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Cellular Respiration: Harvesting Chemical Energy

Cellular Respiration: Harvesting Chemical Energy. Chapter 9. Cells are open systems. Energy flows into most ecosystems as sunlight. Photosynthetic organisms trap light energy and transform it into chemical bond energy. Cells use chemical bond energy to make ATP.

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Cellular Respiration: Harvesting Chemical Energy

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  1. Cellular Respiration: Harvesting Chemical Energy Chapter 9

  2. Cells are open systems • Energy flows into most ecosystems as sunlight. • Photosynthetic organisms trap light energy and transform it into chemical bond energy. • Cells use chemical bond energy to make ATP. • Chemical elements essential for life are recycled, but energy is not. • How do cells harvest chemical energy?

  3. “Spendable Energy”

  4. Cellular respiration and fermentation are catabolic (energy-yielding) pathways • Fermentation -- An ATP-producing process in which both electron donors and acceptors are organic compounds; anaerobic process (without oxygen). • Cellular respiration -- An ATP-producing process in which the ultimate electron acceptor is an inorganic molecule, such as oxygen. • Most efficient catabolic pathway is aerobic (with oxygen). • Carbohydrates, proteins and fats can all be metabolized as fuel, but cellular respiration is most often described as the oxidation of glucose: • C6H12O6 + 6O2 ——>6CO2 + 6H2O + Energy • (ATP + Heat)

  5. Cells must recycle the ATP they spend for work • Respiration transfers the energy stored in food molecules to ATP. • ATP (adenosine triphosphate) -- Nucleotide with unstable phosphate bonds that the cell hydrolyzes for energy; enzymes that catalyze this reaction are called ATPases. • ATP + H2O  ADP + phosphate + energy • ADP + H2O  AMP + phosphate + energy • Removal of a phosphate yields 7 kcal of energy per mole of ATP. • Phosphate groups from ATP are transferred to other compounds to do cellular work (phosphorylation); otherwise energy would be just be lost as heat.

  6. Cells must recycle the ATP they spend for work (continued) • The compound receiving the phosphate group from ATP is said to be phosphorylated and becomes energized; enzymes catalyzing these reactions are kinases. • Cells must replenish the ATP supply to continue cellular work. Respiration provides the energy to regenerate ATP from ADP and inorganic phosphate.

  7. Addition of a phosphate group activates an enzyme

  8. An Introduction to Redox Reactions • Oxidation/reduction reactions -- Chemical reactions which involve a transfer of electrons from one reactant to another (redox for short); use of chemical energy in living things involves redox rxns. • Oxidation -- loss of electrons: • Fe  Fe+3 + 3 e- (Iron has been oxidized) • Reduction -- gain of electrons: • O + 2 e-  O-2 (Oxygen has been reduced). • LEO the lion says GER.

  9. An Introduction to Redox Reactions continued • Electron transfer requires both a donor and acceptor, so when one reactant is oxidized the other is reduced: • Fe + O2 Fe2O3 • In this case, Fe is the reducing agent or reducer; O is the oxidizing agent or oxidizer (has a high electronegativity). • Transfer of electrons may not be complete, but instead may just change the degree of sharing in covalent bonds: • CH4 + O2 CO2 + H2O

  10. Electron Acceptors • C6H12O6 + 6O2 ——>6CO2 + 6H2O + Energy (ATP + Heat) • Hydrogens stripped from glucose are not transferred directly to oxygen, but are first passed to a special electron acceptor. • Nicotinamide adenine dinucleotide (NAD+ / NADH) -- A dinucleotide that functions as a coenzyme in the redox reactions of metabolism.

  11. Electron acceptors cont. • Flavin adenine dinucleotide (FAD / FADH2) – see NADH. • NAD+= Oxidized coenzyme (net positive charge); NADH = Reduced coenzyme (electrically neutral). • Coenzyme -- Small nonprotein organic molecule that is required for certain enzymes to function. • Dinucleotide -- A molecule consisting of two nucleotides.

  12. NAD+ / FAD

  13. Mitochondrion Review

  14. Respiration: an overview • There are three metabolic stages of cellular respiration: • 1. Glycolysis • 2. Krebs Cycle • 3. Electron transport chain (ETC) • Glycolysis occurs in the cytosol of the cell; splits glucose (6C) into two pyruvate (3C) molecules. • Krebs Cycle occurs in the mitochondrial matrix; breaks down pyruvateinto carbon dioxide. • Electron transport chain is located at the inner membrane of the mitochondrion, where ATP synthesis or oxidative phosphorylation takes place.

  15. Glycolysis: a closer look • Glycolysis (Glyco = sugar; lysis = break) Occurs whether or not oxygen is present; yields 2 ATP. • Overall reaction: glucose + 2 ATP  2 pyruvate + 4 ATP • Substrate-level phosphorylation -- ATP production by direct enzymatic transfer of phosphate to ADP.

  16. Glycolysis steps • Step 1: Glucose enters the cell, and carbon six is phosphorylated as 1 ATP is used. • Glucose  glucose-6-phosphate • Step 2: Rearrangement of glucose-6-phosphate to its isomer, fructose-6-phosphate. • Step 3: Carbon one of fructose-6-phosphate is phosphorylated using another ATP to form fructose-1,6-diphosphate.

  17. Glycolysis cont. • Step 4: Aldolase cleaves the 6-carbon fructose into two 3-carbon sugars. • Fructose-1,6-phosphate  2 3-phosphoglyceraldehyde • Step 5: Phosphoglyceraldehyde is phosphorylated on carbon one; NADH is formed. • 2 3-phosphoglyceraldehyde + 2 NAD+  2 1,3 diphosphoglycerate + 2 NADH • Step 6: ATP is produced by substrate-level phosphorylation. • 2 1,3 diphosphoglycerate + 2 ADP  2 3-phosphoglycerate + 2 ATP

  18. End of Glycolysis (really) • Step 7: Phosphate group on carbon three is transferred to carbon two. • 2 3-phosphoglycerate  2 2-phosphoglycerate • Step 8: Enzymatic removal of a water molecule. • 2 2-phosphoglycerate  2 2-phosphoenolpyruvate + H2O • Step 9: ATP is produced by substrate-level phosphorylation; pyruvate formed. • 2 2-phosphoenolpyruvate  2 pyruvate

  19. Glycolysis animations • http://www.northland.cc.mn.us/biology/Biology1111/animations/glycolysis.html • http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html • http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/metabolism/cellresp/glycol_an.html

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