How Cells Release Chemical Energy Chapter 8. Cellular or Aerobic Respiration Cellular respiration evolved to enable organisms to utilize energy stored.
Slide 1 How Cells Release Chemical Energy Chapter 8 Slide 2 Cellular or Aerobic Respiration Cellular respiration evolved to enable organisms to utilize energy stored in glucose. Taps the energy found in the bonds of organic compounds (carbohydrates, lipids, proteins) Slide 3 Comparison of the Main Pathways Aerobic respiration Aerobic metabolic pathways (using oxygen) are used by most eukaryotic cells Ends in the mitochondria Aerobes use oxygen as the final electron acceptor Fermentation Anaerobic metabolic pathways (occur in the absence of oxygen) are used by prokaryotes and protists in anaerobic habitats Anaerobes use nitrate or sulfate as the final e - acceptor Slide 4 Comparison of the Main Pathways Aerobic respiration and fermentation both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvate After glycolysis, the two pathways diverge Fermentation is completed in the cytoplasm, yielding 2 ATP per glucose molecule Aerobic respiration is completed in mitochondria, yielding 36 ATP per glucose molecule (liberates the most energy) Slide 5 Overview of Aerobic Respiration Three sequential stages Glycolysis Acetyl-CoA formation and Krebs cycle Electron transfer phosphorylation (ATP formation) C 6 H 12 O 6 (glucose) + O 2 (oxygen) CO 2 (carbon dioxide) + H 2 O (water) Coenzymes NADH and FADH 2 Slide 6 Fig. 8-3b, p. 125 Slide 7 Animation: Overview of aerobic respiration Slide 8 8.2 Glycolysis Glucose Breakdown Starts Glycolysis starts and ends in the cytoplasm of all prokaryotic and eukaryotic cells An energy investment/input of ATP (2) starts glycolysis Requires a continous supply of glucose and NAD + ( reduced in both glycolysis and the kreb cycle) Slide 9 Glycolysis Two ATPs are used to split glucose (6 carbon compound) and form 2 PGAL (3 carbon compound) Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH Four ATP are formed by substrate-level phosphorylation (net yield of 2 ATP) Slide 10 Fig. 8-4a (2), p. 126 Slide 11 Fig. 8-4b (1), p. 127 Glycolysis ATP-Requiring Steps glucose A An enzyme transfers a phosphate group from ATP to glucose, forming glucose- 6-phosphate. ATP ADP B A phosphate group from a second ATP is transferred to the glucose- 6- phosphate. The resulting molecule is unstable, and it splits into two three- carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). glucose-6-phosphate ATP ADP fructose-1,6-bisphosphate (intermediate) So far, two ATP have been invested in the reactions. Slide 12 Fig. 8-4b (2), p. 127 ATP-Generating Steps 2 PGAL C Enzymes attach a phosphate to the two PGAL, and transfer two electrons and a hydrogen ion from each PGAL to NAD +. Two PGA (phosphoglycerate) and two NADH are the result. 2 NAD + + 2 P i NADH 2 reduced coenzymes D Enzymes transfer a phosphate group from each PGA to ADP. Thus, two ATP have formed by substrate- level phosphorylation. 2 ADP ATP The original energy investment of two ATP has now been recovered. 2 ATP produced by substrate-level phosphorylation 2 PEP E Enzymes transfer a phosphate group from each of two intermediates to ADP. Two more ATP have formed by substrate-level phosphorylation. 2 ADP ATP Two molecules of pyruvate form at this last reaction step. 2 ATP produced by substrate-level phosphorylation F Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule. 2 pyruvate Net 2 ATP + 2 NADH to second stage 2 PGA Slide 13 8.3 Second Stage of Aerobic Respiration The second stage of aerobic respiration finishes breakdown of glucose that began in glycolysis Occurs in mitochondria (inner compartment) begans with the chemical pyruvate to continue respiration Includes two stages: acetyl CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule) Slide 14 The Krebs Cycle Krebs cycle A sequence of enzyme-mediated reactions that break down 1 acetyl CoA (transition stage) into 2 CO 2 Oxaloacetate (last intermediate) is used and regenerated 3 NADH and 1 FADH 2 are formed, 1 ATP is formed Substrate level phosphorylation occurs Electrons and hydrogens are transferred to coenzymes in both glycolysis and the Kreb cycle Energy, CO 2, and H + are released Cycle turns 2x to break down glucose Slide 15 AcetylCoA Formation transition from glyco A An enzyme splits a pyruvate molecule into a two-carbon acetyl group and CO 2. Coenzyme A binds the acetyl group (forming acetylCoA). NAD + combines with released hydrogen ions and electrons, forming NADH. NADH pyruvate CO 2 NAD + coenzyme A acetylCoA Fig. 8-6, p. 129 B The Krebs cycle starts as one carbon atom is transferred from acetyl CoA to oxaloacetate. Citrate forms, and coenzyme A is regenerated. oxaloacetate coenzyme A citrate Krebs Cycle FADH 2 F The coenzyme FAD com- bines with hydrogen ions and electrons, forming FADH 2. FAD NADH G NAD + combines with hydrogen ions and electrons, forming NADH. NAD + C A carbon atom is removed from an intermediate and leaves the cell as CO 2. NAD + combines with released hydrogen ions and electrons, forming NADH. NADH CO 2 NAD + Pyruvates three carbon atoms have now exited the cell, in CO 2. D A carbon atom is removed from another intermediate and leaves the cell as CO 2, and another NADH forms. NADH CO 2 NAD + ATP E One ATP forms by substrate-level phosphorylation. ADP + P i H The nal steps of the Krebs cycle regenerate oxaloacetate. Stepped Art Slide 16 Animation: The Krebs Cycle - details Slide 17 8.4 Aerobic Respirations Big Energy Payoff Many ATPs are formed during the third and final stage of aerobic respiration (Chemiosmotic Theory = production of ATPs) Electron transfer phosphorylation Occurs in mitochondria (inner membrane) Results in attachment of phosphate to ADP to form ATP Generates a hydrogen concentration (build up of hydrogen ions between 2 membranes) gradient Slide 18 Electron Transfer Phosphorylation Slide 19 Summary: The Energy Harvest Typically, the breakdown of one glucose molecule yields 36 ATP Glycolysis: 2 ATP Acetyl CoA formation and Krebs cycle: 2 ATP Electron transfer phosphorylation: 32 ATP Slide 20 Animation: Third-stage reactions Slide 21 8.5 Anaerobic Energy-Releasing Pathways Fermentation pathways break down carbohydrates without using oxygen (Ex. Bacteria that causes botulism) The final steps in these pathways regenerate NAD + Slide 22 Fermentation Pathways Glycolysis is the first stage of fermentation Forms 2 pyruvate, 2 NADH, and 2 ATP Pyruvate turns to lactic acid using e - from NADH (Lactate/Lactose Fermentation) Pyruvate is converted to ethanol producing acetaldehyde and CO 2 (Alcoholic Fermentation) Slide 23 Two Pathways of Fermentation Alcoholic fermentation Pyruvate produce acetaldehyde and CO 2 when converted to ethanol Acetaldehyde receives electrons and hydrogen from NADH, forming NAD + and ethanol Lactate fermentation Pyruvate receives electrons and hydrogen from NADH, forming NAD + and lactate (muscle cells, sour cream, etc) Slide 24 Lactate Fermentation lactate Alcoholic Fermentation 2 CO 2 acetaldehyde Glycolysis glucose 2 NAD + 2 ATP 2 NADH 4 pyruvate ATP Fig. 8-9, p. 132 2 NADH 2 NAD + ethanol glucose Glycolysis 2 NAD + 2 ATP 2 NADH pyruvate 4 ATP 2 NADH 2 NAD + Stepped Art Slide 25 8.6 The Twitchers Slow-twitch muscle fibers (red muscles) make ATP by aerobic respiration Have many mitochondria Dominate in prolonged activity Fast-twitch muscle fibers (white muscles) make ATP by lactate fermentation Have few mitochondria and no myoglobin Sustain short bursts of activity Human muscles have a mixture of both fibers Slide 26 8.7 Alternative Energy Sources in the Body In humans and other mammals, the entrance of glucose and other organic compounds into an energy-releasing pathway depends on the kinds and proportions of carbohydrates (our main source of energy), fats and proteins in the diet Slide 27 The Fate of Glucose at Mealtime and Between Meals When blood glucose concentration rises, the pancreas increases insulin (hormone) secretion Cells take up glucose faster, more ATP is formed When blood glucose concentration falls, the pancreas increases glucagon (hormone) secretion (between meals) Stored glycogen is converted to glucose, the brain continues to receive glucose Triglycerides are tapped as an energy alternative. Slide 28 Energy From Fats About 78% of an adults energy reserves is stored in fat (mostly triglycerides) Excess glucose(carbs) in the diet = FAT; acetyl Co A exits the Kreb Cycle and enters a pathway that makes fatty acids Enzymes cleave fats into glycerol and fatty acids Glycerol products enter glycolysis Fatty acid products enter the Krebs cycle Slide 29 Energy from Proteins Enzymes split dietary proteins into amino acid subunits, which enter the bloodstream Used to build proteins or other molecules Excess amino acids are broken down into ammonia (NH 3 ) and various products that can enter the Krebs cycle Slide 30 Fig. 8-12, p. 135 FOOD FATSCOMPLEX CARBOHYDRATESPROTEINS fatty acids glycerolglucose, other simple sugars amino acids acetylCoA PGAL acetylCoA Glycolysis NADHpyruvate oxaloacetate or another intermediate of the Krebs Krebs Cycle NADH, FADH 2 Electron Transfer Phosphorylation