Electron Transport Chain Flashcards
i) The ETC and OP both occur within the mitochondria. However, each process occurs within specific regions of the mitochondria.
Describe the location where the ETC occurs in the mitochondria by referring to one or more of the mitochondrial matrix, inner membrane, intermembrane space and outer membrane.
Complexes I to IV that make up the electron transport chain (enzymes) are embedded within the inner membrane of the mitochondria. Hence, the electron transport chain takes place in the inner membrane of the mitochondria where the complexes are located.
Describe the location where OP occurs in the mitochondria by referring to one or more of the mitochondrial matrix, inner membrane, intermembrane space and outer membrane.
In some way the matrix, inner membrane and intermembrane space are all involved in OP. H+ are pumped from the matrix into the intermembrane space through complexes I, II and IV of the ETC (located in the inner membrane). Once enough of the H+ have built up in the intermembrane space (between the inner and outer mitochondrial membrane) the H+ move back into the matrix via ATP synthase (generating ATP), which is located in the inner membrane.
What is the starting input (reactant) for ETC and what is the final output (end products)? Why is the final output important?
The starting input for the ETC are the electron rich coenzymes NADH and FADH2 which off-load their electrons (via a redox reaction) to begin the ETC.
Oxygen is also required by the ETC as the final electron acceptor (accepts electrons from complex IV). When oxygen accepts electrons from complex IV it becomes H2O (oxygen also accepts some H+ ), which is the output of the ETC. It is important that oxygen is available as the final electron acceptor (to become H2O) as it allows more electrons to be passed along the chain of complexes in the ETC. If oxygen is not available for use as the final electron acceptor, the ETC stops, resulting in OP also stopping, which means the generation of ATP ceases. The cells begin to die when ATP can not be made.
Where does the input required for the ETC come from?
The electron rich coenzymes come from a variety of metabolic reactions (glycolysis, conversion of pyruvate into acetyl CoA) but the main source of NADH and FADH2 is the citric acid cycle (CAC). In each CAC, 3 NADH and 1 FADH2 are generated before being taken to the ETC.
What makes the ETC coupled to oxidative phosphorylation (OP)? Hint: refer to the movement of H+ into the intermembrane space and what the H+ is used for thereafter.
The ETC is coupled to OP because OP can not occur without the ETC. As the ETC passes electrons along the chain, the protein complexes of the ETC allow H+ to move from the matrix into the intermembrane space. OP relies on the H+ in the intermembrane space to move back into the matrix through the ATP synthase protein.
As H+ moves through ATP synthase the energy generated is used to attach a phosphate unit to ADP which regenerates ATP. The majority of the cells ATP is generated through OP.
What is the final output OP? Why is this output important?
The final output of OP is ATP. The more H+ that move through ATP synthase during OP, the more ATP is made. ATP is used to power the cellular processes that keep the cell alive.
Important reactions of the electron transport chain (ETC) and oxidative phosphorylation (OP):
Electrons are removed from the NADH and FADH2 coenzymes by complexes I and II of the ETC, respectively. The electrons from the electron rich coenzymes are passed along the chain of complexes within the ETC. How are electrons stripped from the NADH coenzyme by complex I?
The electrons are stripped off NADH via a redox reaction. One of the components of complex I (FMN) (reduced) accepts the electrons from NADH, causing NADH to be oxidised to the electron poor NAD+ . The electrons are then passed along the ETC.
NAD+ is taken back to the CAC to be used in the redox reaction steps.
Electrons are passed down the electron transport chain from complexes I/II to complex III and then onto complex IV. Describe the component of the ETC that allows the electrons to be passed between the complexes of the ETC?
Mobile electron carriers (CoQ and cytochrome C) are responsible for carrying electrons between the static complexes within the ETC.
CoQ carries electrons from complexes I and II to complex III, whereas Cyt C carries electrons from complex III to complex IV.
Oxygen is the final electron acceptor of the ETC, where oxygen accepts electrons from complex IV itself becoming H2O. Why is it important that oxygen is present to function as the final electron acceptor of the ETC?
In order for more electrons to move through the ETC, the previous electrons must be taken off the ETC. In the final step of the ETC, oxygen accepts electrons from complex IV. After oxygen has accepted the electrons, more electrons can move through the ETC.
Without oxygen, the ETC would stop as the electrons would be stuck in the chain, preventing more electrons from moving through the chain. This would also stop H+ from moving into the intermembrane space, so OP and ATP production would also stop. Once ATP production stops the cell begins to die.
As electrons are passed along the complexes within the ETC, the complexes I, III and IV open their H+ channels. What happens once these H+ channels are open? How does this process link the ETC to OP?
The open H+ channels of the ETC complexes allows H+ to move from the matrix into the intermembrane space. As more electrons move through the ETC, more H+ are pumped into the intermembrane space (between the inner and outer membrane). When the concentration of H+ is high in the intermembrane space, OP begins where the H+ move through ATP synthase back into the matrix.
Once there is a high concentration of H+ in the intermembrane space, the H+ move through ATP synthase back into the matrix. How does the movement of H+ through ATP synthase regenerate ATP?
Once H+ start to move through ATP synthase (from the intermembrane space into the matrix) an H+ gradient is generated. The H+ gradient (movement) generates energy, which is harnessed by the ATP synthase enzyme to regenerate ATP.
A phosphate unit is attached to an ADP in the presence of energy.
ADP + Pi + energy (H+ movement) → ATP
The more H+ that move through ATP synthase during OP, the more ATP is made.
