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 through the inner membrane through an integral membrane protein called ATP synthase Figure 4. 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 4. 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.
The electron transport chain and the production of ATP through chemiosmosis are collectively called oxidative phosphorylation. 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.
The ATP synthases in mitochondria, chloroplasts, and Bacteria are all structurally similar, and their amino acid sequence similarities are consistent with a common evolutionary origin Watt et al. Lesser degrees of similarity, and more distant evolutionary relationships, exist with Archaeal ATP synthases and with vacuolar membrane ATPases.
Indeed, Bacterial and mitochondrial ATP synthases can work in reverse to hydrolyze ATP and pump protons across the membrane to increase the membrane proton gradient see end of video above. What creates the proton gradient across the membrane? Chemiosmosis — this is really important! We have seen how ATP synthase acts like a proton-powered turbine, and uses the energy released from the down-gradient flow of protons to synthesize ATP.
The process of pumping protons across the membrane to generate the proton gradient is called chemiosmosis. Chemiosmosis is driven by the flow of electrons down the electron transport chain , a series of protein complexes in the membrane that forms an electron bucket brigade. Each of these protein complexes accepts and passes on electrons down the chain, and pumps a proton across the membrane for each electron it passes on. Ultimately, the last complex in the electron transport chain passes the electrons to molecular oxygen O2 to make water, in the case of aerobic respiration.
We define respiration as the passage of electrons down the electron transport chain. We breathe respire oxygen because oxygen is the terminal electron acceptor , the end of the line for our mitochondrial electron transport chain.
The video below shows the details of the electron transfer reactions, and how they are coupled to pumping protons across the membrane. This is a form of active transport, because the electron transfers release free energy that is used to pump protons against their concentration gradient. Many bacteria can use other terminal electron acceptors when oxygen is unavailable; we say that they carry on anaerobic respiration, when the electron transport chain functions in the absence of oxygen, using an alternative terminal electron acceptor.
A molecule that loses electrons is oxidized ; a molecule that gains electrons is reduced. Different molecules have different tendencies to gain or lose electrons, called the redox potential. A redox reaction between a pair of molecules with a large difference in redox potential results in a large release of free energy. In aqueous environments, the transferred electrons pick up protons.
Living cells are the original hydrogen fuel cells. Cellular energy metabolism features a series of redox reactions. NADH is a high-energy molecule. The membrane electron transport chain and chemiosmosis is a strategy for cells to maximize the amount of ATP they can make from the large amounts of free energy available in NADH.
The electron transport chain subdivides the oxidation of NADH by O2 to a series of lower energy redox reactions, which are used to pump protons across the membrane.
Anaerobic respiration in bacteria The amount of energy released by these redox reactions, and thus the amount of energy available for ATP synthesis, depends on the redox potential of the terminal electron acceptor. Oxygen O2 has the greatest redox potential, and thus aerobic respiration results in the most ATP synthesized. Bacteria and Archaea can use other terminal electron acceptors with lower redox potential when oxygen is not available.
This anaerobic respiration produces less ATP. Bacteria can modify their electron transport chains to use a variety of electron donors and electron acceptors, and will switch to the best available in their environment. In marine sediments, microbial communities stratify according to redox potential. The deeper, more anoxic layers use electron acceptors with progressively lower reducing potential. As you will see later in this tutorial, it is the free energy from these redox reactions that is used to drive the production of ATP.
This flowchart shows the major steps involved in breaking down glucose from the diet and converting its chemical energy to the chemical energy in the phosphate bonds of ATP, in aerobic oxygen-using organisms. Note: In this flowchart, red denotes a source of carbon atoms originally from glucose , green denotes energy-currency molecules, and blue denotes the reducing agents that can be oxidized spontaneously. In the discussion above, we see that glucose by itself generates only a tiny amount of ATP.
How does this work? As discussed in an earlier section about coupling reactions, ATP is used as free-energy currency by coupling its spontaneous dephosphorylation Equation 3 with a nonspontaneous biochemical reaction to give a net release of free energy i. This set of coupled reactions is so important that it has been given a special name: oxidative phosphorylation. In addition, we must consider the reduction reaction gaining of electrons that accompanies the oxidation of NADH.
Oxidation reactions are always accompanied by reduction reactions, because an electron given up by one group must be accepted by another group. In this case, molecular oxygen O 2 is the electron acceptor, and the oxygen is reduced to water Equation 10, below. The molecular changes that occur upon oxidation are shown in red.
In this tutorial, we have seen that nonspontaneous reactions in the body occur by coupling them with a very spontaneous reaction usually the ATP reaction shown in Equation 3. But we have not yet answered the question: by what mechanism are these reactions coupled? Every day your body carries out many nonspontaneous reactions.
As discussed earlier, if a nonspontaneous reaction is coupled to a spontaneous reaction, as long as the sum of the free energies for the two reactions is negative, the coupled reactions will occur spontaneously. How is this coupling achieved in the body?
Living systems couple reactions in several ways, but the most common method of coupling reactions is to carry out both reactions on the same enzyme. Consider again the phosphorylation of glycerol Equations Glycerol is phosphorylated by the enzyme glycerol kinase, which is found in your liver. The product of glycerol phosporylation, glycerolphosphate Equation 2 , is used in the synthesis of phospholipids. Glycerol kinase is a large protein comprised of about amino acids.
X-ray crystallography of the protein shows us that there is a deep groove or cleft in the protein where glycerol and ATP attach see Figure 6, below. Because the enzyme holds the ATP and the glycerol in place, the phosphate can be transferred directly from the ATP to glycerol. Instead of two separate reactions where ATP loses a phosphate Equation 3 and glycerol picks up a phosphate Equation 2 , the enzyme allows the phosphate to move directly from ATP to glycerol Equation 4.
The coupling in oxidative phosphorylation uses a more complicated and amazing! This is a schematic representation of ATP and glycerol bound attached to glycerol kinase. The enzyme glycerol kinase is a dimer consists of two identical subuits.
There is a deep cleft between the subunits where ATP and glycerol bind. Since the ATP and phosphate are physically so close together when they are bound to the enzyme, the phosphate can be transferred directly from ATP to glycerol. Hence, the processes of ATP losing a phosphate spontaneous and glycerol gaining a phosphate nonspontaneous are linked together as one spontaneous process.
Neglecting any differences in difficulty synthesizing or accessing these molecules by biological systems, rank the molecules in order of their efficiency as a free-energy currency i. In order to couple the redox and phosphorylation reactions needed for ATP synthesis in the body, there must be some mechanism linking the reactions together.
In cells, this is accomplished through an elegant proton-pumping system that occurs inside special double-membrane-bound organelles specialized cellular components known as mitochondria.
A number of proteins are required to maintain this proton-pumping system and catalyze the oxidative and phosphorylation reactions. There are three key steps in this process:. Note: Steps a and b show cytochrome oxidase, the final electron-carrier protein in the electron-transport chain described above.
When this protein accepts an electron green from another protein in the electron-transport chain, an Fe III ion in the center of a heme group purple embedded in the protein is reduced to Fe II. Cells use a proton-pumping system made up of proteins inside the mitochondria to generate ATP. Before we examine the details of ATP synthesis, we shall step back and look at the big picture by exploring the structure and function of the mitochondria, where oxidative phosphorylation occurs.
The mitochondria Figure 8 are where the oxidative-phosphorylation reactions occur. Mitochondria are present in virtually every cell of the body. They contain the enzymes required for the citric-acid cycle the last steps in the breakdown of glucose , oxidative phosphorylation, and the oxidation of fatty acids.
This is a schematic diagram showing the membranes of the mitochondrion. The purple shapes on the inner membrane represent proteins, which are described in the section below. An enlargement of the boxed portion of the inner membrane in this figure is shown in Figure 8, below. The mitochondrial membranes are crucial for this organelle's role in oxidative phosphorylation. As shown in Figure 8, mitochondria have two membranes, an inner and an outer membrane. The outer membrane is permeable to most small molecules and ions, because it contains large protein channels called porins.
The inner membrane is impermeable to most ions and polar molecules. The inner membrane is the site of oxidative phosphorylation. Recall the discussion of protein channels in the " Maintaining the Body's Chemistry: Dialysis in the Kidneys " Tutorial. As shown in Figure 8, inside the inner membrane is a space known as the matrix ; the space between the two membranes is known as the intermembrane space.
This charge difference is used to provide free energy G for the phosphorylation reaction Equation 8. Electrons are not transferred directly from NADH to O 2 , but rather pass through a series of intermediate electron carriers in the inner membrane of the mitochondrion.
This allows something very important to occur: the pumping of protons across the inner membrane of the mitochondrion. As we shall see, it is this proton pumping that is ultimately responsible for coupling the oxidation-reduction reaction to ATP synthesis. Two major types of mitochondrial proteins see Figure 9, below are required for oxidative phosphorylation to occur.
Both classes of proteins are located in the inner mitochondrial membrane. The electron carriers can be divided into three protein complexes NADH-Q reductase 1 , cytochrome reductase 3 , and cytochrome oxidase 5 that pump protons from the matrix to the intermembrane space, and two mobile carriers ubiquinone 2 and cytochrome c 4 that transfer electrons between the three proton-pumping complexes.
Gold numbers refer to the labels on each protein in Figure 9, below. Because electrons move from one carrier to another until they are finally transferred to O 2 , the electron carriers shown in Figure 9,below are said to form an electron-transport chain. Figure 9, below, is a schematic representation of the proteins involved in oxidative phosphorylation. To see an animation of oxidative phosphorylation, click on "View the Movie. This is a schematic diagram illustrating the transfer of electrons from NADH, through the electron carriers in the electron transport chain, to molecular oxygen.
Please click on the pink button below to view a QuickTime animation of the functions of the proteins embedded in the inner mitochondrial membrane that are necessary for oxidative phosphorylation. Click the blue button below to download QuickTime 4. Ubiquinone Q 2 and cytochrome c Cyt C 4 are mobile electron carriers. Ubiquinone is not actually a protein. All of the electron carriers are shown in purple, with lighter shades representing increasingly higher reduction potentials.
The path of the electrons is shown with the green dotted line. ATP synthetase red has two components: a proton channel allowing diffusion of protons down a concentration gradient, from the intermembrane space to the matrix , and a catalytic component to catalyze the formation of ATP.
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