A Guide to the Natural World David Krogh © 2011 Pearson Education, Inc. Chapter 7 Lecture Outline Vital Harvest: Deriving Energy from Food Biology Fifth Edition

© 2011 Pearson Education, Inc. 7.1 Energizing ATP

© 2011 Pearson Education, Inc. Energizing ATP The molecule adenosine triphosphate (ATP) supplies the energy for most of the activities of living things. For ATP to be produced, a third phosphate group must be added to adenosine diphosphate (ADP) through a redox reaction.

© 2011 Pearson Education, Inc. Storing and Releasing Energy The substance that loses electrons in a redox reaction is said to have been oxidized, while the substance that gains electrons is said to have been reduced.

© 2011 Pearson Education, Inc. NAD Electrons are carried between one part of the energy-harvesting process and another by electron carriers, the most important of which is nicotinamide adenine dinucleotide, or NAD. In its “empty” state, this molecule exists as NAD +.

© 2011 Pearson Education, Inc. NAD Through a redox reaction, NAD + picks up one hydrogen atom and another single electron from food, thus becoming NADH. It will retain this form until it drops off its energetic electrons (and a proton) in a later stage of the energy-harvesting process.

© 2011 Pearson Education, Inc. The Electron Carrier NAD + Figure 7.3 empty loaded empty proton (oxidized) goes to pick up more electrons used in later stage of respiration used in later stage of respiration 1. NAD + within a cell, along with two hydrogen atoms that are part of the food that is supplying energy for the body. 2. NAD + is reduced to NAD by accepting an electron from a hydrogen atom. It also picks up another hydrogen atom to become NADH. (reduced) 3. NADH carries the electrons to a later stage of respiration then drops them off, becoming oxidized to its original form, NAD +.

© 2011 Pearson Education, Inc. Storing and Releasing Energy Figure Energy from food is then stored as a phosphate bond in ATP. 3. Energy is then released when the phosphate bond is broken, and can be used to fuel our everyday activities. energy in energy out 1. Energy from food is required to push a third phosphate group onto ADP. energy hill ATP P + ADP P + ADP

© 2011 Pearson Education, Inc. 7.2 The Three Stages of Cellular Respiration

© 2011 Pearson Education, Inc. Cellular Respiration Cellular respiration is the harvesting of energy from food. It has three stages: 1.Glycolysis 2.Krebs Cycle (Citric Acid Cycle) 3.Electron Transport Chain (ETC)

© 2011 Pearson Education, Inc. Cellular Respiration Some organisms rely solely on glycolysis for energy harvesting. For most organisms, however, glycolysis is a primary process of energy extraction only in certain situations, when quick bursts of energy are required. But, it is a necessary first stage to the Krebs cycle and the ETC.

© 2011 Pearson Education, Inc. Cellular Respiration Glycolysis takes place in the cell’s cytosol, while the Krebs cycle and the ETC take place in cellular organelles, called mitochondria, that lie within the cytosol.

© 2011 Pearson Education, Inc. Cellular Respiration Glycolysis yields two net molecules of ATP per molecule of glucose, as does the Krebs cycle.

© 2011 Pearson Education, Inc. Cellular Respiration Suggested Media Enhancement: Cellular Respiration To access this animation go to folder C_Animations_and_Video_Files and open the BioFlix folder.

© 2011 Pearson Education, Inc. Cellular Respiration The net yield in the ETC is a maximum of about 32 ATP molecules per molecule of glucose.

© 2011 Pearson Education, Inc. Cellular Respiration Glycolysis and the Krebs cycle are critical in that they yield electrons that are carried to the ETC for the final high-yield stage of energy harvesting.

© 2011 Pearson Education, Inc. Figure 7.4 (a) In metaphorical terms insert 1 glucose glycolysis 2 energy tokens 2 energy tokens 32 energy tokens Krebs cycle electron transport chain Just as the video games in some arcades can use only tokens (rather than money) to make them function, so our bodies can use only ATP (rather than food) as a direct source of energy. The energy contained in food—glucose in the example—is transferred to ATP in three major steps: glycolysis, the Krebs cycle, and the electron transport chain. Though glycolysis and the Krebs cycle contribute only small amounts of ATP directly, they also contribute electrons (on the left of the token machine) that help bring about the large yield of ATP in the electron transport chain. Our energy-transfer mechanisms are not quite as efficient as the arcade machine makes them appear. At each stage of the conversion process, some of the original energy contained in the glucose is lost to heat. As with the arcade machine, the starting point in this example is a single molecule of glucose, which again yields ATP in three major sets of steps: glycolysis, the Krebs cycle, and the electron transport chain (ETC). These steps can yield a maximum of about 36 molecules of ATP: 2 in glycolysis, 2 in the Krebs cycle, and 32 in the ETC. As noted, however, glycolysis and the Krebs cycle also yield electrons that move to the ETC, aiding in its ATP production. These electrons get to the ETC via the electron carriers NADH and FADH 2, shown on the left. Oxygen is consumed in energy harvesting, while water and carbon dioxide are produced in it. Glycolysis takes place in the cytosol of the cell, but the Krebs cycle and the ETC take place in cellular organelles, called mitochondria, that lie within the cytosol. (b) In schematic terms glycolysis glucose cytosol reactants products glucose derivatives Krebs cycle electron transport chain 2 ATP 32 ATP 36 ATP maximum per glucose molecule mitochondrion

© 2011 Pearson Education, Inc. 7.3 First Stage of Respiration: Glycolysis

© 2011 Pearson Education, Inc. First Stage of Respiration: Glycolysis Glycolysis begins with a single molecule of glucose. The ultimate products are two molecules of NADH (which move to the ETC, bearing their energetic electrons) and two molecules of ATP (which are ready to be used).

© 2011 Pearson Education, Inc. Glycolysis Glycolysis also produces two molecules of pyruvic acid—the derivatives of the original glucose molecule—which move on to the Krebs cycle.

© 2011 Pearson Education, Inc. Figure 7.5 glycolysis 2 ATP glucose 2 NADH Steps in glycolysis Krebs cycle electron transport chain 1. Delivered by the bloodstream, glucose enters a cell and immediately has a phosphate group from ATP attached to it. Because this process, called phosphorylation, attaches the phosphate to the sixth carbon of glucose, it now goes under the name glucose-6-phosphate. Note that one molecule of ATP has been used in this step. ATP ledger now reads: -1 ATP 2. Glucose 6-phosphate is rearranged to become a molecule called fructose-6-phosphate. 3. Another molecule of ATP is used to add a second phosphate to fructose-6-phosphate, which now becomes fructose-1,6-diphosphate. ATP ledger now reads: -2 ATP 4. The single, six-carbon sugar fructose-1,6-diphosphate now becomes two molecules of a 3-carbon sugar, glyceraldehyde-3-phosphate, each with a phosphate group attached. From here on out, glycolysis happens in duplicate: What happens to one of the glyceraldehyde molecules happens to the other. 5. An enzyme brings together glyceraldehyde-3- phosphate, the electron carrier NAD +, and a phosphate group. The glyceraldehyde-3-phosphate molecule is oxidized by NAD +, which in its new form, NADH, moves to the electron transport chain bearing its electron cargo. The oxidation of NAD + is energetic enough that it allows the phosphate group to become attached to the main molecule, now called 1,3-diphosphoglyceric acid. Because everything is happening in duplicate, two NADH molecules are produced. 6. 1,3-diphosphoglyceric acid loses one of its phosphate groups, thus becoming 3-phosphoglyceric acid. The reaction is energetic enough to push this phosphate group onto an ADP molecule, yielding ATP. Because of duplication, two ATP are produced. ATP ledger now reads: 0 ATP 7. In two reactions, 3-phosphoglyceric acid becomes phosphoenolpyruvic acid, which generates more ATP as it transfers its phosphate group to ADP. Two more ATP molecules are produced. The phosphate transfer turns phosphoenolpyruvic acid into pyruvic acid—the derivative of the original glucose that now will enter the Krebs cycle. ATP ledger now reads: +2 ATP molecules in molecules out ATP 2 ADP glucose glucose-6-phosphate fructose-6-phosphate fructose-1,6-diphosphate glyceraldehyde-3-phosphate 1,3-diphosphoglyceric acid 3-phosphoglyceric acid pyruvic acid ADP Black balls are carbons and gold ovals are phosphate groups ADP 2 ATP

© 2011 Pearson Education, Inc. 7.4 Second Stage of Respiration: The Krebs Cycle

© 2011 Pearson Education, Inc. Intermediate Step There is a transition step in respiration between glycolysis and the Krebs cycle. In it, each pyruvic acid molecule that was produced in glycolysis combines with coenzyme A, thus forming acetyl coenzyme A (acetyl CoA), which enters the Krebs cycle.

© 2011 Pearson Education, Inc. Intermediate Step There are also two other products of this reaction: 1.One molecule of carbon dioxide, which diffuses to the bloodstream. 2.One more molecule of NADH, which moves to the ETC.

© 2011 Pearson Education, Inc. Transition Between Glycolysis and the Krebs Cycle Figure 7.7 glycolysis pyruvic acid mitochondrion to electron transport chain acetyl coenzyme A coenzyme A inner compartment Krebs cycle cytosol

© 2011 Pearson Education, Inc. The Krebs Cycle For each starting molecule of glucose, two molecules of pyruvic acid go through this step. Thus, the step’s product per molecule of glucose is two molecules of carbon dioxide, two NADH, and two acetyl CoA.

© 2011 Pearson Education, Inc. The Krebs Cycle In the Krebs (or citric acid) cycle, the derivatives of the original glucose molecule are oxidized. The result is that more energetic electrons are transported by the electron carriers NADH and to the ETC.

© 2011 Pearson Education, Inc. The Krebs Cycle The net energy yield of the Krebs cycle per molecule of glucose is: six molecules of NADH two molecules of FADH 2 two molecules of ATP

© 2011 Pearson Education, Inc. Figure 7.8 glycolysis Krebs cycle Steps in the Krebs cycle 1. Acetyl CoA combines with the four-carbon oxaloacetic acid and the CoA fragment separates from this compound. The result is the energetic six-carbon molecule citric acid, which will now be oxidized. 2. A citric acid derivative is oxidized by NAD + ; the resulting NADH carries electrons to the ETC. An intermediate molecule then loses a CO 2 molecule. Citric acid is now alpha-ketoglutaric acid. 3. Alpha-ketoglutaric acid loses a CO 2 molecule and the resulting four-carbon molecule is oxidized by NAD An alpha-ketoglutaric acid derivative is split, releasing enough energy to attach a phosphate to ADP, making it ATP. Alpha-ketoglutaric acid has become succinic acid. 5. Succinic acid is oxidized by FAD, losing two complete hydrogen atoms to it. The resulting FADH 2 then moves to the ETC. In a series of steps, succinic acid is transformed into malic acid. 6. Malic acid is oxidized by NAD +. This step transforms malic acid to oxaloacetic acid—the molecule that enters the first step of the Krebs cycle. 2 ATP electron transport chain acetyl coenzyme A oxaloacetic acidcitric acid malic acid succinic acid  -ketoglutaric acid derivative 1. FAD ADP ATP  -Ketoglutaric acid

© 2011 Pearson Education, Inc. 7.5 Third Stage of Respiration: The Electron Transport Chain

© 2011 Pearson Education, Inc. Third Stage of Respiration: The Electron Transport Chain The ETC is a series of molecules located within the mitochondrial inner membrane.

© 2011 Pearson Education, Inc. The Electron Transport Chain On reaching the ETC, the electron carriers NADH and FADH 2 are oxidized by molecules in the chain.

© 2011 Pearson Education, Inc. The Electron Transport Chain Each carrier in the chain is then reduced by accepting electrons from the carrier that came before it.

© 2011 Pearson Education, Inc. The Electron Transport Chain The last electron acceptor in the ETC is oxygen.

© 2011 Pearson Education, Inc. The Electron Transport Chain The movement of electrons through the ETC releases enough energy to power the movement of hydrogen ions (H + ions) through the three ETC protein complexes. They move from the mitochondrion’s inner compartment to its outer compartment.

© 2011 Pearson Education, Inc. The Electron Transport Chain The movement of these ions down their concentration and charge gradients, back into the inner compartment through an enzyme called ATP synthase, drives the synthesis of ATP from ADP and phosphate.

© 2011 Pearson Education, Inc. The Electron Transport Chain In the inner compartment, oxygen accepts the electrons from the ETC and hydrogen ions, thus forming water.

© 2011 Pearson Education, Inc. The Electron Transport Chain Figure 7.9 inner membrane Mitochondrion outer compartment inner compartment Electron transport chain ATP synthesis outer compartment inner compartment ATP synthase inner membrane ATP

© 2011 Pearson Education, Inc. The Electron Transport Chain If oxygen is not present to accept the ETC electrons, the entire energy-harvesting process downstream from glycolysis comes to a halt.

© 2011 Pearson Education, Inc. 7.6 Other Foods, Other Respiratory Pathways

© 2011 Pearson Education, Inc. Other Foods, Other Respiratory Pathways Different nutrients and their derivatives can be channeled through different pathways in cellular respiration in accordance with the needs of an organism.

© 2011 Pearson Education, Inc. Many Respiratory Pathways Proteins and lipids enter the metabolic pathway for ATP production at different points than does glucose.

© 2011 Pearson Education, Inc. Figure 7.10 food proteinscarbohydratesfats amino acids sugarsglycerolfatty acids glucose glycolysis pyruvic acid acetyl CoA Krebs cycle NH 3 (ammonia) electron transport chain