FADH₂ → synthesis of 3 moles of ATP (P/O = 1.5)

Question 24

In 4 steps, explain how a high [ATP] regulates oxidative phosphorylation

Answer 24

⇒ Low [ADP] is low means no substrate for ATP synthase

⇒ Inward flow of H⁺ stops

⇒ [H⁺] in intermitochondrial space increases

⇒ Stops electron transport

Question 25

Explain the inhibition of oxidative phosphorylation

Answer 25

Inhibitors block electron transport e.g. cyanide prevents acceptance of electrons by O₂

Question 26

In three steps, explain how the uncoupling of oxidative phosphorylation occurs

Answer 26

⇒ Uncouplers increase the permeability of inner mitochondrial membrane to H⁺

⇒ Dissipate proton gradient (reducing PMF)

⇒ No drive for ATP synthesis

Question 27

Identify an example of an uncoupler

Answer 27

Fatty acids

Question 28

Thermogenin (UCP1) is a naturally-occurring uncoupling protein found in brown adipose tissue.

In 6 steps, explain its role in response to cold

Answer 28

⇒ Noradrenaline activates lipase

⇒ Lipase releases fatty acids from triacylglycerol

⇒ Fatty acids activate UCP1

⇒ UCP1 uncouples electron transport from ATP synthesis

⇒ H⁺ are transported back into mitochondria

⇒ Energy of PMF is released as extra heat

Question 29

Where is brown adipose tissue found and what is its function?

Answer 29

• Newborn infants – to maintain heat, particularly around vital organs
• Hibernating animals – to generate heat to maintain body temperature

Question 30

Compare and contrast oxidative phosphorylation with substrate level phosphorylation in terms of the following:

• Requirements
• Energy coupling
• O₂ dependency
• ATP synthesis

Answer 30