For the sake of simplicity, these points are not described in this tutorial. In the section above, we see that the oxidation-reduction process is a series of electron transfers that occurs spontaneously and produces a proton gradient. Why are the electron tranfers from one electron carrier to the next spontaneous? What causes electrons to be transferred down the electron-transport chain?
As seen in Table 2, below, and Figure 7a, in these carriers, the species being oxidized or reduced is Fe, which is found either in a iron-sulfur Fe-S group or in a heme group. Table 2 shows that the electrons are transferred through the electron-transport chain because of the difference in the reduction potential of the electron carriers. As explained in the green box below, the higher the electrical potential e of a reduction half reaction is, the greater the tendency is for the species to accept an electron.
Hence, in the electron-transport chain, electrons are transferred spontaneously from carriers whose reduction results in a small electrical potential change to carriers whose reduction results in an increasingly larger electrical potential change. An oxidation-reduction reaction consists of an oxidation half reaction and a reduction half reaction. Every half reaction has an electrical potential e.
By convention, all half reactions are written as reductions, and the electrical potential for an oxidation half-reaction is equal in magnitude, but opposite in sign, to the electrical potential for the corresponding reduction i. The electrical potential for an oxidation-reduction reaction is calculated by. For example, for the overall reaction of the oxidation of NADH paired with the reduction of O 2 , the potential can be calculated as shown below.
The electrical potential e rxn is related to the free energy D G by the following equation:. Thus, the higher the electrical potential of a reduction half reaction, the greater the tendency for the species to accept an electron. Just as in the box above, the electrical potential for the overall reaction electron transfer between two electron carriers is the sum of the potentials for the two half reactions.
As long as the potential for the overall reaction is positive the reaction is spontaneous. Hence, from Table 2 below, we see that cytochrome c 1 part of the cytochrome reductase complex, 3 in Figure 9 can spontaneously transfer an electron to cytochrome c 4 in Figure 9. The net reaction is given by Equation 16, below. We can also see from Table 2 that cytochrome c 1 cannot spontaneously transfer an electron to cytochrome b Equation 19 :.
Table 2 lists the reduction potentials for each of the cytochrome proteins i. Note that each electron transfer is to a cytochrome with a higher reduction potential than the previous cytochrome. As described in the box above and seen in Equations , an increase in potential leads to a decrease in D G Equation 13 , and thus the transfer of electrons through the chain is spontaneous. To view the cytochrome molecules interactively using RASMOL, please click on the name of the complex to download the pdb file.
As we shall see below, this huge concentration gradient leads to the production of ATP. We have seen that the electron-transport chain generates a large proton gradient across the inner mitochondrial membrane. But recall that the ultimate goal of oxidative phosphorylation is to generate ATP to supply readily-available free energy for the body. How does this occur? In addition to the electron-carrier proteins embedded in the inner mitochondrial membrane, a special protein called ATP synthetase Figure 9, the red-colored protein is also embedded in this membrane.
ATP synthetase uses the proton gradient created by the electron-transport chain to drive the phosphorylation reaction that generates ATP Figure 7c. ATP synthetase is a protein consisting of two important segments: a transmembrane proton channel, and a catalytic component located inside the matrix.
Recall from the Kidney Dialysis tutorial that particles spontaneously diffuse from areas of high concentration to areas of low concentration. Thus, since the diffusion of protons through the channel component of ATP synthetase is spontaneous, this process is accompanied by a negative change in free energy i.
Then, using the free energy released by the spontaneous diffusion of protons through the channel segment, a bond is formed between the ADP and a free phosphate group, creating an ATP molecule. A scientist has created a phospholipid-bilayer membrane containing ATP-synthetase proteins. Briefly, explain your answer. What effect do you expect these toxins to have on the production of ATP?
Summary In this tutorial, we have learned that the ability of the body to perform daily activities is dependent on thermodynamic, equilibrium, and electrochemical concepts. These activities, which are typically based on nonspontaneous chemical reactions, are performed by using free-energy currency. The common free-energy currency is ATP, which is a molecule that easily dephosphorylates loses a phosphate group and releases a large amount of free energy.
As the coupled reactions occur i. As seen in Figure 4, the breakdown of glucose glycolysis obtained from the food we eat cannot by itself generate the large amount of ATP that is needed for metabolic energy by the body.
These redox molecules are used in an oxidative-phosphorylation process to produce the majority of the ATP that the body uses. Oxidative phosphorylation occurs in the mitochondria, and the two reactions oxidation of NADH or FADH 2 and phosphorylation to generate ATP are coupled by a proton gradient across the inner membrane of the mitochondria Figure 9.
As seen in Figures 7 and 9, the oxidation of NADH occurs by electron transport through a series of protein complexes located in the inner membrane of the mitochondria. This electron transport is very spontaneous and creates the proton gradient that is necessary to then drive the phosphorylation reaction that generates the ATP.
Hence, oxidative-phosphorylation demonstrates that free energy can be easily transferred by proton gradients. Oxidative-phosphorylation is the primary means of generating free-energy currency for aerobic organisms, and as such is one of the most important subjects in the study of bioenergetics the study of energy and its chemical changes in the biological world.
Alberts, B. In Molecular Biology of the Cell, 3rd ed. Becker, W. In The World of the Cell, 2nd ed. Fasman, G. 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 mitochondrial membrane. The NADH generated from glycolysis cannot easily enter mitochondria. Another factor that affects the yield of ATP molecules generated from glucose is 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. Other molecules that would otherwise be used to harvest energy in glycolysis or the citric acid cycle may be removed to form nucleic acids, amino acids, lipids, or other compounds.
Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose. What happens when the critical reactions of cellular respiration do not proceed correctly? Mitochondrial diseases are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells.
Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases.
Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics.
Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial disease, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disease.
The citric acid cycle is a series of chemical reactions that removes high-energy electrons and uses them in the electron transport chain to generate ATP. One molecule of ATP or an equivalent is produced per each turn of the cycle. In the fourth protein complex, the electrons are accepted by oxygen, the terminal acceptor. The oxygen with its extra electrons then combines with two hydrogen ions, further enhancing the electrochemical gradient, to form water.
If there were no oxygen present in the mitochondrion, the electrons could not be removed from the system, and the entire electron transport chain would back up and stop. The mitochondria would be unable to generate new ATP in this way, and the cell would ultimately die from lack of energy.
This is the reason we must breathe to draw in new oxygen. This is the only place where oxygen is required during the processes of aerobic respiration. In the electron transport chain, the free energy from the series of reactions just described is used to pump hydrogen ions across the membrane. Hydrogen ions diffuse from the intermembrane space through the inner membrane into the mitochondrial matrix through an integral membrane protein called ATP synthase Figure 2.
This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient from the intermembrane space, where there are many mutually repelling hydrogen ions to the matrix, where there are few. This flow of hydrogen ions across the membrane through ATP synthase is called chemiosmosis. Chemiosmosis Figure 2 is used to generate 90 percent of the ATP made during aerobic glucose catabolism.
The result of the reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the electron transport system, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen ions attract hydrogen ions protons from the surrounding medium, and water is formed.
Cytochrome proteins have a prosthetic heme group. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, which makes each complex.
Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes.
Cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time. The fourth complex is composed of cytochrome proteins c, a, and a 3. This complex contains two heme groups one in each of the cytochromes a and a 3 and three copper ions a pair of Cu A and one Cu B in cytochrome a 3. The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced.
The reduced oxygen then picks up two hydrogen ions from the surrounding medium to produce water H 2 O. The removal of the hydrogen ions from the system also contributes to the ion gradient used in the process of chemiosmosis.
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. Describe how the energy obtained from the electron transport chain powers chemiosmosis and discuss the role of hydrogen ions in the synthesis of ATP. Chemiosmosis : In oxidative phosphorylation, the hydrogen ion gradient formed by the electron transport chain is used by ATP synthase to form ATP.
If the membrane were open to diffusion by the hydrogen ions, the ions would tend to spontaneously diffuse back across into the matrix, driven by their electrochemical gradient. However, many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through a membrane protein called ATP synthase.
This protein acts as a tiny generator turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of this molecular machine harnesses the potential energy stored in the hydrogen ion gradient to add a phosphate to ADP, forming ATP. Chemiosmosis is used to generate 90 percent of the ATP made during aerobic glucose catabolism. The production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation.
It is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule.
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