Lecture 10: Oxidadtive Phosphorylation

Question 1

Give a basic overviews of how ATP is generated..

Answer 1

Most ATP is derived from the re-oxidation of the NADH + H+ and FADH2
-this occurs via the electron transport chain
-synthesis of ATP then occurs via the process of oxidative phosphorylation
-the energy released during the oxidation of pyruvate has been trapped in NADH + H+ or FADH2
Re-oxidation:
-the energy trapped in these molecules can be released by oxidation with O2
So NADH + H+ + 1/2 O2 ->(220kj) NAD+ + H2O
What actually happens in the mitochondria?
Electrons are removed from NADH + H+; these electrons are then passed through a series of redox substances then finally to O2.
The ultimate electron accepter is O2
-transfer of electrons from NADH + H+ to O2 occurs via a series of intermediate electron carriers

Question 2

Electron transport chain

List the elements of the transport chain

Answer 2

NADH + H+ -> enzyme FMN -> coenzyme Q -> cytochrome b -> cytochrome c1 -> cytochrome c -> cytochrome (a + a3) -> 1/2 O2
From the first cytochrome to the last cytochrome this atom can exist in either oxidised form (Fe3+) or the reduced form (Fe2+)
-FADH2 enters the chain later than NADH + H+ consequently it produces less ATP than NADH
Refer to the slides on PowerPoint 10 under electron transport chain

Question 3

Where is the electron transport chain located?

Answer 3

The ETC components are arranged in complexes within the inner mitochondrial membrane

Question 4

Explain how the electron transport chain works as a coupled process

Answer 4

There are several intermediate steps in the ETC, at each step there is a yield of free energy. For some steps there is a sufficient energy released for synthesis of ATP from ADP (via oxidative phosphorlyation)
-electrons DO NOT flow down the chain to O2 unless ADP is simultaneously phosphorylated to ATP (coupled process)

Question 5

Respiratory control

How is the rates of oxidative phosphorylation controlled?

Answer 5

Regulation of the rates of ETC and oxidative phosphorylation by ADP levels is known as respiratory control.
-electrons don’t flow from fuel molecules to O2 unless ATP synthesis is needed, meaning that fuel molecules are not catabolised unnecessarily.

Question 6

Uncoupling

Explain what uncoupling is and why it occurs

Answer 6

ETC and oxidative phosphorylation are coupled process which can be uncoupled with certain drugs. In these cases, electron transport occurs without ATP production.
-occurs in new born mammals, brown adipose tissue, also in hibernating mammals.
2,4-dinitrophenol: an artificial substance used in dyes etc.
-in cells, it shuttles protons across membranes
-this acts against the proton gradients across the mitochondrial membrane
-energy is therefore lost as heat instead of ATP

Question 7

Oxidative phosphorylation
Coupled process:
How is electron flow coupled to ATP generation?

Answer 7

It is the chemiosmotic theory.
It is suggested that the flow of electrons down the ETC drives H+ ions across the mitochondrial membrane space.
This creates an electrochemical gradient.
Protons are pumped across the inner mitochondrial membrane as electrons flow through the ETC
-the H+ gradient is a potential source of energyv
-it takes energy to create a gradient (the energy released by the re-oxidation of NADH)
-therefore it should be possible to recapture this energy when the gradient is reversed.
This reaction is catalysed by ATP synthase
ATP + H2O -> ADP + Pi + H+
A build up of H+ will drive the reaction in the direction of ATP synthesis
ATP + H2O <- ADP + Pi + H+

Question 8

Oxidative phosphorylation:

Answer 8

Protons are pumped across the inner mitochondrial membrane as electrons flow through the ETC.
These protons are used to drive the phosphorylation of ADP to ATP by ATP synthase
-transfer of electrons down the ETC to O2 results in transport of H+ from the mitochondrial membrane to the inter-membrane space
-Re-entry of H+ via ATP synthase is accompanied by ATP synthesis (oxidative phosphorylation)

Question 9

What factors about the mitochondria effects the energy production of that cell?

Answer 9

The concentration of the mitochondria in that cell, and the extend of the cristae in those mitochondria is proportional to the energy production of that cell

• a cell with high energy requirements will have a large number of mitochondria with well developed cristae eg muscle cells
• this increases the surface area of the inner mitochondrial membrane on which electron transport and oxidative phosphorylation can occur