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Chapter 8

Chapter 8 . How Cells Release Stored Energy AKA: Cellular Respiration. How do cells make ATP?. ATP is the prime energy carrier for all cells Aerobic Respiration (with oxygen) is the main pathway for energy release from carbohydrates to ATP.

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Chapter 8

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  1. Chapter 8 How Cells Release Stored Energy AKA: Cellular Respiration

  2. How do cells make ATP? • ATP is the prime energy carrier for all cells • Aerobic Respiration (with oxygen) is the main pathway for energy release from carbohydrates to ATP

  3. All energy-releasing pathways start with glycolysis • Glucose is split into two pyruvate molecules • Glycolysis reactions occur in the cytoplasm

  4. Overview of Aerobic Respiration • Aerobic Respiration yields 36 ATP • Anaerobic Respiration (without oxygen) yields 2 ATP Aerobic respiration route: C6H12O6 + 6O2 6CO2 + 6H2O (Reverse equation to photosynthesis)

  5. Three steps to aerobic respiration • 1- Glycolysis: is the breakdown of glucose to pyruvate • Small amount of ATP are generate (2 ATP) • Takes place in the cytoplasm • 2- Kreb Cycle: degrades pyruvate to carbon dioxide, water, ATP, H+ ions and electrons (accepted by NAD+ and FAD) • Takes place in the mitochondrian • Makes 2 ATP

  6. Continue… • 3- Electron Transfer Phosphorylation: processes the H+ ions and electrons to generate high yields of ATP; oxygen is the final electron acceptor • Takes place in the mitochondrion • Yields 32 ATP (this is the real takes place)

  7. Glycolysis: First stage of energy-releasing pathways • 2 ATP is required to start glycosis • Enzymes in the cytoplasm catalyze several steps in glucose breakdown • Glucose is first phosphorylated in energy-requiring steps, then the six-carbon intermediate is split to form two molecules of PGAL (which gives a phosphate to make ATP) • Enzymes remove H+ and electrons from PGAL and transfer them to NAD+ which becomes NADH (used later in the electron transfer)

  8. By substrate-level phosphorylation, four ATP are produced • The end product to glycolysis is: • 2 ATP (net gain) • 2 pyruvates • 2 NADH For each glucose moleucule degraded

  9. Continue… • The pyruvic acid diffuses into the inner compartment of the mitochondrion where a transition reaction occurs that serves to prepare pyruvic acid for entry into the next stage of respiration: • (a) pyruvic acid acetic acid + CO2 (a waste product of cell metabolism) + NADH + • (b) acetic acid + co-enzyme A -> acetyl CoA

  10. Second Stage of the Aerobic Pathway: Kreb Cycle • Takes place in the inner mitochondria matrix • Pyruvate enters the mitochondria and is converted to acetyl-CoA, which then joins oxaloacetate already present from a previous “turn” of the cycle. • During each turn of the cycle, three carbon atoms enter (as pyruvate) and three leave as three carbon dioxide molecules

  11. Functions of the second stage • H+ and e- are transferred to NAD+ and FAD (coenzymes) • Ten coenzymes are loaded with electrons and hydrogen • Two molecules of ATP are produced by substrate-level of phosphorlyation

  12. Continue… • Most of the molecules are recycled to conserve oxaloacetate for continuous processing of acetyl-CoA • Carbon dioxide is produced as a by-product

  13. Third Stage of the Aerobic Pathway • This is where the real work is done • NADH and FADH2 give up their electrons to transfer (enzyme) system embedded in the mitochondrial inner membrane

  14. Electron Transfer Phosphorylation • According to the chemiosmotic model, energy is released in the passage of electrons through components of the transfer series • Oxygen joins with the “spent” electrons and H+ to yield water

  15. Summary of the Energy Harvest Net Gain • Electron transfer 32 ATP • Glycolysis 2 ATP • Kreb Cycle 2 ATP Total 36 ATP (per glucose molecule)

  16. Continue… • Normally, for every NADH produced within the mitochondria and processed by electrons transfer chain, three ATP are produced • FADH2 produced 2 ATP

  17. Continue… • NADH from the cytoplasm cannot enter mitochondrian and must transfer its electrons!! • In most cells (skeletal and brain) the electrons are transferred to FAD and thus yield two ATP (for a total yield of 36) • But in the liver, heart, and kidney cells, NAD+ accepts the electrons to yield three ATP because two NADH are produced per glucose, this total yield of 38 ATP

  18. Anaerobic Respiration • Cellular respiration without using oxygen (or very limited) • Pyruvate from glycolysis is metabolized to produce molecules other than acetyl-CoA • Example: Single Yeast Cells

  19. Fermentation Pathways • With an energy yield of only 2 ATPs • Glycolysis serves the first stage (just like aerobic respiration)

  20. Lactate Fermentation • Certain bacteria (as in bacteria) and muscles cells have the enzymes capable of converting pyruvate to lactate • Example: Muscle Cramps • No additional ATP beyond the net two from glycolysis is produced but NAD+ is regenerated

  21. Alcoholic Fermentation • Fermentation begins with glucose degradation to pyruvate • Cellular enzymes convert pyruvate to acetaldehyde, which then accepts electrons from NADH to become alcohol. • Yeast are valuable in the baking industry (Carbon dioxide byproduct makes dough “rise”) and in alcoholic beverage production

  22. Anaerobic Electron Transfer • Some kinds of bacteria are able to strip electrons from organic compounds and send them through a special electron transfer in their membranes to produce ATP • Example: Such bacteria include those that reduce sulfate to hydrogen sulfide (foul smelling gas) and those that convert nitrate to nitrite

  23. Alternative Energy Sources in the Human Body • Excess carbohydrate intake is stored as glycogen in the liver and muscle for future use. • Free glucose is used until it runs low, then glycogen reserves are tapped

  24. Energy from Fats • Excess fats (including those made from carbohydrates) are stored away in cells of adipose tissue • Fats are digested into glycerol, which enters glycolysis, and fatty acids, which enter the Kreb Cycle • Fatty acids have more carbon and hydrogen atoms, they degraded more slowly and yield greater amounts of ATP

  25. Energy from Proteins • Amino acids are released by digestion and travel in the blood • After the amino group is removed, the amino acid is removed, the amino acid remanant is fed into in the Kreb Cycle

  26. Perspective on the Molecular Unity of LIfe • Photosynthesis and cellular respiration are intimately connected • Life is not some mysterious force, but a series of chemical reactions under highly integrated control.

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