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How Cells Release Stored Energy Cell respiration When a molecule has hydrogen, it has more energy and removing them releases the energy. The energy produced from the "burning" of glucose is used to make ATP. In chemistry this process is called the oxidation of glucose. The hydrogen carriers NAD and FAD are used to help release the energy in glucose by moving hydrogens and electrons around. Glucose • A simple sugar (C6H12O6) • Atoms held together by covalent bonds In-text figure Page 136 Summary Equation for Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose carbon oxygen dioxide water •Glycolysis -in cytoplasm -does not use oxygen Occurs in Two Stages • Energy-requiring steps – ATP energy activates glucose and its six-carbon derivatives – Actually uses 2 ATP’s • Energy-releasing steps – The products of the first part are split into 2 three carbon pyruvate molecules – 4 ATP and 2NADH form – 4 ATP’s form – 2 ATP’s used = 2 net ATP’s made Glycolysis is often called anaerobic respriation because it does not need oxygen. This process occurs in the cytoplasm. Two net molecules of ATP are made for cell use. It involves glucose being converted to two molecules of pyruvic acid (or pyruvate). This process is not very efficient at converting the energy of glucose into ATP as only 2 ADP are phosphorylated instead of 32 as in Krebs and chemiosmosis. Energy-Requiring Steps of Glycolysis 2 ATP invested Energy-Requiring Steps glucose ATP ADP P glucose-6-phosphate P fructose-6-phosphate ATP ADP P P fructose1,6-bisphosphate P PGAL P PGAL Figure 8.4(2) Page 137 P NAD+ Pi P PGAL NADH NAD+ Pi PGAL NADH EnergyReleasing Steps P P P P 1,3-bisphosphoglycerate 1,3-bisphosphoglycerate ADP ADP ATP ATP Phosphorylations P P 3-phosphoglycerate 3-phosphoglycerate P P 2-phosphoglycerate H2 O P 2-phosphoglycerate PEP PEP P ADP ADP ATP ATP pyruvate H2 O pyruvate Figure 8.4 Page 137 Glycolysis: Net Energy Yield Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP, 2 pyruvic acid and 2 NADH After glycolysis: -If oxygen is present, the pyruvic acid will go into the mitochondria where the Kreb’s cycle (or citric acid cycle) is performed followed by the ETC. This allows the rest of the energy stored in the hydrogen to be extracted. -If no there is no oxygen, then the Kreb's cycle can not be completed (or started in some organisms). A cell can continue doing glycolysis in the absence of oxygen to produce some ATP, BUT it must regenerate NAD to keep glycolysis going. This is called anaerobic respiration (or fermentation). Pyruvate will form either lactic acid (muscles) or ethanol (bacteria, yeast or plants). In either case NAD is regenerated so that glycolysis can continue. Fermentation Pathways • Begin with glycolysis • Do not break glucose down completely to carbon dioxide and water • Yield only the 2 ATP from glycolysis • Steps that follow glycolysis serve only to regenerate NAD+ Lactate Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 NAD+ 2 ADP 2 4 NADH ATP energy output 2 pyruvate 2 ATP net LACTATE FORMATION electrons, hydrogen from NADH 2 lactate GLYCOLYSIS Alcoholic Fermentation C6H12O6 2 ATP energy input 2 NAD+ 2 ADP 2 4 NADH ATP 2 pyruvate energy output 2 ATP net ETHANOL FORMATION 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol Anaerobic Electron Transport • Carried out by certain bacteria • Electron transfer chain is in bacterial plasma membrane • Final electron acceptor is compound from environment (such as nitrate), not oxygen • ATP yield is low After glysolysis, if oxygen is present aerobic respiration proceeds in the mitochondria. PreparatorySteps • Preparatory reactions – Pyruvate is decarboxylated and oxidized (releasing carbon dioxide) – NAD+ is reduced – Acetyl Co-A is formed – Ex. 1 pyruvate coenzyme A (CoA) NAD+ NADH O acetyl-CoA O carbon dioxide Ex. 2 The Krebs Cycle Overall Reactants Overall Products • • • • • • • • • Acetyl-CoA 3 NAD+ FAD ADP and Pi Coenzyme A 2 CO2 3 NADH FADH2 ATP The enzymes for Kreb's is found in the inner compartment of the mitochondria. Summary of Krebs- Occurs in Mitochondria 2X's Pyruvate---> 3 CO2 6 CO2 1 ADP ---> 1 ATP 2 ATP 4 NAD ---> 4 NADH 8 NADH 1 FAD ---> 1 FADH2 2 FADH2 (also, oxaloacetate regenerates so cycle can continue) Coenzyme Reductions during First Two Stages • Glycolysis • Preparatory reactions • Krebs cycle 2 NADH 2 FADH2 + 6 NADH • Total 2 FADH2 + 10 NADH 2 NADH Electron Transfer Phosphorylation • Occurs in the mitochondria • Coenzymes deliver electrons to electron transfer chains • Electron transfer sets up H+ ion gradients • Flow of H+ down gradients powers ATP formation (chemiosmotic) The purpose of chemiosmosis is to extract the energy found in NADH and FADH2 to make more ATP. This involves the cristae. There are electron transport chains that are used. The electrons from the NADH and FADH2 are used to move on the electron transport chain. As the electrons move down the electron transport chain, H+ ions are pumped across the membrane. The electrons from one NADH can pump 6 H+ across the membrane, but the electrons from FADH2 can only pump 4 H+ across the membrane. Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT The outer compartment of the mitochondria becomes positive and the inside becomes negative like a battery. This "battery" can do work. The hydrogen ions can cross an F1 particle and make ATP. It takes 2 H+ to cross the F1 particle to provide enough energy to make ATP. Making ATP: Chemiosmotic Model ATP INNER COMPARTMENT ADP + Pi Summary 8 NADH2 x 6 H = 48 H+ 2 FADH2(Krebs)x 4 H = 8 H+ 2 FADH2(glyc.) X 4 H = 8 H+ 64 H+ 64 H+ --> 32 ATP Importance of Oxygen • Electron transport phosphorylation requires the presence of oxygen • Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water Summary of Energy Harvest (per molecule of glucose) • Glycolysis – 2 ATP formed by substrate-level phosphorylation • Krebs cycle and preparatory reactions – 2 ATP formed by substrate-level phosphorylation • Electron transport phosphorylation – 32 ATP formed CYTOPLASM 2 glucose ATP 4 Glycolysis e- + H+ (2 ATP net) 2 pyruvate 2 NADH e- + H + 2 CO2 Overview of Aerobic Krebs Respiration Cycle 2 NADH 8 NADH 2 FADH2 e- ATP e - + H+ e- + 4 CO2 H+ 2 Electron Transfer Phosphorylation H+ 32 ATP ATP water e- + oxygen Typical Energy Yield: 36 ATP Figure 8.3 Page 135 Efficiency of Aerobic Respiration • 686 kcal of energy are released • 7.5 kcal are conserved in each ATP • When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP • Efficiency is 270 / 686 X 100 = 39 percent (at best) • Most energy is lost as heat **Fats/Lipids = Glycerol and 3 fatty acids Glycerol is converted to PGAL and respired in glycolysis. The fatty acids are chopped into 2 carbon acetyl groups and used in the Krebs or citric acid cycle. **Proteins = amino acids The amino acids are sent to the liver where the liver removes the amine group. The left over acid is then used at some point in the Krebs cycle. FOOD fats glycogen complex carbohydrates proteins simple sugars amino acids Alternative Energy Sources fatty acids glycerol glucose-6-phosphate NH3 GLYCOLYSIS PGAL pyruvate acetyl-CoA Other organic molecules can be involved in respiration. Figure 8.11 Page 145 KREBS CYCLE urea carbon backbones Evolution of Metabolic Pathways • When life originated, atmosphere had little (none) free oxygen • Earliest organisms used anaerobic pathways • Later, noncyclic pathway of photosynthesis increased atmospheric oxygen • Cells arose that used oxygen as final acceptor in electron transport and were more efficient Processes Are Linked sunlight energy PHOTOSYNTHESIS water + carbon dioxide sugar molecules oxygen AEROBIC RESPIRATION In-text figure Page 146 Linked Processes Photosynthesis Aerobic Respiration • Energy-storing pathway • Energy-releasing pathway • Releases oxygen • Requires oxygen • Requires carbon dioxide • Releases carbon dioxide • Makes carbs • Breaks carbs