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How Cells Harvest Chemical Energy

How Cells Harvest Chemical Energy. Chapter 6. Overview. Photosynthesis Aerobic respiration Anaerobic respiration Alternate sources of energy. Components of a Reaction. Reactants Intermediates Products. A. B. C. Endergonic vs. Exergonic Reactions. Endergonic = Energy-requiring

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How Cells Harvest Chemical Energy

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  1. How Cells Harvest Chemical Energy Chapter 6

  2. Overview • Photosynthesis • Aerobic respiration • Anaerobic respiration • Alternate sources of energy

  3. Components of a Reaction Reactants Intermediates Products A B C

  4. Endergonic vs. Exergonic Reactions Endergonic = Energy-requiring Exergonic = Energy-releasing

  5. Redox Reactions One molecule gives up electrons = oxidized One molecule gains electrons = reduced H+ atoms released simultaneously (are attracted to negative charge of electrons) Coenzymes pick up e-s & H+ from substrates & deliver to e- transfer chains

  6. Electron Transfer Chains Membrane-bound groups of enzymes/molecules Accept & give up e-s in sequence E-s enter chain at higher energy level than when they leave it (lose energy at each descending step of chain) e-s

  7. Substrate-Level Phosphorylation Formation of ATP by direct transfer of Pi group to ADP from intermediate

  8. NAD+ & FAD Coenzymes 1. Accept e-s & H+ from intermediates that form during glucose catabolism Become reduced = NADH & FADH2 • NADH and FADH2 give up e-s & H+ to e- transfer chains during final stages of aerobic respiration Become oxidized = NAD+ & FAD

  9. Autotrophs “Self-nourishing” Synthesize own food Obtain energy & organic compounds (e.g. C) from the physical environment

  10. Chemoautotrophs Have no enzymes to allow for complex metabolic reactions Obtain energy & C from simple inorganic & organic compounds e.g. CH4, H2S

  11. Photoautotrophs Contain light-sensitive molecules Can split H2O & use electrons Process releases lots of oxygen, which reacts rapidly with metals & creates toxic free radicals

  12. Early photoautotrophs existed when there was lots of Fe & metals everywhere Released O2 oxidized these metals & rusted them out O2 could then be released freely

  13. Over a few thousand years, O2 levels in sea & atmosphere increased Survival of the fittest: Most anaerobes died out because couldn’t neutralize toxic O2 radicals Chemoautotrophs with little or no O2 tolerance restricted to extreme & anoxic environments

  14. As O2 accumulated in the atmosphere, O atoms combined to form O3 = ozone layer (protects against lethal UV radiation from sun) Life was able to move out from the “darks” & live under open sky = diversification = evolution

  15. Photosynthesis The process by which photoautotrophs use light energy from the sun to make glucose, which can then be converted into ATP 12H2O + 6CO2 6O2 + C6H12O6 + 6H2O

  16. Respiration = breathing Cellular respiration = getting energy from food

  17. Organisms need usable energy in order to survive Obtained energy is converted into ATP chemical bond energy Can be used to do work e.g. metabolism

  18. Anaerobes Can’t tolerate O2 Make ATP via fermentation 1 glucose → 2 ATP e.g. first organisms, some prokaryotes & eukaryotes Clostridium difficile

  19. Aerobes Require O2 Make ATP via aerobic respiration (many also use anaerobic pathways) 1 glucose → 36+ ATP (vital for survival of large organisms) e.g. most eukaryotes, some prokaryotes

  20. Facultative Anaerobes Normally use aerobic pathways (i.e. use O2) Can switch to anaerobic pathways when O2 levels are low Entamoeba histolytica

  21. Mitochondria Membrane-bound organelles in most eukaryotic cells (# differs depending on cell type) Power source of cells • Production of ATP in presence of O2 • Convert NADH and FADH2 into ATP energy via oxidative phosphorylation Allow cell to produce lots of ATP simultaneously • Without mitochondria, complex animals wouldn’t exist

  22. Mitochondrion Structure Outer membrane Selectively permeable Inner membrane Highly impermeable Contains ATP synthase Has membrane potential Cristae ↑ surface area of inner membrane, which ↑ capacity to generate ATP Matrix Contains 100s of enzymes which oxidize pyruvate and fatty acids, and control the Krebs cycle

  23. Cellular Respiration The oxidation of food molecules (e.g. glucose) into CO2 & H2O Energy released is captured as ATP Used for all endergonic activities of cell Enzymes catalyze each step Intermediates formed at one step become substrates for enzyme at next step

  24. 2 phases of cellular respiration: Glycolysis: Glucose → 2 pyruvates Occurs in all cells Oxidation of pyruvate into CO2 and H2O: Energy-releasing pathways differ depending on cell & its needs

  25. 40% of energy from glucose is harvested Rest (60%) is lost as heat A working muscle uses 10 million ATP per second!

  26. Aerobic Respiration C6H1206 + 6O2→ 6CO2 + 6H2O Breakdown of glucose in presence of O2 3 stages of reactions: • Glycolysis • Krebs Cycle • Electron Transfer Phosphorylation

  27. Glycolysis • Glucose → 2 pyruvates • Occurs in cytosol Krebs Cycle • Pyruvate → CO2 + H2O + e-s • Occurs in mitochondria Electron Transfer Phosphorylation • Formation of lots of ATP

  28. Stage I: Glycolysis Glucose → 2 pyruvates “Universal energy-harvesting process of life” Initial energy-releasing mechanism for all cells Occurs in cytosol Coupled endergonic & exergonic reactions

  29. Endergonic Steps of Glycolysis Requires input of 2 ATP ATP #1 phosphorylates glucose Glucose → intermediate ATP #2 transfers Pi to intermediate Intermediate → PGAL + DHAP DHAP converts into PGAL = 2 PGAL enter next stage

  30. Exergonic Steps of Glycolysis Each PGAL gives 2 e-s + H+ to NAD+ 2 NAD+→ 2 NADH Intermediates each give Pi to ADP 2 ADP → 2 ATP (substrate-level phosphorylation) Pays back 2 ATP used in endergonic steps Intermediates each release H+ + OH 2 intermediates → 2 PEP Each PEP gives Pi to ADP 2 ADP → 2 ATP (substrate-level phosphorylation) 2 PEP → 2 pyruvate

  31. Sum Total of Glycolysis Glucose → 2 pyruvate + 2 NADH + 2 ATP From here, pyruvate can enter: • Aerobic pathway (Krebs cycle) • Anaerobic pathway (fermentation) (depends on cell & environmental conditions)

  32. Stage II: Krebs Cycle Pyruvate → CO2 + H2O (+ e-s) a.k.a. citric acid cycle Occurs in mitochondria Main function is to supply Stage III with e-s (in order to reduce NAD+ & FAD in stage III)

  33. Mitochondrial membrane proteins transport pyruvate into inner compartment Enzymes take 1 C from pyruvate C + O2→ CO2 Intermediates + coenzyme A → acetyl-CoA NAD+ is reduced into NADH

  34. Acetyl-CoA enters Krebs cycle Transfers 2 Cs to oxaloacetate → citrate Rearrangement of intermediates occurs 2 C released → 2CO2 3 NAD++ H+ + e-s → 3 NADH ADP + Pi→ ATP FAD + H+ + e-s → FADH2 Oxaloacetate regenerates so that cycle can run again

  35. In total, one turn of the cycle: 3 NADH + 1 FADH2 + 1 ATP Cycle repeats again for 2nd pyruvate molecule Remember 1 glucose → 2 pyruvates After both pyruvates are broken down: 6 NADH + 2 FADH2 + 2 ATP

  36. Sum Total of the Krebs Cycle With 2 NADH from acetyl-CoA formation: 2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2 CO2 released into surroundings NADH & FADH2 deliver e-s and H+ to 3rd stage

  37. Stage III: Electron Transfer Phosphorylation H+ + e-s→ H2O + ATP E-s delivered to electron transfer chains (ETCs) in inner mitochondrial membrane E- flow in ETCs drives phosphorylation of ADP → ATP (lots of it!)

  38. a.k.a. oxidative phosphorylation ATP formed by oxidation of NADH & FADH2 Responsible for high ATP yield

  39. NADH & FADH2 give e-s to ETCs Simultaneous release of H+ Energy released at each transfer of ETC At 3 transfers, released energy pumps H+ across mitochondrial membrane into outer compartment Concentration & electric gradients result across inner membrane = membrane potential

  40. H+ re-enters inner compartment by flowing down concentration gradient through ATP synthases Causes reversible change in shape of ATP synthases ADP + Pi→ ATP (oxidative phosphorylation) At end of ETCs, O2 picks up e-s & H+→ H2O

  41. Sum Total of ET Phosphorylation H+ + e-s→ H2O + 32 ATP In liver, heart, and kidney: • e-s from NADH delivered to different ETC entry point • H+ gradient makes 3 ATP (instead of 1) • Results in 34 ATP total

  42. Animation of ET Phosphorylation • http://vcell.ndsu.nodak.edu/animations/etc/movie.htm • http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter9/animations.html#

  43. In oxygen-starved cells, e-s have nowhere to go so get gridlocked No e- flow = no H+ gradients = no ATP forms Results in cell death because not enough ATP to sustain metabolic processes

  44. 3 different categories of poisons interfere with cellular respiration: • ETC Blockers • Inhibitors • Uncouplers

  45. ETC Blockers Block ETC at various steps of chain Starves cells of energy by prohibiting ATP synthesis

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