Lecture 29: The Electron Transport Chain

Question 1

Oxidative phosphorylation refers to the coupled process of what two pathways?

Answer 1

electron transport through the electron transport chain
(ETC)

AND

the phosphorylation of ADP to ATP by ATP-synthase

Question 2

What couples the two pathways in oxidative phosphorylation?

Answer 2

They are coupled by a proton gradient

Question 3

Where in the mitochondria does the ETC take place?

Answer 3

The ETC is in the inner membrane of mitochondria

Question 4

What is oxygen in the ETC?

Answer 4

(oxygen is the terminal electron acceptor)

Last in line of the ETC and drives the process fowards

Question 5

Describe an experiment that shows the electron transport chain is in the inner mitochondrial membrane.

Answer 5

Isolate mitochondria from cells:

Treat with strong detergent:
Solubilize all membranes
ETC does not work
→ ETC in a membrane

Treat with mild detergent:
Only removes outer membrane ETC still works
→ ETC is in the inner mitochondrial membrane

Question 6

What is the role of electron carriers in the electron transport chain?

Answer 6

Movement of e through the ETC involves carriers undergoing a series of redox reactions

Each carrier accepts electron(s) (is reduced) in one redox reaction and then donates electron(s) (is oxidized) in another redox reaction

As electrons move to carriers with a higher reduction potential (oxygen has the highest reduction potential) energy is released (the ΔG0’ is negative)

The energy released in the ETC is used to translocate protons across the mitochondrial inner membrane

Question 7

The ETC oxidizes reduced coenzymes. Where do these reduced coenzymes come from?

Answer 7

Glycolysis:
2 NADH (per glucose)

Pyruvate Oxidation:
1 NADH per pyruvate, 2 NADH (per glucose)

β-Oxidation:
1 NADH and 1 FADH₂ (per cycle of β-oxidation)

Citric Acid Cycle:
3 NADH and 1 FADH₂ (per acetyl-CoA)

Question 8

The energy released by the movements of electrons up their reduction potential through the ETC causes what to be translocated across the mitochondrial inner membrane? Include the number for each complex

Answer 8

PROTONS baby

Complex I:
4 protons (H⁺) are pumped across the membrane per pair of electrons from NADH.

Complex II:
0 protons are pumped directly at this complex. Instead, electrons from FADH₂ enter the chain, bypassing
Complex I.

Complex III:
4 protons (H⁺) are pumped across the membrane per pair of electrons that pass through this complex.

Complex IV:
2 protons (H⁺) are pumped across the membrane per pair of electrons that pass through this complex.

Question 9

If the electron transport chain is inhibited, what are the
consequences?

Answer 9

• No proton gradient formed (ATP not made)
• Build-up of reduced co-enzymes (NADH and FADH2 )
  so no oxidizing power for other pathways

Question 10

At which complex of the ETC is NADH oxidized? How many electrons does this release into the ETC and how many protons are pumped across the membrane at this complex?

Answer 10

• NADH is oxidized at Complex I
• Two e released into the ETC
• Four protons are pumped for each NADH oxidized

Question 11

At which complex of the ETC is FADH2 from the citric acid cycle oxidized? How many electrons does this release into the ETC and how many protons are pumped across the membrane at this complex?

Answer 11

• FADH2 is oxidized at Complex II
• SDH reaction is shared with the citric acid cycle
• Two electrons released into the ETC
• No protons are pumped

Question 12

What mobile electron carrier do Complex I and II pass electrons to?

Answer 12

• Complex I and Complex II both pass two electrons to UQ/ CoQ
• UQ can move within the inner mitochondrial membrane
• A coenzyme (organic but not from a vitamin)
• Required by last carrier in Complex 1 and first carrier in Complex 3
• Exists in two forms
• Carries 2 hydrogen atoms (reducing equivalents)
• Co-Q undergoes two-electron redox reactions (like NADH and FADH2), but can accept or release one electron at a time

Question 13

Where does UQ/CoQ deliver the electrons?

Answer 13

UQ/CoQ accepts electrons from both Complex I (from NADH) and Complex II (from FADH₂).

After accepting the electrons, UQ/CoQ becomes ubiquinol (CoQH₂), the reduced form.

UQ/CoQ then shuttles these electrons to Complex III, where they continue down the chain,

Question 14

What electron carrier does Complex III pass electrons to?

Answer 14

• Complex III releases one electron at a time to Cytochrome c

Question 15

This passing of electrons at Complex III causes how many protons to be pumped across the inner membrane?

Answer 15

Complex III pumps four protons across the inner membrane - (for one coenzyme/ two electrons)

Question 16

What reaction takes place at Complex IV?

Answer 16

• Complex IV accepts one electron at a time from Cytochrome c
• Reduces oxygen to water (terminal electron acceptor)
• For 1 NADH/ FADH2 (2 electrons): 2 H+ pumped
• For 1 NADH/ FADH2 (2 electrons): ½O2 + 2H+ → H2O

Biologically the last carrier in Complex IV waits until it has four electrons (oxidation of two coenzymes): O2 + 4H+ → 2H2O

Question 17

How does cytochrome C deliver ‘how many?’ electrons at a time to Complex IV?

Answer 17

• Moves on outer surface of the inner mitochondrial membrane
• Cytochrome c carries one electron at a time from Complex III to Complex IV
• Cytochrome c carries one electron via reversible Fe2+/Fe3+ redox reactions

Question 18

For each NADH oxidized in the ETC, how many protons are pumped?

Answer 18

NADH: 4 (CI) + 4 (CIII) + 2 (CIV) = 10 protons pumped

Question 19

For each FADH2 oxidized in the ETC, how many protons are pumped?

Answer 19

FADH2: 4 (CIII) + 2 (CIV) = 6 protons pumped