3.

• Glycolysis


  Can occur as long as there is a sufficient supply of NAD+ to drive the redox reactions required for activity since it does not require oxygen

  
• Options for energy generation in the absence of oxygen

  • Anaerobic Cell Respiration
  • Fermentation
• Anaerobic Cell Respiration


  • Other molecules such as sulfur or nitrogen as final electron acceptors; mostly occurs with bacteria; does use the ETC
• Fermentation


  • Inefficient breakdown of sugars without oxygen; does not use the ETC or Krebs cycle; all events remain in the cytoplasm

  
• Neither anaerobic respiration nor fermentation is as efficient as cellular respiration and not all organisms are capable of anaerobic respiration. Often fermentation is the only alternative.
• Fermentation


  A metabolic pathway which converts pyruvic acid in order to regenerate low energy electron carrier precursors (NAD+)

  
• Fermentation does not require oxygen and occurs in the cytoplasm in all cells
• ATP generated in fermentation is by substrate-level phosphorylation ONLY
• Types of fermentation


  • Lactic Acid Fermentation
  • Ethanol Fermentation
  • Mixed Acid Fermentation
  • Butanediol Fermentation
  • Butanol Fermentation
  • Propionic Acid Fermentation
• Lactic Acid Fermentation


  Pyruvic acid (generated during glycolysis) is converted into lactic acid to regenerate the NAD+ molecules needed to continue glycolysis

  
• Types of Lactic Acid Fermentation

  • Homolactic Fermentation
  • Heterolactic Fermentation
• Ethanol Fermentation


  Pyruvic acid is converted into acetaldehyde by pyruvate decarboxylase with CO2 produced as a byproduct, then subsequently ethanol via alcohol dehydrogenase
• Ethanol buildup can become toxic, with most yeasts becoming toxic around 16-17% v/v. Higher alcohol content must be distilled.
• Other Alternatives to Lactic Acid and Ethanol Fermentation

  • Mixed Acid Fermentation
  • Butanediol Fermentation
  • Butanol Fermentation
  • Propionic Acid Fermentation
• Photosynthesis


  Consists of two main sets of reactions: Light Reactions and Light-Independent (Calvin Cycle) Reactions

  
• Photosynthesis Reactions


  • Light Reactions
  • Light-Independent (Calvin Cycle) Reactions

  
• Light Reactions


  Reactions which take place only in the presence of light and involve the capture of light energy by photosynthetic pigments in the thylakoids which is used to split water and generate ATP and high energy molecules (NADPH) for Calvin cycle; O2 waste

  
• Light-Independent (Calvin Cycle) Reactions


  Reactions which take place in the stroma and do not require light; result in the production of sugars from CO2, a process called carbon fixation

  
• Photosystem


  A reaction center complex (a type of protein complex with a special pair of chlorophyll a molecules) surrounded by light-harvesting complexes

  
• Photosystems


  • Light-harvesting complexes (which contain chlorophyll bound to proteins) transfer the energy of light photons to the reaction center
  • Excited electrons are accepted by a primary electron acceptor in the reaction center complex which initiates the light reactions cascade

  
• Photosystems in the Light Reactions

  • Photosystem II (PSII)
  • Photosystem I (PSI)

  
• Microbes employ a wide array of light-capturing pigments including chlorophyll, bacteriochlorophyll, carotenoids, phycoerythrin, and phycocyanin
• Electron Transport Chain


  The 3 proteins in the membrane that pass electrons via paired oxidation-reduction reactions are plastoquinone (Pq), a cytochrome complex, and plastocyanin (PC). Energy released in the e- transfer powers formation of H+ gradient.
• Calvin Cycle / Light-Independent Reactions

  Take carbon dioxide (CO2) gas and convert it into glucose (C6H12O6) using ATP and NADPH

  
• Steps of the Calvin Cycle


  • Carbon Fixation / Carboxylation
  • Reduction
  • Regeneration

  
• Several alternative CO2 fixation pathways are known to exist including the Reductive (Reverse) TCA Cycle and the Reductive Acetyl-CoA Pathway
• Types of Photosynthesis


  • Oxygenic Photoautotrophy
  • Anoxygenic Photoautotrophy
  • Anoxygenic Photoheterotrophy
  • Rhodopsin-based Phototrophy

  
• Oxygenic Photoautotrophy


  • Photosynthesis involving oxygen production which occurs in oxygen-rich conditions. Uses both photosystems. "Classic" photosynthesis.

  
• Cyanobacteria


  • The only known oxygenic photosynthetic bacteria which perform photosynthesis on flattened sacs of membrane folds called thylakoids
  • Employ photoactive molecules including bacteriochlorophyll, phycoerythrin, and phycocyanin for light reactions and undergo the Calvin cycle
  • Contain polyhedral bodies called carboxysomes rich in RuBisCO and carbonic anhydrase to concentrate and sequester carbon for the Calvin cycle

  
• Anoxygenic Photoautotrophy


  • Photosynthesis driving CO2 fixation processes through the use of either water or often sulfur compounds as an electron donor. These organisms use 1 photosystem to generate ATP and do NOT produce oxygen as a byproduct.

  
• Anoxygenic Photoheterotrophy


  • Photosynthesis involving non-CO2 fixation processes to convert organic compounds (such as acetate) into useable carbon sources. These pathways also only utilize 1 photosystem for light harvesting. Does NOT produce oxygen gas.

  
• Rhodopsin-based Phototrophy


  The simplest type of phototrophy which uses a light-driven proton pump to fuel ATP synthesis. The proton pump contains rhodopsin proteins containing the photoactive pigment, retinal.