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. Electron Carriers 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.
Proteins with these molecules are called flavoproteins. Cytochromes — use iron atoms as part of a heme group to carry 1 electron at a time. Iron-sulfur Fe-S proteins , such as ferredoxin — use iron atoms not part of heme group to carry 1 electron at a time. Electron Transport Chain 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.
Electron Transport Chain. Study Questions How are metabolism, catabolism, and anabolism defined? What are the 3 major types of work carried out by cells? What is free energy? Under the terms of the licence agreement, an individual user may print out a PDF of a single entry from a reference work in OR for personal use for details see Privacy Policy and Legal Notice.
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Several of these transitions are shown, including the oxidation succinate to fumarate which is mechanistically coupled to the reduction of ubiquinone to ubiquinol in the inner mitochrondrial membranes. Each of these intermediate electron transfers must be thermodynamically favorable on its own in order for respiration to proceed. Yet they are favorable enough to pump a proton across the cell or mitochondrial membrane. This is the energetic basis for chemiosmosis: cells store quanta of energy too small for ATP synthesis in the proton gradient across a membrane.
Though ATP is often claimed to be the energy currency of the cell, in fact, for the energetic balance of the cell the carriers of reducing power are themselves no less important.
We can use the redox potential to connect these two molecular protagonists, and estimate an upper bound on the number of ATP molecules that can be produced from the oxidation of NADH produced, for example, in the TCA cycle.
For the reaction considered above of NADH oxidation by oxygen, the maximal associated free energy that can be extracted is thus. In the cell, oxidation of NADH proceeds through several steps in respiration and results in the transfer of 10 protons across the membrane against the electro-chemical potential BNID These proton transfers correspond to yet another way of capturing biochemical energy.
Why should one discuss redox potentials of half reactions and not free energies of full reactions? The units themselves owe their origins to the ability in the field of electrochemistry to measure in the lab the voltage difference, i.
The usefulness of redox potentials for half reactions lies in the ability to assemble combinations of different donors and acceptors to assess the thermodynamic feasibility and energy gain of every considered reaction. Just as we speak of the pH of a solution, at first guess, we might imagine that it would be possible to speak of an apparently analogous redox potential of the cell. Knowing the concentration of the reduced and oxidized forms of a given reaction pair defines their pool redox potential via the relation.
As can be seen the first is relatively oxidized and the second relatively reduced with a ratio among them usually much larger than 1. In the mitochondrial matrix a ratio of fold more of the oxidized form is reported BNID as shown in Table 1. A cell is not at equilibrium and there is weak coupling between different redox pairs.
A The relevant half-reactions and potentials from Table P2 are as follows:. Thus the equilibrium lies far to the right, favoring a discharged battery as anyone who has ever tried unsuccessfully to start a car after letting it sit for a long time will know. The reaction is as follows:. Unfortunately, these criteria apply only to systems in which all reactants and products are present in their standard states, a situation that is seldom encountered in the real world. A coulomb C relates electrical potential, expressed in volts, and energy, expressed in joules.
The product of the cell potential and the total charge is the maximum amount of energy available to do work, which is related to the change in free energy that occurs during the chemical process.
Learning Objectives To understand the relationship between cell potential and the equilibrium constant. To use cell potentials to calculate solution concentrations.
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