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4. Respiration

4. Respiration. Review. Photosynthesis is the process of incorporating energy from light into energy-rich molecules like glucose. Respiration is the opposite process extracting that stored energy from glucose to form ATP (from ADP and P i ). The chemical equation describing this process is

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4. Respiration

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  1. 4. Respiration

  2. Review • Photosynthesis is the process of incorporating energy from light into energy-rich molecules like glucose. • Respirationis the opposite process extracting that stored energy from glucose to form ATP (from ADP and Pi). • The chemical equation describing this process is • If you replace the energy with light and reverse the equation, it will describe photosynthesis.

  3. Glycolysis • Glycolysisis the decomposition (lysis) of glucose(glyco) to pyruvate(or pyruvic acid).

  4. 5 • Nine intermediate products are formed, and, of course, each one is catalyzed by an enzyme. • In six of the steps, magnesium ions (Mg2+) are cofactors that promote enzyme activity. 6 1 7 2 8 3 9 4 5

  5. The steps are summarized as follows. 1. 2 ATP are added.The first several steps require the input of energy. This changes glucose in preparation for subsequent steps. 2. 2 NADH are produced.NADH (Nicotinamide adenine dinucleotide) is a coenzyme, accepting 2 electrons from the substrate molecule. Like NADPH in photosynthesis, it is an energy-rich molecule. (You can keep the two coenzymes NADH and NADPH associated with the correct processes by using the P in NADPH as a reminder of the P in photosynthesis. The P in NADPH, however, actually represents phosphorus.) 3. 4 ATP are produced. 4. 2 pyruvate are formed. Remember that PGAL = (glyceride 3-phosphate, or glyceraldehyde-3-phospate)

  6. In summary, glycolysis takes 1 glucose and turns it into 2 pyruvate, 2 NADH, and a net of 2 ATP (made 4 ATP, but used 2 ATP).

  7. The Krebs Cycle • The Krebs cycle details what happens to the pyruvate end product of glycolysis.

  8. The Krebs Cycle • Although the Krebs cycle is described for 1 pyruvate, remember that glycolysis produces 2 pyruvate. • In the Figure, the “× 2” next to the pyruvate and the Krebs cycle is a reminder to multiply the products of this cycle by 2 to account for the products of a single glucose.

  9. The Krebs Cycle • 1. Pyruvate to acetyl CoA. In a step leading up to the actual Krebs cycle, pyruvate combines with coenzyme A (CoA) to produce acetyl CoA. In that reaction 1 NADH and 1 CO2 are also produced. • 2. Krebs Cycle: 3 NADH, 1 FADH2, 1 ATP, CO2.The Krebs cycle begins when acetyl CoA combines with OAA (oxaloacetic acid) to form citric acid. There are 7 intermediate products. Along the way, 3 NADH and 1 FADH2 (Flavin adenine dinucleotide) are made and CO2 is released. FADH2, like NADH, is a coenzyme, accepting electrons during a reaction. • Because the first product made from acetyl CoA is the 3-carbon citric acid, the Krebs cycle is also known as the citric acid cycleor the tricarboxylic acid (TCA) cycle. • The CO2 produced by the Krebs cycle is the CO2 animals exhale when they breathe.

  10. There are 7 intermediate products. 1 7 2 6 5 3 4

  11. Oxidative Phosphorylation • Oxidative phosphorylation is the process of extracting ATP from NADH and FADH2.

  12. Oxidative Phosphorylation • Electrons from NADH and FADH2 pass along an electron transport chain analogous to electron transport chains in photophosphorylation. • These electrons pass from one carrier protein to another along the chain, losing energy at each step. • Cytochromesand various other modified proteins participate as carrier proteins in this chain. One of these cytochromes, cytochrome c, is often compared among species to assess genetic relatedness.

  13. Oxidative Phosphorylation • The last electron acceptor at the end of the chain is oxygen. • The 1⁄2O2 accepts the two electrons and, together with 2 H+, forms water. • NADH provides electrons that have enough energy to phosphorylate 3 ADP to 3 ATP. • FADH2 produces 2 ATP.

  14. How Many ATP? -2 ATP • How many ATP are made from the energy released from the breakdown of 1 glucose? • Glycolysis produces 2 ATP and 2 NADH. • When 2 pyruvate (from 1 glucose) are converted to 2 acetyl CoA, 2 more NADH are produced. • From 2 acetyl CoA, the Krebs cycle produces 6 NADH, 2 FADH2, and 2 ATP. • If each NADH produces 3 ATP during oxidative phosphorylation, and FADH2 produces 2 ATP, the total ATP count from 1 original glucose appears to be 38 (Table 4-1).

  15. The actual number, however, is 36. This is becauseglycolysis occurs in thecytoplasm and each NADH produced there must be transported into the mitochondria for oxidative phosphorylation. The transport of NADH across the mitochondrial membrane reduces the yield of these NADH to only 2 ATP. Not 6 ATP

  16. Mitochondria • The Krebs cycle and the conversion of pyruvate to acetyl CoA occur in the mitochondrial matrix(the fluid part). • The electron transport chain proteins are embedded in the cristae(singular, crista). • The cristae are internal convoluted membranes that separate the mitochondrion into an inner compartment that contains the matrix and an outer compartment between the cristae and the outer mitochondrial membrane. • Note how the spatial arrangement of the respiratory processes in the mitochondrion is similar to the spatial arrangement of photosynthetic processes in the chloroplasts. • In chloroplasts, the carrier proteins of electron transport chains are embedded in the inner membranes, the thylakoids, while the enzymes for the Calvin-Benson cycle are in the stroma.

  17. In the cytoplasm, glycolysis produces 2 pyruvate, 2 NADH, and 2 ATP. • In order for ATP to be extracted from the pyruvate and NADH, these molecules must be shipped across the mitochondrial membrane and into the matrix. • Within the mitochondria, pyruvate (after conversion to acetyl CoA) enters the Krebs cycle. • The 2 NADH begin oxidative phosphorylation with the electron transport chain in the cristae. • These NADH, however, to produce a net of only 2 ATP each because 1 ATP is required to move each of them into the mitochondria.

  18. Chemiosmotic Theory • Electrons from NADH and FADH2 lose energy as they pass along the electron transport chain in oxidative phosphorylation. • That energy is used to phosphorylate ADP to ATP. • Chemiosmotic theory describes how that phosphorylation occurs. • The process is analogous to ATP generation in chloroplasts (Figure 4-3).

  19. Chemiosmotic Theory • 1. H+ accumulate in the outer compartment.The Krebs cycle produces NADH and FADH2 in the matrix. As these two molecules move through the electron transport chain, H+ (which is only a proton) are pumped from the matrix across the cristae and into the outer compartment (between the cristae and the mitochondrial outer membrane).

  20. 2. A pH and electrical gradient across the crista membrane is created.The accumulation of H+ in the outer compartment creates a proton gradient (equivalent to a pH gradient) and an electric charge (or voltage) gradient. These gradients are potential energy reserves in the same manner as water behind a dam is stored energy.

  21. 3. ATP synthases generate ATP.Channel proteins (ATP synthases) in the cristae allow the protons in the outer compartment to flow back into the matrix. The protons moving through the channel generate the energy for these channel proteins to produce ATP. It is similar to how turbines in a dam generate electricity when water flows through them.

  22. Anaerobic Respiration What if oxygen is not present? • If oxygen is not present, there is no electron acceptor to accept the electrons at the end of the electron transport chain. • If this occurs, then NADH accumulates. • Once all the NAD+ has been converted to NADH, the Krebs cycle and glycolysis both stop (both need NAD+ to accept electrons). • Once this happens, no new ATP is produced, and the cell soon dies.

  23. Anaerobic respirationis a method cells use to escape this fate. • The pathways in plants and animals, alcoholicand lactate fermentation, respectively, are slightly different, but the objective of both processes is to replenish NAD+ so that glycolysis can proceed once again. • Anaerobic respiration occurs in the cytoplasm alongside glycolysis.

  24. Alcoholic Fermentation • Alcoholic fermentation(or sometimes, just fermentation) occurs in plants, fungi (such as yeasts), and bacteria. • The steps, illustrated in the Figure, are as follows: 1. Pyruvate to acetaldehyde.For each pyruvate, 1 CO2 and 1 acetaldehyde are produced. The CO2 formed is the source of carbonation in fermented drinks like beer and champagne. 2. Acetaldehyde to ethanol.The important part of this step is that the energy in NADH is used to drive this reaction, releasing NAD+. For each acetaldehyde, 1 ethanol is made and 1 NAD+ is produced. The ethanol (ethyl alcohol) produced here is the source of alcohol in beer and wine.

  25. It is important that you recognize the objective of this pathway. At first glance, you should wonder why the energy in an energy-rich molecule like NADH is removed and put into the formation of ethanol, essentially a waste product that eventually kills the yeast (and other organisms) that produce it. • The goal of this pathway, however, does not really concern ethanol, but the task of freeing NAD+to allow glycolysis to continue. Recall that in the absence of O2 , all the NAD+ is bottled up in NADH. This is because oxidative phosphorylation cannot accept the electrons of NADH without oxygen. • The purpose of the fermentation pathway, then, is to release some NAD+ for use by glycolysis. • The reward for this effort is 2 ATP from glycolysis for each 2 converted pyruvate. This is not much, but it’s better than the alternative—0 ATP.

  26. Lactate Fermentation • There is only one step in lactate fermentation. A pyruvate is converted to lactate (or lactic acid) and in the process, NADH gives up its electrons to form NAD+. • As in alcoholic fermentation, the NAD+ can now be used for glycolysis. • When O2 again becomes available, lactate can be broken down and its store of energy can be retrieved. • Because O2 is required to do this, lactate fermentation creates what is often called an oxygen debt.

  27. Oxygen Debt“الدين الأوكسجيني“ • When you’re exercising and your skeletal muscles aren’t getting enough oxygen, the body must tap into itsanaerobic metabolism through the Lactate Fermentation. • The accumulation of lactic acid in muscles contributes to muscle fatigue and muscle cramping.  • If you keep pushing yourself, and building up youroxygen deficit, your performance will deteriorate.

  28. Once you stop exercise, you will continue to pant because your body needs oxygen to convert lactic acid that was created during exercise touseful energy through aerobic respiration • This is known as oxygen debt following exercise.  • The greater the accumulation of lactic acid, the moreadditionaloxygen will be required by your body. • Off course, the stronger your heart is, the shorter that recovery time will be.

  29. Cardiac stress test • Doctors mimic this through something known as a cardiac stress testto see what your heart can do. • Doctors want to see how hard they could push you and make you run on a treadmill as hard as you can before you have to tell them to stop the treadmill. • Once they stop the treadmill, you will continue to pant because your body needs to repay theoxygen debtthat was created during exercise.  • The doctor will note how quickly your heart rate returns back to normal because it’s a direct indication of the power and strength of your heart.

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