Lecture 5 (W3) - Electron Transport Chain (ETC)

Question 1

What are the 5 main parts / structures of the mitochondria?

Answer 1

1. Outer membrane
2. Inner membrane
3. Cristae – the folds inside the inner membrane
4. Inter membeane space – between the inner and outer membrane
5. Mitcohondrial matrix – space inside the inner membrane

Question 2

What is the main composition of the inner membrane of the mitochondria?

Answer 2

75% protein complexes ; 25% lipids

Question 3

Why is the mitochondria inner membrane non-permeable to large molecules, and only permeable to small molecules such as CO₂, H₂O, O₂? [2]

Answer 3

1. The inner membrane is embedded with large protein complexes, thus there is no space for other molecules to diffuse freely.
2. If the membrane were freely permeable to ions and protons, the proton gradient would collapse, preventing ATP synthesis by ATP synthase.

Question 4

The electron transport chain has a series of redox reactions. What are the overall reactions involved in the ETC? Write their equations.

Answer 4

Oxidation : NADH → NAD⁺ + H⁺ + 2e^-

Oxidation : FADH₂ → FAD⁺ + 2H⁺ + 2e^-

Reduction : 1/2 O₂ + 2e^- + 2H⁺ → H₂O

Question 5

What is the electron transport chain?

Answer 5

• The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria.
• Electrons are passed from one member of the transport chain to another in a series of redox reactions.
• Energy released in these reactions is captured as a proton gradient.

Question 6

What is the goal of the ETC?

Answer 6

To generate a proton gradient from energy stored in electron acceptors, to drive ATP synthesis by ATP synthase

Question 7

What are the 2 overall functions of the ETC?

Answer 7

1. To generate a proton gradient, with high [H⁺] in the inner mitochondria membrane and low [H⁺] in the matrix
2. To regenerate electron carriers NAD⁺ and FAD to be reused in glycolysis / TCA cycle

Question 8

What are the main protein complexes and organic molecules involved in the ETC?

Answer 8

**Protein complexes (4) **
1. Complex I
2. Complex II
3. Complex III
4. Complex IV

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Organic molecules
1. Ubiquinone (Coenzyme Q₁₀)
2. Cytochrome C

Question 9

In order to regulate the ETC, it involves the regulation of which 2 aspects?

Answer 9

1. Regulation of the rate at which ATP is generated
2. Regulation of the speed at which electrons flow through the ETC

Question 10

What is the function of complex I in the ETC?

Answer 10

To accept 2e^- from NADH, transfer them to Coenzyme Q (ubiquinone) and pump H⁺ to the inner mitochondrial membrane.

Question 11

What is the function of complex II in the ETC?

Answer 11

It accepts 2e^- from FADH₂ (which comes from TCA cycle and regenerating FAD. The elctrons are then passed onto coenzyme q (ubiquinone)
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From TCA cycle :
Succinate → fumarate
FAD → FADH₂ + 2e^-

Question 12

Protons are pumped into the intermembrane space in complex II. True or False?

Answer 12

False.The energy drop generated from Complex II to coenzyme Q is not significant enough to drive protons across and into the intermembrane space.

Question 13

What is the function of coenzyme Q, ubiquinone in the ETC?

Answer 13

To receive the electrons from Complex I and Complex II, and transfer them to Complex III.

Question 14

What is the function of complex III in the ETC?

Answer 14

To accept electrons from coenzyme Q and transferring them to Cytochrome C. As electrons are transferred to cytochrome c, the energy released is used to pump protons from the mitochondrial matrix into the inter-membrane space

Question 15

What is the function of cytochrome c in the ETC?

Answer 15

accepts electrons from complex III and transfers them to complex IV.

• in the process of accepting electrons, it gets reduced. The reduced form of cytochrome c is called ferrocytochrome

Question 16

What is the function of complex IV in the ETC?

Answer 16

It is the terminal complex of the ETC. It accepts electrons from ferrocytochrome C and transfers them to oxygen, reducing oxygen into water. The transfer of electrons releases energy and protons are pumped into the intermembrane space.

Question 17

What is standard reduction potential?

Answer 17

The tendency of a chemical species to get reduced (i.e. gain electrons)

Question 18

In the electron transport chain, electrons flow from species with high standard reduction potential to species with low standard reduction potential. True or False?

Answer 18

False.
Electrons flow from species of low standard reduction potential (less likely to be reduced) to speciies of high standard reduction potential (more likely to be reduced).

Question 20

What happens to oxygen at the end of the ETC?

Answer 20

Oxygen acts as the final electron acceptor and gets reduced into water.

Question 22

What is the purpose of having so many complexes in the ETC? Why can’t electrons be directly transferred to oxygen?

Answer 22

• It allows for energy to be released in a stepwise, controlled manner. If electrons were directly transferred to oxygen, this energy would be released all at once as heat, and no ATP will be efficiently produced.
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• Furthermore, oxygen is a strong electron acceptor, and releasing all electrons at once could lead to the formation of reactive oxygen species (ROS), which can damage cellular components (DNA, protein).

Question 23

What are the 2 regulation mechanisms in regulating the ETC?

Answer 23

1. Prosthetic groups (facilitate electron transfer within the ETC)
2. Protein complexes

Question 24

Which 4 prosthetic groups are involved in the regulation of the ETC?

Answer 24

1. Flavin Mononucleotide (FMN) and coenzyme Q
2. Heme
3. Iron-sulfur clusters
4. Copper

Question 25

In the ETC, where are prosthetic groups found?

Answer 25

They are found within protein complexes I,II,III,IV, which are enzymes (oxidoreductases) which catalyse the transfer of electrons to O₂.

Question 26

The heme proesthetic group is found in which molecule in the ETC?

Answer 26

Cytochrome C

Question 27

How do prosthetic groups regulate the ETC?

Answer 27

Prosthetic groups regulate the speed at which electrons flow through the ETC by facilitating electron transfer.

Question 28

In the ETC, the protein part of protein complexes (complex I,II,III,IV) are responsible for electron transfer. True or False?

Answer 28

False, it is the prosthetic groups (mostly metal ions w valence), that are responsible for accepting / transferring electrons.

Question 29

How do protein complexes regulate the ETC?

Answer 29

Each protein complex (e.g. Complex I), is a large complex with several redox centres binded to several different cofactors. Thus, this allows for a more controlled flow of electrons through the ETC (regulation of speed of electron flow).

Question 30

Protein complexes can regulate the ETC by assembly / disassembly (to form / break free from supercomplexes). How does this regulate the ETC?

Answer 30

Under stress or metabolic changes, ETC complexes dynamically assemble or disassemble to adjust electron flow rate and ATP production.

• Supercomplex formation increases electron transfer efficiency, minimizes ROS formation, and optimizes proton pumping.
• Disassembly slows down respiration under low oxygen (hypoxia), oxidative stress, or reduced ATP demand, preventing energy waste and excess ROS.