The reduced substance with the greatest tendency to donate electrons would be found at the top of the tower on the right, while the oxidized substance with the greatest tendency to accept electrons would be found at the bottom of the tower on the left. Redox couples in the middle can serve as either electron donors or acceptors, depending upon what substance they partner with for a reaction.
Larger values are derived when there is the biggest distance between the donor and the acceptor or a bigger fall down the tower. The actual formula is:. The transference of electrons from donor to acceptor does not occur directly, since chemically dissimilar electron donors and acceptors might never interact with one another.
Instead, many cellular intermediates participate in the process, with the possibility for energy capture occurring along the way. These intermediates are called electron carriers and they go back and forth between a reduced form when they are carrying an electron and an oxidized form after they have passed the electron on , without being consumed in the reaction themselves.
In order for the reaction to be energetically favorable for the cell, the carriers must be arranged in order of their standard reduction potential i. It is important to note that some carriers accept both electrons and protons, while other carriers accept electrons only. This fact will become of crucial importance later, in the discussion of how energy is generated.
While there are many different electron carriers, some unique to specific organisms or groups of organisms, let us cover some of the more common ones:. The process starts with an initial electron donor, a substance from outside of the cell, and ends with a final electron acceptor, another substance from outside of the cell. In the middle the electrons are passed from carrier to carrier, as the electrons work their way down the electron tower. In order to make the process more efficient, most of the electron carriers are embedded within a membrane of the cell, in the order that they are arranged on a redox tower.
These electron transport chains are found within the cell membrane of bacteria and archaea, and within the mitochondrial membrane of eukaryotes.
Energetics While some energy is lost as heat in chemical reactions, the measurement of interest for cells is the amount of free energy G , or the energy available to do work. Adenosine triphosphate ATP Adenosine triphosphate or ATP is a high-energy molecule used by all cells for energy currency, partly because it readily donates a phosphoryl group to other molecules. Enzymes In order for a chemical reaction to proceed, chemical bonds must be broken.
Redox Reactions Cells conserve energy in the form of ATP by coupling its synthesis to the release of energy via oxidation-reduction redox reactions , where electrons are passed from an electron donor to an electron acceptor.
Conjugate Redox Pair Electrons do not exist freely in solution, they must be coupled with atoms or molecules. Electron Tower. The extra electrons on the oxygen attract hydrogen ions protons from the surrounding medium, and water is formed. Figure 3. Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease?
What effect would cyanide have on ATP synthesis? The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species.
Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. The NADH generated from glycolysis cannot easily enter mitochondria. Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far.
For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis.
Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways. Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose.
The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them.
The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the chain by either NADH or FADH 2 complete the chain, as low-energy electrons reduce oxygen molecules and form water.
The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways. Improve this page Learn More. Skip to main content.
Module 6: Metabolic Pathways. Search for:. Electron Transport Chain Learning Outcomes Describe the respiratory chain electron transport chain and its role in cellular respiration. Practice Question Figure 2. DNP is an effective diet drug because it uncouples ATP synthesis; in other words, after taking it, a person obtains less energy out of the food he or she eats.
Possible Answers: It serves as the terminal electron acceptor. It is essential for transporting pyruvate into the mitochondria. Correct answer: It serves as the terminal electron acceptor. Explanation : Oxygen serves as the terminal electron acceptor for the electron transport chain. Example Question 85 : Cellular Respiration. Which of the following processes requires an electron acceptor?
Possible Answers: The electron transport chain requires a nitrogen electron acceptor. The citric acid cycle requires an oxygen electron acceptor. The electron transport chain requires an oxygen electron acceptor.
Glycolysis requires a nitrogen electron acceptor. Correct answer: The electron transport chain requires an oxygen electron acceptor.
Explanation : Oxygen acts as the terminal electron acceptor in the electron transport chain ETC. Example Question 86 : Cellular Respiration. Possible Answers: Oxidative inhibition.
Removing the F0 subunit from the ATP synthase complex. Correct answer: Dispersion of the proton gradient. Explanation : In ATP synthesis, the proton gradient is an interconvertible form of energy in electron transport. The other answer choices are not directly related to the generation of the proton gradient.
Possible Answers: It is converted to starch. It is used to make water from hydrogen ions and oxygen. Correct answer: It is converted to heat. Example Question 88 : Cellular Respiration. Along what structure do electrons in the electron transport chain ETC move? Possible Answers: The mitochondrial matrix. Correct answer: The inner membrane of the mitochondria. Explanation : The events of the electron transport chain take place on the inner membrane of the mitochondria.
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