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.
If the problem continues, please let us know and we'll try to help. An unexpected error occurred. Previous Video 8. This range of ATP is approximate for three reasons. Second, NADH produced in glycolysis cannot pass through the mitochondrial membrane and therefore must pass its high-energy electrons to other electron carriers within the mitochondria, and, depending on the cell type, produce FADH2 or NADH, yielding either 1.
Third, the energy produced by respiration is also used to power other activities, like the transport of pyruvate through the mitochondrial membrane, yielding about 30 or 32 ATPs. Cellular respiration produces ATP molecules per glucose molecule. Although most of the ATP results from oxidative phosphorylation and the electron transport chain ETC , 4 ATP are gained beforehand 2 from glycolysis and 2 from the citric acid cycle.
The ETC is embedded in the inner mitochondrial membrane and comprises four main protein complexes and an ATP synthase. This distribution of protons generates a concentration gradient across the membrane.
Four protons are needed to synthesize 1 ATP. Since a single NADH produces 2. Importantly, glycolysis occurs in the cytosol and the ETC is located in the mitochondria in eukaryotes.
The mitochondrial membrane is not permeable to NADH, hence the electrons of the 2 NADH that are produced by glycolysis need to be shuttled into the mitochondria. Given the different ATP yield depending on the electron carrier, the total yield of cellular respiration is 30 to 32 ATP per glucose molecule. Da Poian, A. Nature Education 3 9 Lane, N.
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Login processing Chapter 8: Cellular Respiration. Chapter 1: Scientific Inquiry. Chapter 2: Chemistry of Life. Chapter 3: Macromolecules. Chapter 4: Cell Structure and Function. Chapter 5: Membranes and Cellular Transport.
Two carbon atoms come into the citric acid cycle from each acetyl group. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not contain the same carbon atoms contributed by the acetyl group on that turn of the pathway.
The two acetyl-carbon atoms will eventually be released on later turns of the cycle; in this way, all six carbon atoms from the original glucose molecule will be eventually released as carbon dioxide. It takes two turns of the cycle to process the equivalent of one glucose molecule. These high-energy carriers will connect with the last portion of aerobic respiration to produce ATP molecules.
One ATP or an equivalent is also made in each cycle. Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is both anabolic and catabolic. You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP.
Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these pathways. Rather, it derives from a process that begins with passing electrons through a series of chemical reactions to a final electron acceptor, oxygen.
These reactions take place in specialized protein complexes located in the inner membrane of the mitochondria of eukaryotic organisms and on the inner part of the cell membrane of prokaryotic organisms. The energy of the electrons is harvested and used to generate a electrochemical gradient across the inner mitochondrial membrane.
The potential energy of this gradient is used to generate ATP. The entirety of this process is called oxidative phosphorylation. The electron transport chain Figure 4. Oxygen continuously diffuses into plants for this purpose. In animals, oxygen enters the body through the respiratory system. Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced.
There are four complexes composed of proteins, labeled I through IV in Figure 4. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes.
In each transfer of an electron through the electron transport chain, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions across the inner mitochondrial membrane into the intermembrane space, creating an electrochemical gradient. 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 affect would cyanide have on ATP synthesis?
As they are passed from one complex to another there are a total of four , the electrons lose energy, and some of that energy is used to pump hydrogen ions from the mitochondrial matrix into the intermembrane space.
In the fourth protein complex, the electrons are accepted by oxygen, the terminal acceptor.
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