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Cellular respiration (glycolysis, TCA and ETC)

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A step by step discussion of vital processes in the release of energy from biomolecules by cells.

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Cellular respiration (glycolysis, TCA and ETC)

  1. 1. NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM TOPIC; •CELLULAR RESPIRATION Lecturer: Dr. G. Kattam Maiyoh 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  2. 2. Learning Objectives • Explain why cells need breakdown biomolecules (E.G. glucose) • Describe the basic steps in; – Glycolysis, – The TCA cycle, – The electron transport chain (ETC) • Summarize the energy yield of all above steps cellular respiration 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  3. 3. Overview of Cellular Respiration 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  4. 4. Overview of Cellular Respiration • Cellular respiration is the step-wise release of energy from carbohydrates and other molecules; energy from these reactions is used to synthesize ATP molecules. • This is an aerobic process that requires oxygen (O2) and gives off carbon dioxide (CO2), and involves the complete breakdown of glucose to carbon dioxide and water. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  5. 5. • Metabolism refers to all the chemical reactions of the body – some reactions produce the energy stored in ATP that other reactions consume – all biological molecules will eventually be broken down and recycled or excreted from the body 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  6. 6. Catabolism and Anabolism • Catabolic reactions breakdown complex organic compounds – providing energy (exergonic) – glycolysis, Krebs cycle and electron transport • Anabolic reactions synthesize complex molecules from small molecules – requiring energy (endergonic) • Exchange of energy requires use of ATP (adenosine triphosphate) molecule. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 25-6
  7. 7. ATP Molecule & Energy a b • Each cell has about 1 billion ATP molecules that last for less than one minute • Over half of the energy released from ATP is converted to heat GKM/NSB 211: DIGESTIVE SYSTEM 11/20/13 NUTRITION AND METABOLISM/2013 25-7
  8. 8. Mechanisms of ATP Generation • Phosphorylation is the addition of phospahate group. – bond attaching 3rd phosphate group contains stored energy • Mechanisms of phosphorylation – within animals • substrate-level phosphorylation in cytosol • oxidative phosphorylation in mitochondria – in chlorophyll-containing plants or bacteria • photophosphorylation. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND25-8 METABOLISM/2013
  9. 9. Phosphorylation in Animal Cells • In cytoplasm (1) • In mitochondria (2, 3 & 4) 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 25-9
  10. 10. • (Insert Fig. 7.4a) 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 Sld 38
  11. 11. Carbohydrate Metabolism--In Review • In GI tract – polysaccharides broken down into simple sugars – absorption of simple sugars (glucose, fructose & galactose) • In liver – fructose & galactose transformed into glucose – storage of glycogen (also in muscle) • In body cells --functions of glucose – oxidized to produce energy – conversion into something else – storage energy as triglyceride in fat 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM 25-11 NUTRITION AND METABOLISM/2013
  12. 12. Glucose Movement into Cells • In GI tract and kidney tubules, Na+/glucose symporters • Most other cells, GluT facilitated diffusion transporters move glucose into cells • Glucose 6-phosphate forms immediately inside cell (requires ATP) thus, glucose hidden in cell • Concentration gradient favorable for more glucose to enter 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 25-12
  13. 13. Glucose Catabolism • Cellular respiration – 4 steps are involved – glucose + O2 produces H2O + energy + CO2 • Anaerobic respiration – called glycolysis (1) – Results in formation of acetyl CoA (2) is transitional step to Krebs cycle • Aerobic respiration – Krebs cycle (3) and electron transport chain (4) 11/20/13 GKM/NSB 211: DIGESTIVE SYSTE M NUTRITION AND METABOLISM/2013 25-13
  14. 14. • Each step of cellular respiration requires a separate enzyme. • Some enzymes use the oxidationreduction coenzyme NAD+ (nicotinamide adenine dinucleotide). • When a metabolite is oxidized, NAD+ accepts two electrons plus a hydrogen ion (H+) and NADH results; NAD+ can also reduce a metabolite by giving up electrons. • FAD (flavin adenine dinucleotide) is sometimes used instead of NAD+. GKM/NSB 211: DIGESTIVE SYSTEM 11/20/13 NUTRITION AND METABOLISM/2013
  15. 15. 6 CH OPO 2− 2 3 5 O H 4 OH H OH 3 H H 2 H 1 OH OH glucose-6-phosphate Glycolysis takes place in the cytosol of cells. Glucose enters the Glycolysis pathway by conversion to glucose-6-phosphate. Initially there is energy input corresponding to cleavage of two ~P bonds of ATP. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  16. 16. 6 CH2OH 5 H 4 OH O H OH H 2 3 H OH glucose 6 CH OPO 2− 2 3 5 O ATP ADP H H 1 OH Mg2+ 4 H OH OH 3 H 1 H 2 OH Hexokinase H OH glucose-6-phosphate 1. Hexokinase catalyzes: Glucose + ATP  glucose-6-P + ADP The reaction involves nucleophilic attack of the C6 hydroxyl O of glucose on P of the terminal phosphate of ATP. ATP binds to the enzyme as a complex with Mg++. GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  17. 17. Glycolysis & Fate of Pyruvic Acid • Breakdown of six-carbon glucose molecule into 2 threecarbon molecules of pyruvic acid – 10 step process occurring in cell cytosol – produces 4 molecules of ATP after input of 2 ATP – utilizes 2 NAD+ molecules as hydrogen acceptors 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 25-17
  18. 18. 10 Steps of Glycolysis 11/20/13 GKM/CHE 214/LEC 03/SEM 02/2011 GKM/NSB 211: DIGESTIVE SYSTEM 25-18 NUTRITION AND METABOLISM/2013
  19. 19. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  20. 20. If O2 shortage in a cell •Pyruvic acid is reduced to lactic acid so that NAD+ will be still available for further glycolysis •This process is known as fermentation •Lactic acid rapidly diffuses out of cell to blood •Liver cells remove it from blood & convert it back to pyruvic acid 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  21. 21. Why does fermentation occur? Pyruvate is reduced to lactate when oxygen is not available because fermentation uses NADH and regenerates NAD+. In this way NAD+ is now free to pick up more electrons during early steps of glycolysis; this keeps glycolysis going. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  22. 22. Two types of Anaerobic Respiration Fermentation-yeast Lactic Acid or lactate-muscles 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  23. 23. Advantages and Disadvantages of Fermentation • Fermentation can provide a rapid burst of ATP in muscle cells, even when oxygen is in limited supply. • Lactate, however, is toxic to cells. • Initially, blood carries away lactate as it forms; eventually lactate builds up, lowering cell pH, and causing muscles to fatigue. • Oxygen debt occurs, and the liver must reconvert lactate to pyruvate. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  24. 24. Efficiency of Fermentation • Two ATP produced during fermentation are equivalent to 14.6 kcal; complete oxidation of glucose to CO2 and H2O represents a yield of 686 kcal per molecule of glucose. • Thus, fermentation is only 2.1% efficient compared to cellular respiration. • (14.6/686) x 100 = 2.1% 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  25. 25. Glycolysis summary •Inputs: •Glucose •2 NAD+ •2 ATP •4 ADP + 2 P 11/20/13 •Outputs: •2 pyruvate •2 NADH •2 ADP •2 ATP (net gain) GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  26. 26. Transition Reaction • The transition reaction connects glycolysis to the citric acid cycle, and is thus the transition between these two pathways. • Pyruvate is converted to a C2 acetyl group attached to coenzyme A (CoA), and CO2 is released. • During this oxidation reaction, NAD+ is converted to NADH + H+; the transition reaction occurs twice per glucose molecule. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  27. 27. Formation of Acetyl Coenzyme A • Pyruvic acid enters the mitochondria with help of transporter protein • Decarboxylation – pyruvate dehydrogenase converts 3 carbon pyruvic acid to 2 carbon fragment (CO2 produced) – pyruvic acid is oxidized so that NAD+ becomes NADH • 2 carbon fragment (acetyl group) is attached to Coenzyme A to form Acetyl coenzyme A which enter Krebs cycle – coenzyme A is derived from pantothenic acid (B vitamin). 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 25-27
  28. 28. Krebs Cycle (Citric Acid Cycle) • Citric acid cycle – a cyclical oxidationreduction & decarboxylation reactions occurring in matrix of mitochondria • Gives off CO2 and produce one ATP per cycle; occurs twice per glucose molecule • It finishes the same as it starts (4C) – acetyl CoA (2C) enters at top & combines with a 4C compound – 2 decarboxylation reactions peel 2 carbons off again when CO2 is formed GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  29. 29. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  30. 30. THE TCA The names of the various enzymes in the previous slide are indicated in the figure below GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  31. 31. What happens in the cycle? • During the cycle, oxidation occurs when NAD+ accepts electrons in three sites and FAD accepts electrons once. • A gain of one ATP per every turn of the cycle; it turns twice per glucose. • During the citric acid cycle, the six carbon atoms in glucose become CO2. • The transition reaction produces two CO2, and the citric acid cycle produces four CO2 per molecule of glucose. 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  32. 32. Products of the Krebs Cycle • Energy stored in bonds is released step by step to form several reduced coenzymes (NADH & FADH2) that store the energy • In summary: each Acetyl CoA molecule that enters the Krebs cycle produces yields; – 2 molecules of CO2 • one reason O2 is needed – 3 molecules of NADH + H+ – one molecule of ATP – one molecule of FADH2 • Remember, each glucose produced 2 acetyl CoA molecules 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  33. 33. Citric acid cycle inputs and outputs per glucose molecule •Inputs: •2 acetyl groups •6 NAD+ •2 FAD •2 ADP + 2 P •Outputs: ½ of the above per cycle 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013 •4 CO2 •6 NADH •2 FADH2 •2 ATP
  34. 34. The Electron Transport Chain • Involves a series of integral membrane proteins in the inner mitochondrial membrane capable of oxidation/reduction • Each electron carrier is reduced as it picks up electrons and is oxidized as it gives up electrons • Small amounts of energy is released in small steps • Energy used to form ATP by chemiosmosis GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  35. 35. Chemiosmosis • Small amounts of energy released as substances are passed along inner membrane • Energy used to pump H+ ions from matrix into space between inner & outer membrane • High concentration of H+ is maintained outside of inner membrane • ATP synthesis occurs as H+ diffuses through a special H+ channel in inner membrane GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  36. 36. Steps in Electron Transport • Carriers of electron transport chain are clustered into 3 complexes that each act as proton pump (expel H+) • Mobile shuttles pass electrons between complexes • Last complex passes its electrons (2H+) to a half of O2 molecule to form a water molecule (H2O) GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  37. 37. Proton Motive Force & Chemiosmosis • • Buildup of H+ outside the inner membrane creates + charge – electrochemical gradient potential energy is called proton motive force ATP synthase enzyme within H+ channel uses proton motive force to synthesize ATP from ADP and P GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  38. 38. Energy yields from Glycolysis -TCA • Glycolysis and the citric acid cycle accounts for four ATP. • ETC accounts for 32 or 34 ATP, and the grand total of ATP is therefore 36 or 38 ATP. • Cells differ as to the delivery of the electrons from NADH generated outside the mitochondria. • If they are delivered by a shuttle mechanism to the start of the electron transport system, 6 ATP result; otherwise, 4 ATP result. GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013
  39. 39. • Most ATP is produced by the electron transport system and chemiosmosis. • Per glucose molecule, ten NADH and two FADH2 take electrons to the electron transport system; three ATP are formed per NADH and two ATP per FADH2. • Electrons carried by NADH produced during glycolysis are shuttled to the electron transport chain by an organic molecule. 7-39
  40. 40. A Summary of the Energy Yield of Aerobic Metabolism Figure 25.7
  41. 41. Thank you for listening !! 11/20/13 GKM/NSB 211: DIGESTIVE SYSTEM NUTRITION AND METABOLISM/2013

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