The electron transport chain

Question 1

What is oxidative phosphorylation?

Answer 1

Oxidative phosphorylation is the coupled process of the electron transport through the electron transport chain and the phosphorylation of ADP to ATP by ATP-synthase
They are coupled by a proton gradient - the ETC makes the proton gradient and ATP-synthase uses the proton gradient

Question 2

What is the electron transport chain?

Answer 2

The ETC is the transport of electrons, electrons are passed through series of carriers (one to the other). Electrons come from reduced coenzymes (NADH and FADH2) and are fed into the ETC. These electrons will ultimately reduce molecular oxygen (terminal electron acceptor) to water. Protons are pumped as the electrons are transported through the ETC, builds a proton gradient

Question 3

Where does the ETC occur?

Answer 3

The ETC occurs in the mitochondria (requires oxygen). A proton gradient requires a barrier (to stop proton diffusion), membranes are a barrier to protons (protons cannot diffuse through membranes), use inner mitochondria membrane meaning ETC has access to matrix and reduced coenzymes

Question 4

What are the four complexes in the ETC?

Answer 4

There are four complexes, each complex has a number of carrier, electrons move along carriers in complex and between complexes via mobile carriers, uniquinone/CoQ (found within inner membrane) and cytochrome c (sits on outer surface)

Question 5

How are electrons moved through the ETC?

Answer 5

Each carrier accepts electrons (reduces them) in one redox reaction and then donates electrons (oxidises them) in another redox reaction
When NADH reacts with first carrier, NADH is oxidised to NAD+
Next carrier passes electron to another via a redox reaction, the carriers are reduced when the electron is passed along

Question 6

Relationship between electrons moving and energy released

Answer 6

As electrons move to carriers with a higher reduction potential (move from lower to higher), oxygen has the highest reduction potential, energy is released when each electron moves from a lower reduction potential to a higher reduction potential hence G∆˚’ is negative

Question 7

What is the energy released in ETC used for?

Answer 7

The energy from redox reactions released is used to move protons against their concentration gradient through the complexes and across the mitochondrial inner membrane

Question 8

What is NADH electrons flow through ETC?

Answer 8

NADH electrons are passed into complex I (NAD+ goes back to pathways) from complex I they go to UQ, then passed to complex III, travel through to Cyt c and then to complex IV and pass electrons to O2 to reduce it to water

Question 9

What is FADH2 electrons flow through ETC?

Answer 9

FADH2 electrons are fed into complex II, go to UQ, then passed to complex III, travel through to Cyt c, move to complex IV and pass electrons to O2 to reduce it to water

Question 10

Examples of inhibitors of ETC

Answer 10

Rotenone stops electron transfer from Complex I to Co-Q
Cyanide stops electrons from being passed to a carrier in complex IV
Carbon monoxide binds where O2 binds, at the end of complex IV, hence electrons get stuck
These poisons stop flow of electrons through ETC, this means no proton gradient is formed hence ATP is not made. Build up of reduced coenzymes means no oxidising power for other pathways

Question 11

What happens in complex I of the ETC?

Answer 11

NADH is oxidised to NAD+ at complex I hence two electrons are released into the ETC, two electrons are passed along as a pair
Enough energy is released for four protons to be pumped for each NADH oxidised

Question 12

What happens in complex II of the ETC?

Answer 12

FADH2 is oxidised at complex II to FAD hence two electrons are released into ETC
SDH reaction is shared with the citric acid cycle
No protons are pumped at complex II

Question 13

What is UQ/CoQ?

Answer 13

UQ can move around in the inner membrane until it gets to complex III
CoQ is not from a vitamin. It exists in two forms, in its reduced form it carries two electrons, it its oxidised form it doesn’t
It can also have an intermediate state where it only carries one electron, hence it can accept or release one electron at a time

Question 14

Why is intermediate state of CoQ important?

Answer 14

Complex I and II both pass two electrons to CoQ, CoQ releases one electron at a time to Complex III (Q-cycle) hence why the intermediate state of CoQ is important

Question 15

What happens in complex III of the ETC?

Answer 15

Complex III releases one electron at a time to Cyt c
Complex III pumps four protons across the inner membrane for each two electrons that goes through it

Question 16

What is cytochrome c?

Answer 16

Cyt c moves on outer surface of the inner mitochondrial membrane
It picks up one electron at a time from complex III and shuffles it along to complex IV
Cyt c carries one electron via Fe2+/Fe3+ redox reactions as it is a haem containing proton hence able to pick up electrons

Question 17

What happens in complex IV of the ETC?

Answer 17

Complex IV accepts one electron at a time from Cyt c
Electrons move through carriers in complex IV and redox reactions release enough energy for two protons to go through their concentration gradient
Oxygen accepts electrons at end of complex IV (terminal acceptor) and is reduced to water
Biologically complex IV waits for four electrons to reduce O2 into two H2O rather than doing it with two electrons and using 1/2O2 to make one H2O

Question 18

What is energy accounting?

Answer 18

For each NADH that enters ETC there are 10 protons pumped from matrix into intermembrane space (four from complex I, four from complex III and two from complex IV)
For each FADH2 that enters ETC there are 6 protons pumped from matrix into intermembrane space (four from complex III and two from complex IV)