2

• Catabolic Reactions


  Provides energy (ATP) and building blocks for anabolism

  
• Anabolic Reactions


  Uses energy (ATP) and building blocks to build molecules
• Photosynthesis


  6 CO2 + 6 H2O + Energy à C6H12O6 + 6 O2
• Cellular Respiration


  C6H12O6 + 6 O2 à 6 CO2 + 6 H2O + Energy
• Adenosine triphosphate (ATP)


  A chemical coenzyme used by the cell to store energy generated through catabolic reactions – similar to a cellular rechargeable battery

  
• NADH, NADPH, and FADH2


  Coenzymes which when reduced (carry hydrogen) also carry electrons and are in their high energy states, when oxidized (after losing hydrogen and electrons) they become NAD+, NADPH and FAD
• NAD+ / NADH = ~1000
• NADP+ / NADPH = ~0.1
• Energy Carrier Molecules


  • ATP
  • NADH
  • NADPH
  • FADH2
• Photophosphorylation


  Light energy is used to power the phosphorylation of ADP via the ETC
• Substrate Level Phosphorylation


  A molecule transfers a phosphate directly to ADP rather than coupling an inorganic phosphate (Pi); occurs during glycolysis and Krebs cycle

  
• Oxidative Phosphorylation


  Similar to photophosphorylation with oxidation occurring through the ETC; 90% of ATP is generated by this approach in oxidative phosphorylation step of cellular respiration

  
• Catabolic Pathways to Produce ATP

  • Aerobic Respiration
  • Anaerobic Respiration
  • Fermentation

  
• Glycolysis


  Conversion of glucose into pyruvic acid which gets further modified to acetyl CoA

  
• Krebs Cycle


  Conversion of acetyl CoA in a cyclical process to produce several high energy carrier molecules (NADH, FADH2)

  
• Oxidative Phosphorylation via the Electron Transport Chain

  Synthesis of ATP using the energy harvested from the high energy electron carriers

  
• Embden-Meyerhof-Parnas (EMP) Pathway


  A metabolic pathway step which converts glucose (a 6-carbon sugar, C6H12O6) into two molecules of pyruvic acid (3-carbon sugar)

  
• EMP Pathway


  • It is the most common glycolysis pathway, occurring in animals and most prokaryotes
  • Consists of 2 Phases: Energy Investment Phase and Energy Payoff Phase
  • Starting Material: Glucose (1x)
  • End Product: Pyruvic Acid (2x)
  • Byproducts: NADH (2x) and ATP (2x)
  • Typical Cellular Location: Cytoplasm

  
• Entner-Doudoroff (ED) Pathway


  A catabolic pathway step which also converts glucose (a 6-carbon sugar, C6H12O6) into two molecules of pyruvic acid (3-carbon sugar); is an alternative to the EMP pathway

  
• ED Pathway


  • Differs in 4 enzymes compared to the EMP pathway and produces only 1 ATP per glucose (instead of 2)
  • Examples of organisms which use the ED pathway are typically facultative anaerobes (like E. coli), obligate aerobes, and/or several phototrophic species
  • Very few obligate anaerobes employ the ED pathway
  • Starting Material: Glucose (1x)
  • End Product: Pyruvic Acid (2x)
  • Byproducts: NADH (1x), NADPH (1x), and ATP (1x)
  • Typical Cellular Location: Cytoplasm

  
• Oxidative Pentose Phosphate (OPP) Pathway

  A catabolic process involving the use of glucose-6-phosphate and a variety of other sugar intermediates to produce ribose-5-phosphate, NADPH, and other important pentose molecules

  
• OPP Pathway


  • Often used in parallel with the EMP pathway, but does not consume nor produce ATP directly
  • Permits inter-conversion between sugars and plays a central role in facilitating anabolic pathways
  • NADPH is a primary antioxidant and reducing agent in the cell, driving forward anabolic reactions by donating electrons
  • Ribose-5-phosphate is an important substrate for anabolic reactions to produce DNA, RNA, ATP, and a variety of other key biological molecules
  • Involves two main phases: Oxidative Phase and Non-oxidative Phase
  • Occurs in the cytosol of the cell
  • Plays a more important role in anabolic reactions, though catabolic
  • Many of the reaction steps in the non-oxidative phase are reversible permitting flexibility with sugar use

  
• Though not all organisms are capable of all 3 catabolic pathways, most are able to do the OPP pathway and will either do the EMP or ED pathway
• Conversion of Pyruvate to Acetyl CoA


  • Starting Material: Pyruvic Acid (2x)
  • End Product: Acetyl CoA (2x)
  • Byproducts: NADH (2x) and CO2 (2x)
  • Cellular Location: Cytoplasm (Prokaryotes)

  
• Pyruvate Dehydrogenase Complex (PDC)


  An important conserved element in both prokaryotic and eukaryotic metabolism, playing a role in conversion of acetyl-CoA to acetyl phosphate

  
• Krebs Cycle (or Citric Acid Cycle or TCA Cycle)


  A metabolic pathway step which combines acetyl CoA (a 2-carbon molecule) with oxaloacetate (4-carbon molecule) to generate citric acid, high energy cofactors (NADH and FADH2), ATP, and CO2

  
• Krebs Cycle


  • Has 8 steps, with oxaloacetate regenerated as an end product to be recycled
  • Occurs in the cytoplasm in prokaryotes and in the mitochondria of eukaryotes
  • Starting Material: Acetyl CoA [2x per glucose]
  • End Product: Oxaloacetate (reused)
  • Byproducts: NADH (3x), FADH2 (1x), ATP (1x), CO2 (2x) [2x per glucose]

  
• Glyoxylate Bypass


  A modified TCA cycle employed by some prokaryotes when both carbon and energy are scarce, allowing them to circumvent steps resulting in CO2 losses to save carbon for biosynthesis

  
• Oxidative Phosphorylation


  The last step of cellular respiration where ATP is generated by chemiosmosis of hydrogen ions (H+) in the electron transport chain (ETC)

  
• Electron Transport Chain (ETC)


  • A series of multi-protein complexes which alternate between reduced and oxidized states as they accept and donate electrons, with electrons dropping in free energy as they go down the chain and being finally passed to the final electron acceptor, O2, forming H2O
  • Electron transfer in the ETC causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space, which are then moved back across by facilitated diffusion (chemiosmosis) and pass through ATP synthase to generate ATP
• Glyoxylate bypass


  Allows microbes to circumvent the steps of the TCA cycle which result in CO2 losses (to save for carbon biosynthesis), but result in less high energy electron carriers (1 FADH2 and 1 NADH). In essence, they can trade energy for more carbon availability.

  
• Electron Transport Chain (ETC)


  • A series of multi-protein complexes which alternate between reduced and oxidized states as they accept and donate electrons
  • Electrons drop in free energy as they go down the chain and, in aerobic respiration, are finally passed to the final electron acceptor, O2, forming H2O
• Chemiosmosis


  1. Electron transfer in the ETC causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space

  2. These ions are then moved back across by facilitated diffusion

  3. The pass through ATP synthase which uses the energy to form ATP

  
• Starting Material


  • High energy electron carriers (NADH, FADH2)
  • O2 (oxygen)
• End Product


  • ATP (many)
• Byproducts


  • H2O (several)
• Cellular Location


  Plasma membrane (Prokaryotes) / Mitochondrial Membrane (Eukaryotes)
• Mitchell Extrusion (Chemiosmotic) Hypothesis

  • A concentration gradient forms with more H+ in the inner membrane space
  • An ATP synthase (enzyme) molecule uses the gradient to move the ions (H+) back into the cell generating massive amounts of ATP
  • The electrons reaching this step are bound to oxygen (the final electron acceptor) and oxygen combines with the H+ ions to create water

  
• Bacteria: ~38 ATP per Glucose
• Eukaryotes: ~36 ATP per Glucose