Lecture 9

• • Energy flows into an ecosystem as sunlight and leaves as heat
  • Photosynthesis generates O2 and organic molecules, which are used in cellular respiration
  • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

  
• High energy vs. Low energy

  A) Anabolism

  B) Catabolism

  
• The breakdown of organic molecules is exergonic
• Fermentation is a partial degradation of sugars that occurs without O2
• Aerobic respiration consumes organic molecules and O2 and yields ATP
• Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2
• Redox Reactions: Oxidation and Reduction

  • The transfer of electrons during chemical reactions releases energy stored in organic molecules
  • This released energy is ultimately used to synthesize ATP
• Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions
• In oxidation, a substance loses electrons, or is oxidized
• In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)
• The electron donor is called the reducing agent and the electron acceptor is called the oxidizing agent.
• Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds (e.g. the reaction between methane and O2)
• During cellular respiration, the fuel (e.g. glucose) is oxidized, and O2 is reduced:

  A) oxidized

  B) reduced

  
• Cellular respiration is very exothermic with ∆G = -2880 kJ/mole of glucose
• Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

  • In cellular respiration, glucose and other organic molecules are broken down in a series of steps
  • Electrons from organic compounds are usually first transferred to NAD+, a coenzyme
  • As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration
  • Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

  
• Nicotinamide Adenine Dinucleotide

  
• NADH passes the electrons to the electron transport chain
• Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction. O2 pulls electrons down the chain in an energy-yielding tumble. The energy yielded is used to regenerate ATP.
• Cellular respiration has three stages:

  • Glycolysis
  • Pyruvate oxidation and the Citric Acid cycle
  • Oxidative phosphorylation
• Glycolysis breaks down glucose into two molecules of pyruvate
• Pyruvate oxidation and the Citric Acid cycle completes the breakdown of glucose
• Oxidative phosphorylation accounts for most of the ATP synthesis
• The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions
• Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
• Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
• Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate
• Glycolysis occurs in the cytoplasm and has two major phases

  Energy investment phase

  Energy payoff phase
• In the energy investment phase, there needs to be a glucose transporter

  
• Energy payoff phase

  A) 4 ATP

  B) 2 NADH

  C) 2 Pyruvate

  
• Net for Glycolysis

  A) 2 Pyruvate

  B) 2 ATP

  C) 2 NADH

  
• 1. Hexokinase: phosphorylation makes sugar more reactive; charge traps sugar in cell

  
• 2. Phosphoglucoisomerase: glucose to fructose

  
• 3. Phosphofructokinase (PFK): transferring a second phosphate onto this sugar to form a product called Fructose 1, 6-bisphosphate. And this sugar now with its two phosphates on there, is very reactive.

  
• 4. Aldolase: cuts molecule into 2 three-carbon molecules

  5. Isomerase: needed to generate the next product, Glyceraldehyde-3-phosphate (G3P)

  
• 6. Triose phosphate dehydrogenase: notice we need two G3P

  
• 7. Phosphoglycerokinase: 2 ADP's are phosphorylated by substrate-level phosphorylation to 2 ATP's

  
• 8. Phosphoglyceromutase: 3-phosphoglycerate mutates to 2-phosphoglycerate

  
• 9. Enolase: generates phosphoenolpyruvate (PEP)

  
• 10. Pyruvate kinase: Phosphoenolpyruvate (PEP) converted to pyruvate and 2 molecules of ATP