oxidative phosphorylation Flashcards
What is oxidative phosphorylation?
Oxidative phosphorylation is the process by which electrons from the reduced cofactors NADH and ubiquinol are funneled in a stepwise manner to oxygen. Electrons flow much like electricity through a circuit, with free energy being conserved through the concomitant formation of a proton gradient. In the end the investment of reduced cofactors results in the production of ATP.
Recall that the reduced electron carriers NADH and ubiquinol are produced during glycolysis and the citric acid cycle, as well as fatty acid oxidation pathways. During the cellular process of respiration, oxidative phosphorylation utilizes the chemical energy of these reduced molecules to produce ATP. In nearly all eukaryotes, the ultimate electron acceptor in a series of oxidation-reduction reactions is oxygen within the mitochondrion.
The protons that have accumulated in the intermembrane space are analogous to water that builds up behind a dam. These protons would spontaneously flow back into the matrix following their concentration gradient if the inner membrane were permeable to them.
A complex channel protein known as ATP synthase allows the controlled flow of protons back into the matrix while, at the same time, harnessing the free energy of proton flow to convert ADP and phosphate into ATP. ATP can be thought of as a form of portable free energy, or an energy currency, that can be used for energy-requiring reactions elsewhere in the cell. This is analogous to water flowing through the turbines of a dam to generate electricity.
This process is known as the chemiosmotic theory. The details of how ATP synthase functions at the molecular level will be explored in the next exercise on ATP synthase.
What is the outer membrane of mitochondrion made of? What is its function?
The outer membrane is porous and allows for the free diffusion of small molecules due to the presence of channel proteins called porins. The topology of the outer membrane resembles that of a bacterial outer membrane, in accordance with the mitochondrion’s origin as a bacterial symbiont.
Inside the mitochondrion, what are the membranes made of? What is its function?
The inner membrane is impermeable to most substances, including ions, and encloses a space referred to as the matrix. The inner membrane is convoluted in structure, thereby providing a large surface area for the protein complexes of the oxidative phosphorylation chain.
How does malate-aspartate shuttle connect to the citric acid cycle in the mitochondrion?
Since the inner mitochondrial membrane is impermeable to most molecules, NADH produced from glycolysis in the cytosol must be imported via the biochemical reactions of the malate-aspartate shuttle. The process is a form of currency exchange between one region of the cell and another. ATP, ADP, and phosphate also require transport proteins for their import and export across the inner membrane.
During the transport chain, what happens in Complex I?
(use image below to help you explain)
Electrons from reduced NAD begin their journey at Complex I. Complex I transfers a pair of electrons from NADH to ubiquinone, with several redox centers acting as intermediate acceptors.
First, two electrons are donated to a flavin mononucleotide group. From here, the electrons are then shuttled one at a time through a series of iron-sulfur sites. Finally, ubiquinone accepts the two electrons, in turn, to become fully reduced. Ubiquinone is a mobile electron carrier that is freely diffusible within the lipid bilayer.
For every pair of electrons transferred, complex I concurrently translocates four protons from the matrix to the intermembrane space. A proton wire is formed as the protons are rapidly relayed through hydrogen-bonded amino acids within Complex I. In this manner, a proton differential, and therefore a pH gradient, begins to form across the inner membrane. The energy released from the dissipation of this proton gradient will eventually be used to form ATP.
what happens during the Complex II stage?
Two other things contribute electrons to the transport chain. The first is succinate dehydrogenase from the citric acid cycle, sometimes referred to as Complex II in the transport chain. The second is the glycerol phosphate shuttle. These are not discussed in your textbook as part of the electron transport chain because they do not directly contribute to the proton gradient.
what happens during Complex III?
Use the image below to help explain the process.
Reduced ubiquinone next shuttles its electrons to the mobile carrier cytochrome c through the action of Complex III. Cytochromes are proteins containing a characteristic heme group.
Members of the cytochrome family contain a characteristic heme group that consists of a porphyrin ring surrounding a central iron atom. The iron cycles between oxidized and reduced states as electrons are passed to the cytochrome.
Some cytochromes are part of large protein complexes, but cytochrome c is a peripheral membrane protein that acts as a mobile electron carrier.
Complex III extracts electrons from ubiquinone in a detailed two-step process known as the Q cycle. One electron is passed to an iron-sulfur protein, to cytochrome c1, and then to cytochromec. The second electron follows a different route, passing through cytochrome b and then back to a quinone to produce a semiquinone. Another ubiquinol donates its two electrons to Complex III. One electron passes through the iron-sulfur protein and cytochrome c1, to cytochrome c, and the other electron passes through cytochrome b to the waiting semiquinone. The net result is that two electrons are passed to cytochrome c and four protons are pumped into the intermembrane space to contribute to the proton gradient.
what happens after complex IV?
The final step in the electron transport chain is the reduction of molecular oxygen by electrons derived from cytochrome c. Cytochrome c ferries its electrons to Complex IV, which contains both heme prosthetic groups and copper ions. Ultimately, four electrons must be transferred through complex IV to result in the full reduction of oxygen to two water molecules. The net result is four more protons are pumped across the inner membrane per molecule of oxygen reduced, or two protons per electron pair.
match the images in the grey boxes to illustrate the process of electron transport chain.
NADH2
Which of the component of electron transfer chain is peripherally associiated with the inner mitochondrial membrane?
Cytochrome is the only component that is perpherally associated ith the membrane.
Which of the components of the electron thransport chain that contain a heme group?
complex IV, complex III, cyt c
Which of the following reactions do not contribute to the formation of a proton gradient across the inner mitochondrial membrane?
b. Succinate + Q → fumarate + QH2
An inhibitor of respiration, known as rotenone, scavenges electrons from Complex I, thereby blocking the reduction of ubiquinone by NADH. Which of the following effects would you observe for an organism exposed to rotenone?
D. Oxygen consumption and ATP synthesis will decrease
