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Cellular Respiration

Cellular Respiration. Pp 69 – 73 & 217 - 237. Define cell respiration. Cell respiration is the controlled release of energy from organic compounds in cells to form ATP Glucose is the major substrate for respiration

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Cellular Respiration

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  1. Cellular Respiration Pp 69 – 73 & 217 - 237

  2. Define cell respiration • Cell respiration is the controlled release of energy from organic compounds in cells to form ATP • Glucose is the major substrate for respiration • Adenosine triphosphates (ATP) is the molecule which directly fuels the majority of biological reactions.

  3. Why cell respiration? • Cells require a constant source of energy to perform various tasks e.g. • Movement • Transport • Division

  4. Types of Respiration: (i) Anaerobic Respiration (ii) Aerobic Respiration • Occurs in the absence of Oxygen • Occurs in the cells’ cytoplasm • Yields small amount of ATP (2 molecules) per molecule of glucose • Involves fermentation of pyruvate to lactate in muscles/CO2 & ethanol in plant & yeast • Occurs in presence of Oxygen • Occurs in the cells’ mitochondria • Yields large amount of ATP (38 molecules) per molecule of glucose • Does not involve fermentation

  5. Types of Respiration (i) Anaerobic Respiration (ii) Aerobic Respiration Occurs in the absence of Oxygen Occurs in presence of Oxygen Occurs in the cells’ cytoplasm Occurs in the cells’ mitochondria Yields small amount of ATP (2 molecules) per molecule of glucose Yields large amount of ATP (38 molecules) per molecule of glucose Involves fermentation of pyruvate to lactate in muscles/CO2 & ethanol in plant & yeast Does not involve fermentation

  6. Comparison between Aerobic & Anaerobic Respiration

  7. Adenosine triphosphate (ATP): • ATP the molecule which directly fuels the majority of biological reactions • About 1025 ATP molecules are hydrolysed to ADP and Pi daily • ADP is reduced back to ATP using the free energy from the oxidation of organic molecules

  8. ATP Cycle

  9. in cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP

  10. Glycolysis and Cell Respiration: • Glycolysis occurs in the cytoplasm of the cell • 1 glucose molecules is broken down into 2 pyruvate molecules • There is a net production of 2 ATP molecules • Glycolysis does not require oxygen • Fate of pyruvate depends on presence or absence of oxygen

  11. Anaerobic Cell Respiration:

  12. Summary Equation • The summary equation for cellular respiration is: C6H12O6 + O2 CO2 + H2O + Glucose + oxygen  carbon dioxide + water + ATP ATP

  13. Key players in this process • Glucose: source of fuel • NAD+: electron carrier • Enzymes: mediate entire process • Mitochondria: site of aerobic respiration • ATP: principal end product • Protons/Electrons: sources of potential energy • Oxygen: final electron acceptor

  14. Redox reactions • Reduction: reducing overall positive charge by gaining electrons • Oxidation: loss of electrons

  15. NAD+: an electron carrier • In order for electrons to be passed from one compound to another, an electron carrier is needed • NAD+ is reduced to NADH when picking up electrons • It is oxidized back to NAD+ when losing them

  16. Where do the electrons come from? • Remember all those hydrogen atoms that make up glucose? • Hydrogens are a part of fats, too. • Hydrogen = 1e-, so here, H = e-

  17. Respiration is a controlled release of energy • It’s a highly exergonic, but well-controlled process • Mediated by enzymes, electron carriers • Otherwise, it would be like an explosion • Not compatible with life!

  18. Phosphorylation • Addition of a phosphate group to a molecule; in this case, to ADP, forming ATP • Substrate level phosphorylation vs. oxidative phosphorylation

  19. Substrate-level phosphorylation • An enzyme transfers a phosphate group from a substrate to ADP • Ineffective in generating large amounts of ATP

  20. Oxidative phosphorylation • Refers to phosphorylation that occurs due to redox reactions transferring electrons from food to oxygen • Happens on electron transport chains

  21. Mitochondrion Structure Inner membrane Outer membrane Intermembrane space DNA (mtDNA) Matrix (liquid) Cristae (folds)

  22. Three stages of respiration • Stage 1: Glycolysis (energy investment) • Some ATP is made, some is used • Stage 2: Krebs Cycle (oxidation of pyruvate) • Generation of CO2 • Stage 3: Oxidative Phosphorylation • Generation of most ATP

  23. Three stages of respiration

  24. Stage 1: Glycolysis • Where • Cell’s cytoplasm • Why • To break glucose down into pyruvate, which feeds into the Krebs Cycle • To regenerate NAD, an electron carrier

  25. Glycolysis: How Glucose is phosphorylated. -1 ATP A series of enzymes produces intermediate products. An intermediate is phosphorylated. -1 ATP NAD is reduced to NADH, 1 each per PGAL +2 NADH + H+ This diphosphate compound is unstable and breaks into 2 PGAL. The PGAL molecules generate ATP through substrate-level phosphorylation. +4 ATP Summary: 2 pyruvate produced 2 NADH + H+ produced Net 2 ATP produced

  26. Stage 2: Krebs cycle • Where: • Matrix of mitochondria, but only if O2 present • Why: • To oxidize pyruvate to CO2 • To build up a H+ ion gradient used in electron transport

  27. Krebs Cycle: How Pyruvate is decarboxylated. -1 CO2 Resulting acetic acid (2C) is oxidized by NAD. +1 NADH + H+ Acetyl group (2C) has Coenzyme A added 1 Acetyl co-A: link reaction Acetyl co-A (2C) is added to a 4C base molecule, forming a 6C intermediate NAD oxidizes a 4C to form original 4C molecule: +1 NADH + H+ NAD oxidizes these intermediates CO2 is given off as a byproduct +2 NADH + H+ -1 CO2 To regenerate the original 4C base, ATP is generated and FAD oxidizes an intermediate +1 ATP +1 FADH2

  28. Krebs cycle summary • Per pyruvate that enters: • 1 ATP made • 3 CO2 given off • 4 NADH produced • 1 FADH2 produced • Think: how many pyruvates entered the cycle? • How many times must this cycle happen to break down ONE glucose?

  29. Stage 3: Oxidative phosphorylation • Where: • Inner membrane of mitochondria (on cristae) • Why: • To produce ATP from H+ ion gradient generated during Krebs cycle • Requires oxygen!

  30. Oxidative Phosphorylation: How H+ ions accumulate in the matrix as a result of NADH picking them up during the Krebs cycle. Intermembrane space cytochromes H+ H+ H+ matrix H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

  31. Oxidative Phosphorylation: How e- e- Intermembrane space e- e- e- e- cytochromes The H+ ions diffuse out of the matrix through protein channels into the intermembrane space where they split into H protons and electrons. matrix H+ H+ H+ H+ H+ H+ H+

  32. Oxidative Phosphorylation: How e- e- Intermembrane space H+ e- H+ H+ e- H+ e- H+ H+ e- The accumulated H+ ions then move through a pump called ATP synthase to produce ATP. What powers the pump is the electrochemical gradient produced. cytochromes matrix ATP ADP

  33. Oxidative Phosphorylation: How Intermembrane space e- e- e- e- e- H+ H+ H+ e- H+ While the H+ protons move through ATP synthase, electrons carried by NADH are passed along electron transport chains composed of cytochromes. H+ H+ cytochromes matrix ATP ADP

  34. O H+ H+ Oxidative Phosphorylation: How H+ Now the H+ atoms are in the matrix. The hydrogen atoms and electrons combine with oxygen, the final electron acceptor of oxidative phosphorylation, to form water. H+ H+ H+ H+ H+ cytochromes matrix e- e- e- e- 34 ATP e- ADP e-

  35. Summary: Oxidative Phosphorylation • 34 ATP made • H2O generated • NADH oxidized back to NAD • Very efficient process! Produces a lot of energy.

  36. But what if there’s no oxygen? Remember, the first living organisms lived in an anaerobic environment…

  37. Without oxygen… • NADH cannot be oxidized back to NAD+ • In order for aerobic respiration to occur, NADH must be oxidized and some intermediate compound must be reduced

  38. Fermentation • Includes glycolysis • Also side reactions that allow for NADH to be oxidized back to NAD+ by shuttling electrons to intermediate products such as ethanol and lactate

  39. Alcoholic Fermentation • Glycolysis happens • Pyruvate is then converted to acetaldehyde, CO2 is released • Acetaldehyde is reduced by NADH to ethanol • No additional ATP is made • Occurs in yeasts, some bacteria

  40. Lactic acid fermentation • Glycolysis happens • Pyruvate is reduced by NADH and forms lactate (lactic acid) • No CO2 is released • No additional ATP is formed • Done by bacteria, muscle cells

  41. Other molecules can be used in respiration • Proteins: must be deaminated, then converted to pyruvate • Fats: undergo beta-oxidation • Cells prefer carbs

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