1. Chapter 9 Cellular Respiration

2. Overview: Life Is Work Living cells require energy from outside sources Some animals, such as the chimpanzee, obtain energy by eating plants, and some animals feed on other organisms that eat plants Humans depend on other plants and animals for their source of energy

4. Introduction To perform their many tasks, cells require transfusions of energy from outside sources. –In most ecosystems, energy enters as sunlight. –Photosynthesis generates O 2 and organic molecules, which are used in cellular respiration –Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

5. Concept 9.1 Catabolic Pathways Organic molecules store energy in their arrangement of atoms, within the bonds. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy by breaking those bonds. Some of the released energy is used to do work and the rest is dissipated as heat.

6. One type of catabolic process, fermentation, leads to the partial degradation of sugars in the absence of oxygen. A more efficient and widespread catabolic process, cellular respiration, uses oxygen as a reactant to complete the breakdown of a variety of organic molecules. Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O 2 Catabolic Pathways and Production of ATP

7. Cellular respiration is similar to the combustion of gasoline in an automobile engine. The overall process is: –Organic compounds + O 2 -> CO 2 + H 2 O + Energy Carbohydrates, fats, and proteins can all be used as the fuel. –C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + Energy (ATP + heat)

8. ATP production ATP is the pivotal molecule in cellular energetics. It is the chemical equivalent of a loaded spring. –The close packing of three negatively-charged phosphate groups is an unstable, energy-storing arrangement. –Loss of the end phosphate group “relaxes” the “spring”. The price of most cellular work is the conversion of ATP to ADP an inorganic phosphate (P i ). An animal cell regenerates ATP from ADP and P i by the catabolism of organic molecules.

9. Redox Reactions: Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP

10. The Principle of Redox Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)

11. becomes oxidized (loses electron) becomes reduced (gains electron) becomes oxidized becomes reduced The electron donor is called the reducing agent The electron receptor is called the oxidizing agent Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds

12. Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O 2 is reduced becomes oxidized becomes reduced

13. Stepwise Energy Harvest via NAD + and the Electron Transport Chain In cellular respiration, glucose and other organic molecules are broken down in a series of steps Electrons from organic compounds are usually first transferred to NAD +, a coenzyme As an electron acceptor, NAD + functions as an oxidizing agent during cellular respiration Each NADH (the reduced form of NAD + ) represents stored energy that is tapped to synthesize ATP

15. NADH passes the electrons to the electron transport chain Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction O 2 pulls electrons down the chain in an energy- yielding tumble The energy yielded is used to regenerate ATP

17. The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose has three stages –Glycolysis (breaks down glucose into two molecules of pyruvate) –The citric acid cycle (completes the breakdown of glucose) –Oxidative phosphorylation (accounts for most of the ATP synthesis)

22. Cellular Respiration Overview Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and oxidative phosphorylation

23. Glycolysis occurs in the cytoplasm. –It begins catabolism by breaking glucose into two molecules of pyruvate while making two ATP. The Citric Acid cycle (CAC) occurs in the mitochondrial matrix. –It degrades pyruvate to carbon dioxide while making two more ATP. –AKA Kreb’s cycle, Hans Kreb, 1930’s Several steps in both glycolysis and the CAC transfer electrons from substrates to NAD +, forming NADH. NADH passes these electrons to oxidative phosphorylation where the ETC and ATP synthase combine to make more ATP.

24. Respiration uses the small steps in the respiratory pathway to break the large denomination of energy contained in glucose into the small change of ATP. –The quantity of energy in ATP is more appropriate for the level of work required in the cell. Ultimately 38 ATP are produced per mole of glucose that is degraded to carbon dioxide and water by respiration.

25. Concept 9.2 Glycolysis During glycolysis, glucose, a six carbon-sugar, is split into two, three-carbon sugars. These smaller sugars are oxidized and rearranged to form two molecules of pyruvate. –Oxidation = removal of electrons –Reduction = addition of an electron (confusing ???) Each of the ten steps in glycolysis is catalyzed by a specific enzyme. These steps can be divided into two phases: an energy investment phase and an energy payoff phase.

26. The net yield from glycolysis is 2 ATP and 2 NADH per glucose. –No CO 2 is produced during glycolysis. Glycolysis occurs whether O 2 is present or not.

29. Concept 9.3 Citric Acid Cycle More than 3/4 of the original energy in glucose is still present in two molecules of pyruvate. If oxygen is present, pyruvate enters the mitochondrion where enzymes of the CAC complete the oxidation (removal of electrons) of the organic fuel to carbon dioxide. Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH 2 (another electron carrier) per acetyl CoA.

30. As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the CAC in the matrix. –A carboxyl group is removed as CO 2. –A pair of electrons are transferred from the remaining two-carbon fragment to NAD + to form NADH. –The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA.

31. The Citric Acid Cycle consists of eight steps. –Remember 2 molecules of acetyl CoA enter the CAC.

32. The conversion of pyruvate to Acetyl CoA and the CAC produce large quantities of electron carriers (NADH and FADH 2 ) and two ATP

33. Concept 9.4 Oxidative Phosphorylation So far, only 4 of 36-38 ATP ultimately produced by catabolism of one glucose molecule are derived from substrate-level phosphorylation (Glycolysis and CAC) The vast majority of the ATP comes from the energy in the electrons carried by NADH and FADH 2.

34. The Pathway of Electron Transport The electron transport chain is in the inner membrane (cristae) of the mitochondrion Most of the chain’s components are proteins, which exist in multi-protein complexes The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O 2, forming H 2 O

35. The energy in these electrons is passed on to the electron transport system which then passes the electron to oxygen, forming water.

36. Electrons are transferred from NADH or FADH 2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O 2 The electron transport chain generates no ATP directly It breaks the large free-energy drop from food to O 2 into smaller steps that release energy in manageable amounts

37. As the NADH and FADH 2 enter the ETC, H + are pumped across the cristae into the inner membrane space creating a proton gradient.

39. The energy stored in a H + gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis The H + gradient is referred to as a proton-motive force, emphasizing its capacity to do work

40. A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and P i. ATP synthase uses the energy of the proton gradient to produce 32 – 34 ATP.

41. Final Overview Considering the fate of carbon, one six- carbon glucose molecule is oxidized to six CO 2 molecules. Some ATP is produced by substrate-level phosphorylation during glycolysis (2) and the Krebs cycle (2), but most comes from oxidative phosphorylation in the ETC (34).

44. How efficient is respiration in generating ATP? –Complete oxidation of glucose releases 686 kcal per mole. –Formation of each ATP requires at least 7.3 kcal/mole. –Efficiency of respiration is 7.3 kcal/mole x 38 = 277.4, which is 40% of the released energy. –The other approximately 60% is lost as heat. Cellular respiration is remarkably efficient in energy conversion.

45. Concept 9.5 Fermentation Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic). Anaerobic catabolism of sugars can occur by fermentation. Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD + by transferring electrons from NADH to pyruvate or derivatives of pyruvate.

46. Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O 2, for example sulfate Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP

47. In alcohol fermentation, pyruvate is converted to ethanol in two steps. –First, pyruvate is converted to a two-carbon compound, acetaldehyde by the removal of CO 2. –Second, acetaldehyde is reduced by NADH to ethanol. –Alcohol fermentation by yeast is used in brewing and winemaking.

48. During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). –Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt. –Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O 2 is scarce. The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.

49. Comparing Fermentation with Anaerobic and Aerobic Respiration All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food In all three, NAD + is the oxidizing agent that accepts electrons during glycolysis The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O 2 in cellular respiration Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

51. Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways

52. The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle

54. Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle