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Cellular Respiration

Cellular respiration and fermentation are crucial catabolic pathways that generate energy for life. This process involves the breakdown of organic molecules using oxygen, resulting in the production of ATP, the cell's energy currency. Learn about the different phases, including glycolysis, the Krebs cycle, and oxidative phosphorylation, and how they work together to produce energy.

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Cellular Respiration

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  1. Cellular Respiration

  2. Life is work, which requires E • In most ecosystems, E enters as sunlight • Light E trapped in O-molecules is available to both photosynthetic organisms and others that eat them

  3. Cellular respiration and fermentation are catabolic, energy-yielding pathways • E is stored in molecules • Enzymes catalyze the hydrolysis of high E molecules to low E products • Some of the released E is used to do work; the rest is lost as heat

  4. Fermentation – leads to the partial degradation of sugars in the absence of O2 • Cellular respiration – A more efficient and widespread catabolic process; uses O2 as a reactant to complete the breakdown of a variety of organic molecules • Carbohydrates, fats, and proteins can all be used as the fuel

  5. ATP: pivotal molecule in cellular energetics • It is the chemical equivalent of a loaded spring: • The close packing of 3 negatively-charged phosphate groups is an unstable, E-storing arrangement • Loss of the end phosphate group “relaxes” the “spring”

  6. Most cellular work converts ATP to ADP and inorganic phosphate (Pi) • Animal cells regenerate ATP from ADP and Pi by the catabolism of organic molecules

  7. The transfer of the last phosphate group from ATP to another molecule is phosphorylation • The receiving molecule changes shape, performing work (transport, mechanical, or chemical) • When the phosphate groups leaves the molecule, the molecule returns to its original shape

  8. Catabolic pathways relocate e-s stored in food molecules, releasing E used to synthesize ATP • Redox rxns: • Oxidation: loss of e-s • Reduction: addition of e-s • More generally: Xe- + Y  X + Ye- • X, the e- donor, is the reducing agent which is oxidized and reduces Y • Y, the e- recipient, is the oxidizing agent which is reduced and oxidizes X • Redox reactions require both a donor and acceptor

  9. O2: very potent oxidizing agent • An electron loses E as it shifts from a less electronegative atom to a more electronegative one • A redox rxn that relocates e-s closer to O2 releases chemical E that can do work • To reverse the process, E must be added to pull an e- away from an atom

  10. e-s “fall” from organic molecules to O2 during cellular respiration • C6H12O6 + 6O2 6CO2 + 6H2O • Glucose is oxidized, O2 is reduced, and e-s lose potential E • Molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of “hilltop” e-s that “fall” closer to O2

  11. The cell has a rich reservoir of e-s associated with hydrogen, especially in carbohydrates and fats • However, these fuels don’t spontaneously combine with O2 because they lack the activation E • Enzymes lower the barrier of activation E, allowing these fuels to be oxidized slowly • The “fall” of e-s during respiration is stepwise, via an ETC and NAD+

  12. Glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme • At key steps, H atoms are stripped from glucose and passed first to a coenzyme, like NAD+ (nicotinamide adenine dinucleotide) • Dehydrogenase enzymes strip two H atoms from the fuel, pass two e-s and one proton to NAD+ and release H+

  13. O2 and H2 could either combust and lose all of the E as heat, or through a step-by-step process, the E could be harnessed to form ATP

  14. Mitochondrion • Double membrane-bound organelle • Outer membrane • Inter- membrane space • Inner membrane • Matrix • Cristae: folds of inner membrane

  15. Cellular Respira-tion • 3 phases: • Glyco- lysis • Krebs cycle • ETC

  16. Substrate-level phosphorylation • Enzymes transfer phosphate from O-molecule to ADP • Occurs in glycolysis and the Krebs cycle

  17. Oxidative phosphorylation • Occurs due to the ETC • The transfer of e-s down the ETC to O2 powers the phosphorylation of ADP to ATP • Produces almost 90% of the ATP generated by respiration

  18. Glycolysis • Occurs in the cytoplasm • Glucose (6-C sugar) is split ultimately into two pyruvates • Process occurs regardless of the presence of O2 • No CO2 is produced • Net E yield per glucose: • 2 ATP • 2 NADH

  19. Glycolysis • Each of the ten steps in glycolysis is catalyzed by a specific enzyme • Kinase: phosphorylates • Isomerase: rearranges molecules to form isomers • Dehydrogenase: oxidizes O-molecules • 10 steps can be divided into two phases: • an E investment phase • an E payoff phase

  20. E investment phase • ATP provides activation E by phosphorylating glucose with 2 ATPs

  21. E payoff phase

  22. Krebs Cycle • Pyruvate can now enter the mitochondrion • The Krebs cycle can only accept a 2-C molecule • So, a multi-enzyme complex breaks down the 3-C pyruvate to the 2-C acetate • This process is known as pyruvic acid breakdown

  23. Pyruvic Acid Breakdown • An enzyme rips off a carboxyl group in the form of CO2 to make acetate • In the process, NAD+ is reduced to NADH • Coenzyme A then grabs the acetate, making acetyl CoA, and carries it off to the Krebs cycle

  24. Pyruvic Acid Breakdown

  25. Pyruvic Acid Breakdown

  26. Krebs Cycle • Occurs in the matrix of the mitochondrion • The acetate from acetyl CoA bonds to oxaloacetate to form citrate • The cycle ultimately recycles oxaloacetate, releasing CO2, ATP, NADH, and FADH2 in the process

  27. Krebs Cycle

  28. Krebs cycle, per glucose, produces a total of: • 8 NADH • 2 FADH2 • 2 ATP • 6 CO2 • Krebs cycle, per pyruvate, produces half of the above totals

  29. ETC • Only 4 of the 38 ATPs that are formed in Cell Resp are formed by substrate-level phosphorylation • The other 34 come from the E from the e-s carried by NADH and FADH2 • ETC is a chain of proteins found in the inner membrane of the mitochondrion • It’s folded (cristae) to increase surface area

  30. ETC • Thousands of ETCs are found on the cristae of a single mitochondrion • NADH and FADH2 are oxidized as they dump their e-s into the ETC • FADH2 has less free E  it dumps its e-s later in the ETC  fewer H+ move across the membrane

  31. ETC

  32. ETC • The e-s drop in free E as they are passed along the ETC • This loss of E drives H+ across the inner membrane from the matrix to the inter-membrane space (start of chemiosmosis) • The high [H+] creates the proton-motive force: ability of the proton gradient to do work

  33. ETC • Each NADH contributes enough E to generate a maximum of 3 ATP • In some eukaryotic cells, NADH produced in the cytosol by glycolysis may only be worth 2 ATP • The e-s must be shuttled to the mitochondrion • In some shuttle systems, the e-s are passed to NAD+, in others the e-s are passed to FAD • Each FADH2 can be used to generate about 2 ATP

  34. ETC

  35. ETC • The high proton concentration flows through ATP synthase from inter-membrane space to the matrix • ATP synthase makes ATP from ADP and Pi

  36. ETC

  37. ETC • Chemiosmosis is not unique to mitochondria • Plant cells use a very similar system in the chloroplasts • Prokaryotes generate H+ gradients across their plasma membrane

  38. Anaerobic Respiration • Up to 38 ATPs can be generated when O2 is present • What happens when there is no O2? • Glucose can still be oxidized to make ATP…much less ATP

  39. Anaerobic Respiration • Glycolysis still takes place in the cytosol, which still produces: • 2 (net) ATP • 2 NADH • 2 pyruvate • Pyruvate is harmful to cells and must be broken down

  40. Anaerobic Respiration • The process is called fermentation • Alcoholic fermentation • Lactic acid fermentation

  41. Alcoholic Fermentation • The NADH made in glycolysis is used to convert pyruvate into ethanol and CO2 • Glucose  pyruvate  acetaldehyde + CO2 ethanol

  42. Alcoholic Fermentation • Utilized by yeast in the absence of O2 • CO2 produced by fermentation allows bread to rise • Ethanol utilized in production of beer and wine

  43. Alcoholic Fermentation

  44. Lactic Acid Fermentation • The NADH made in glycolysis is used to convert pyruvate into lactate • Glucose  pyruvate  lactate

  45. Lactic Acid Fermentation • Some fungi and bacteria are used to make cheese, yogurt, sour cream, sauerkraut, and pickles • Muscle cells switch from aerobic to anaerobic when O2 is depleted • NADH and FADH2cannot dump their e-s into the ETC • Krebs cycle stops making NADH and FADH2, i.e., the cycle stops • Pyruvate is not broken down

  46. Lactic Acid Fermentation • Lactate may cause muscle fatigue, but it is ultimately converted back into pyruvate in the liver

  47. Aer- vs. Anaerobic Respiration • Both perform glycolyis • Both generate ATP • Aerobic generates 38 ATP; both substrate-level and oxidative phosphorylation • Anaerobic generates 2 ATP; only substrate-level phosphorylation • Both use NADH as e- carrier

  48. Facultative Anaerobes • Yeast and many bacteria (and humans at the cellular level) can perform either type

  49. Obligates • Obligate aerobes: must live where O2 is present • Obligate anaerobes: must live where O2 is not present

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