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Cell Respiration Chapter 9

Cell Respiration Chapter 9. Cellular respiration is the combustion of fuels (principally glucose ) for the release of energy to perform cell functions. The reactions that characterize respiration maintain order by supplying energy for the struggle against entropy.

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Cell Respiration Chapter 9

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  1. Cell Respiration Chapter 9

  2. Cellular respiration is the combustion of fuels (principally glucose) for the release of energy to perform cell functions. • The reactions that characterize respiration maintain order by supplying energy for the struggle against entropy.

  3. The energy released during respiration is stored in ATP molecules, which are in turn broken down by the cell and the released energy is used for movement, reproduction, transport, and (perhaps most importantly) to drive cellular reactions. • This energy release* occurs in small quantities (units of ATP) by enzyme-controlled reactions that occur in series (metabolic pathways). *Please note that cellular respiration does not produce energy, it simply releases it from food molecules

  4. To summarize, making ATP is the fundamental function of respiration.Following that, the ATP is used to perform cellular work.

  5. The Summary Chemical Equation for Respiration • C6H12O6 + 6O2 --------> 6CO2 + 6H2O + Energy (38 ATPs) • Cells may “burn” organic molecules other than glucose, but glucose is most frequentlymetabolized.

  6. A typical eukaryotic cell transfers only about 38% of glucose’s energy to ATP molecules. (Most of the rest is converted to heat.) In the absence of oxygen (anaerobic respiration), onlyabout 2% of glucose’s energy is transferred to ATP.

  7. Please recall that there are only 2 ways to synthesize ATP during cellular respiration*: • 1. Substrate-level Phosphorylation (4 ATPs) • 2. Chemiosmotic Phosphorylation (34 ATPs) • (ATP can also be synthesized by photophosphorylation during photosynthesis.

  8. How do cells extract energy from organic food molecules? • Most of the energy available to a cell is contained in the specificarrangement of electrons in the chemical bonds that hold glucose (or any food fuel) together. • During cellular respiration, glucose is broken apart in a series of steps. As these steps occur, electrons are rearranged and pulled away from the food molecules by the coenzymes NAD+ and FAD. The total energy yield from cellular respiration (38 ATPs) comes from substrate level phosphorylation andtheuse of these electrons to drive ATP synthesis (chemiosmosis).

  9. In each step, molecules start where they have more energy stored in their electrons, and end up at a lower energy level. • •Thus, a cell transfers energy from glucose to ATP by coupling exergonic reactions with endergonic ones.

  10. Below, rather than seeing electron movements, you see changes in hydrogen atom distribution. BUT, these hydrogen movements represent electron transfers because each hydrogen atom consists of one proton and one electron. Loss of hydrogen atoms C6H12O6+ 6O2 --------> 6CO2 + 6H2O + Energy (ATPs) Gain of hydrogen atoms

  11. As we will see, the O2 serves as the ultimate recipient of electrons. This is because its atoms have a strong tendency to pull electrons away from other atoms. • As a result, the electrons stripped from glucose finally end up in H2O.

  12. Glucose is a rich source of electrons. These electrons are removed during Glycolysis (the 1st reaction series in Cellular Respiration), the Transition Reactions (2nd reaction series), and Kreb's Cycle (the 3rd series - a "cycle") and transferred to the mitochondrial crista where they are "cashedin" for ATPs during the Electron Transport Chain (Chemiosmosis).

  13. The model above reveals that 34 of 38 ATPs generated during cellular respiration are produced by chemiosmosis. Substrate-level phosphorylation accounts for the other 4 ATPs.

  14. The Mitochondrion - The ATP • The membrane surrounding the mitochondrion is called the mitochondrial membrane orthe outer membrane. • •The membrane within the interior of themitochondrion is called the inner membrane. Theprominences that result from the infoldings and outfoldingsof this membrane are thecristae. The space inside the inner membrane is the matrix.

  15. Thus, the inner membrane compartmentalizes the interior of the mitochondrion into separate spaces with different compositions and particle concentrations. • A second function of the inner membrane is to embed the proteins that perform the cellular work of pumping protons. Five proteins, collectively, make up the Electron Transport Chain (ETS), and three of them are proton pumps. • The ETS proteins (Complexes I and II) accept the electrons that were stripped from glucose and shuttled to the inner membrane by the coenzymes NAD+ and FAD.

  16. Note that the five proteins are arranged in a sequence, one after another,so that electron transfers can occur efficiently. • The three integral proteins are proton pumps, and the two mobile characters (Ubiquinone and Cytochrome C) move rapidly alongthe membrane, shuttling electronsbetween the three large complexes. • •Also embedded in the inner membrane is the ATP Synthase Complex (or Port).

  17. During chemiosmosis, electrons transferred from glucose (as it is decomposed) by the reduced coenzymes NADH and FADH2 are passed from one proteincomplex to the next, releasing energy that is used to pump protons from the matrix into the intermembrane space.

  18. Overview of the Four Stages of Cellular Respiration

  19. Stage I: Glycolysis • 2 ATPs worth of energy are used (input) to phosphorylate glucose. • Upon the splitting ofbiphosphorylated glucose, enough energy isgiven off to generate 4 ATPs worth of energy. • The energy yield of glycolysis, then, is 2 net ATPs*.

  20. Additionally, 2 NAD+ molecules accept 2 electrons each (and 1proton) to become 2 NADH – these 2 NADHs will be “cashed in” during chemiosmosis! • (Note: Each NADH “stores” enough energy to generate 3 ATPs during chemiosmosis.) • Two molecules of 3-C pyruvate are the “products” of glycolysis.

  21. Both pyruvates (3-C) enter the mitochondrion and proceed through the Transition Reactions (Stage 2) in oxygen’s presence. • •In oxygen’s absence, fermentation occurs! (Fermentation will be discussed at the end of this body of notes) • •Glycolysis, then, is a series of ten* reactions (each catalyzed by a specificenzyme) that occur to break down glucose to pyruvate. It results in a net yield of 2 ATPs and 2 NADH.

  22. Glycolysis Reactants and Products • UsedProduced • 1 Glucose (C6H12O6) 2 Pyruvic Acids (C3H4O3) • 4 ADP 4 ATP • 4 Phosphate Groups 2 Phosphate Groups • 2 ATP 2 ADP • 2 NAD+ 2 NADH • 2 H+ • 2 H2O

  23. Glycolysis may be regarded as a series of preparatory reactions, to be followed by either fermentation (under anaerobic conditions) or by cellular respiration (when oxygen is available) -The Transition Reactions, Krebs Cycle, The Electron Transport Chain (ETC). • •Note that no CO2 formation has occurred (yet). All carbon atoms are still accounted for. http://www.youtube.com/watch?v=3GTjQTqUuOw

  24. Stage II: The Transition Reactions (AKA Pyruvate Oxidation) • The glycolysis end-product pyruvate is transported into the interior of the mitochondrion. • Within the mitochondrial matrix, pyruvate is oxidized to the two-carboncompound, acetate (CH3CO-), freeing one molecule of CO2. (The lowenergy carboxyl group is stripped away and converted to CO2.) • Remember: glycolysis produced 2 molecules of pyruvate – the other pyruvate is also oxidized, releasing a 2nd molecule of CO2.

  25. The remaining two-carbon fragment is oxidized to form acetate, releasing 2 electrons – these electrons are enzymatically joined to NAD+ to make NADH (a hydrogen proton is also added). • In this process, the acetate is linked to a coenzyme, called coenzyme A (CoA*) producing the energy-rich compound called acetyl coenzyme A (famously known as Acetyl CoA). *CoA, like other coenzymes, acts like a taxicab to shuttle acetate to Krebs Cycle.

  26. During the Transition Reactions: • 2 molecules of by-product CO2are produced. • 2 molecules ofNAD+arereduced to NADH. Thesemolecules will be“cashed in” during chemiosmosis (Stage IV). (Note: Each NADH “stores” enough energy to generate 3 ATPs during chemiosmosis.) • Acetyl CoA will feed its acetate to Krebs Cycle for further oxidation

  27. Stage III: The Krebs Cycle http://www.phschool.com/science/biology_place/biocoach/cellresp/continuer3.html • A cyclic series of eight reactions, each catalyzed by a specific enzyme. • During each turn of the Krebs Cycle, one acetate (2-C) is fed from Acetyl CoA to oxaloacetate (4-C) to formcitrate (6-C). • Through the cycle, citrate (6-C) is oxidized back to oxaloacetate (4-C), releasing 2 CO2 molecules. • This means that the cycle starts and ends with oxaloacetate, and the cycle keeps “turning” as long as acetate is “dumped in”.

  28. One glucose molecule, when oxidized during Glycolysis and theTransition Reactions, produces 2 acetates, so the cycle turnstwice for each glucose molecule metabolized

  29. Each single turn of the cycle produces 1 ATP (via substrate-levelphosphorylation)

  30. Each turn of the cycle strips enough (6) electrons to reduce 3NAD+ to 3 NADH. (Note: Each NADH “stores” enough energy to generate 3 ATPs duringchemiosmosis.)

  31. Each turn of the cycle strips an additional 2 electrons to convert FAD to FADH2. (Note: Each FADH2 “stores” enough energy to generate 2 ATPs during chemiosmosis.)

  32. Energy Summary of Krebs cycle (2 Turns) • 6 NADH (Note: Each NADH “stores” enough energy to generate 3 ATPs during chemiosmosis.) • 2 FADH2 (Note: Each FADH2 “stores” enough energy to generate 2 ATPs during chemiosmosis.) • 2 ATPs via substrate-level phosphorylation

  33. The Krebs Cycle is also known as the Tri Carboxylic Acid cycle (TCA) and the Citric Acid Cycle. • The Krebs Cycle occurs within the mitochondrion. • The Krebs Cycle depends upon continued NAD+ and FAD availability. If NADH and FADH2 are not oxidized during chemiosmosis, The Krebs Cycle stops http://www.youtube.com/watch?v=-cDFYXc9Wko

  34. Here’s a Visual To Help Make The Krebs Cycle More Easily Understood

  35. Just Kidding!!!!!! That Was A Joke!!!!!! HERE’S A Krebs Cycle Diagram That Makes Sense Of The Process

  36. IV: The Electron Transport Chain (ETC) • Stated simply, the electrons harvested from Stage I (Glycolysis*), Stage II (TheTransition Reactions), and Stage III (Krebs Cycle) are transported by NADH and FADH2to an Electron Transport Chain embedded in the inner mitochondrial membrane.

  37. The electrons shuttled by these two coenzymes are transferred to protein complexes embedded in the inner mitochondrial membrane. As they are passed from one protein to the next in oxidation-reduction exchanges, the energy is used to pump protons (that were also stripped from glucose and transported by the coenzymes) through the membrane into theintermembranespace.

  38. The protons (in their highly concentrated state) MUST follow the Law ofDiffusion, but their only exit is through a protein complex called an ATP synthase complex (or “ATP synthase port”). • As hydrogen ions (AKA protons) diffuse through the ATP synthase complexes, the energy from this downhill flow is used to drive the synthesis of ATP from ADP and free inorganic phosphate (Pi) functional groups. • The last protein in the embedded ETC passes its 2 electrons to an oxygen atom. This highly electronegative oxygen atom picks up 2 protons from the mitochondrial matrix and becomes water.

  39. Because the mitochondrial membrane is impermeable to NADH, the 2 NADHs produced during glycolysis (outside of the mitochondrion) must “drop off” their electrons at the outer mitochondrial membrane to be actively transported to the mitochondrial matrix. This task requires 2 ATPs worth of energy. These 2 ATPs, then, are subtracted from the gross eukaryotic cellular respiration ATP yield (38 – 2) to result in a net yield of 36 ATPs in eukaryotic cells. Prokaryotic cells that utilize chemiosmosis* do not have mitochondria, so the need for this extra energy expenditure is eliminated and the yield is 38 ATPs.

  40. Prokaryotes lack both chloroplasts and mitochondria, but some bacteria have evolved a method of chemiosmosis by utilizing their plasma membrane as a "hydrogen ion separator". • The ATP that is generated by chemiosmosis in this way can be used by the bacterium to pump nutrients and waste products across the membrane AND to rotate their flagella. http://www.youtube.com/watch?v=xbJ0nbzt5Kw

  41. The Total Net Energy Yield of Cell Respiration • StageNet Energy Yield • Glycolysis 2 ATPs* • Kreb’sCycle2 ATPs* • Electron Transport Chain32 or 34*** ATPs* • ************************************************** • Total Net Energy Yield of36 or 38*** ATPs • Aerobic Respiration • * Substrate-level Phosphorylation • ** Chemiosmotic Phosphorylation • ***36 in Eukaryotic Cells, 38 in Prokaryotic Cells

  42. The ATP Yield In Prokaryotic Cells

  43. The ATP Yield In Eukaryotic Cells The discrepancy occurs here! See Pg. 11!

  44. Fermentation is an Anaerobic Alternative to Aerobic Respiration • Yeast Fermentation • •Single-celled fungi normally respire aerobically, but can survive anaerobically on 2 ATPs* of energy from each glucose molecule. *(These two ATPs result from glycolysis) • In the anaerobic reaction series, glucose is the starting material, and ethanol and 2 CO2 are the products, along with the 2 ATP molecules generated during glycolysis.

  45. This process is called alcoholic fermentation and it occurs in yeast and some species of bacteria under anaerobic conditions. • NADH is oxidized to provide NAD+ for additional glycolysis. • Glycolysis will not occur in the absence of NAD+ • The value of fermentation to yeast and bacteria is seen in the oxidation of NADH, so the yeast cell has a constant supply of NAD+ to complete more glycolysis so it has ATPs to stay alive!

  46. Even though ethanol is energy-rich, it is toxic to the organism that producesit. Yeasts release their alcohol wastes to their surroundings, but die if the alcohol becomes too concentrated.

  47. Animal Cell Fermentation • Many types of bacterial, fungal, and animal cells can survive in oxygen’s absence by utilizing lactic acid fermentation. • In this category of fermentation, lactic acid is the end product.

  48. Like alcohol, lactic acid is energy-rich. Upon its production in mammals, it is picked up by blood and carried to the liver for econstitutioninto pyruvate. • No CO2 is given off.

  49. This is the type of respiration used by muscle cells in oxygen’s absence. • •The value of lactic acid fermentation is seen in the oxidation of NADH, so the animal cell has a constant supply of NAD+to complete more glycolysis so it has ATPs to stay alive!

  50. The accumulation of lactic acid in muscle cells causes pain and fatigue. This is the familiar “burn” experienced during anaerobic exercise. • The dairy industry uses lactic acid fermentation to make cheese and yogurt! • •Net Yield = 2 ATPs http://www.dnatube.com/video/5078/Fermentation-Anaerobic-respiration-Lactic-Acid-and-Ethanol

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