PHYSIOLOGY

• Phototrophy

  Photosynthesis - conversion of light energy into chemical energy; the most important biological process on earth
• Types of phototrophy

  • Photoautotrophy - use CO2 as their carbon source
  • Photoheterotrophy - rely on other organic compounds, mostly anoxygenic
• More than six photosynthetic transport systems have evolved in bacteria while eukaryotes obtain the ability to photosynthesize with chloroplasts
• Photosynthesis reactions

  • Light Dependent reaction - for generation of ATP and NADPH
  • Light Independent reaction - use ATP and NADH (or NADPH) for fixation of carbon
• Types of photosynthesis

  • Oxygenic Photosynthesis - utilize water as electron donor and release oxygen
  • Anoxygenic Photosynthesis - utilize inorganic compounds as electron donors; does not produce oxygen
• Chlorophyll and Bacteriochlorophyll

  Tetrapyrroles related to cytochromes, with magnesium instead of iron
• Existence of different forms of chlorophyll and bacteriochlorophyll allows phototrophs to utilize different wavelengths
• Reaction centers and Antenna pigments

  • In oxygenic phototrophs and in purple anoxygenic phototrophs, chlorophyll/bacteriochlorophyll molecules do not exist freely in the cell but are attached to proteins and housed within membranes to form "photocomplexes"
  • Photocomplexes consist of 50 to 100 chlorophyll/bacteriochlorophyll molecules
  • Reaction centers participate directly in the reactions that lead to energy conservation (transfer electrons across the photosynthesis membrane)
  • Antenna pigments surround the reaction centers and absorb light, funneling some of the energy to the reaction center
• Photosynthetic membranes in chloroplasts and chromatophores

  • Light gathering pigments exist within the membranes of the cell
  • In eukaryotes, photosynthesis takes place in the chloroplasts within thylakoids
  • Grana - stacks of thylakoids
  • Stroma - matrix space that surrounds the thylakoids
  • Lumen - spaces within the thylakoids
  • Chromatophores - membrane vesicles
  • Lamellae - membrane stacks
  • Thylakoids - lamellar membranes
  • Chlorosomes - structure for capturing energy at low light
• Additional photosynthetic pigments

  • Carotenoids - most widespread accessory pigments in phototrophs, function primarily as protective agents against photooxidation
  • Phycobiliproteins and phycobilisomes - found in cyanobacteria and red algae, main light harvesting systems
• Types of phycobiliproteins

  • Phycoerythrin
  • Phycocyanin
  • Allophycocyanin
• Phycobilisomes facilitate energy transfer to reaction centers, allowing cyanobacteria to grow at lower light intensities
• Anoxygenic photosynthesis

  • Light reactions generate a proton motive force to generate ATP and key parts of this process in the photosynthetic light reaction
  • Two classes of reaction centers: iron-sulfur type (FES-type) and quinone (Q-type)
• Electron flow in purple bacteria


  1. Use Q-type reaction center which contains three polypeptides, designated L, M, and H

  2. These polypeptides bind to a special pair of bacteriochlorophyll

  3. Two bacteriochlorophyll a (special pair)

  4. Two additional bacteriochlorophyll a

  5. Two bacteriopheophytin a

  6. Two quinones

  7. One carotenoid molecule

  8. When electrons are excited, the P870 will reduce a molecule of bacteriochlorophyll a within the reaction center

  9. Bacteriochlorophyll a proceeds to reduce bacteriopheophytin a

  10. Cytochrome c2 is a periplasmic cytochrome that functions as an electron shuttle between the membrane-bound bc1 complex and the reaction center
• Photophosphorylation


  Generation of ATP through photosynthetic electron flow
• Types of photophosphorylation

  • Cyclic photophosphorylation - electrons move within a closed loop, no net input or consumption of electrons, they simply travel a circuitous route
  • Non-cyclic photophosphorylation - generates both ATP and NADPH
• Reducing power for purple bacteria can come from many sources, in particular reduced sulfur compounds such as H2S
• When H2S is formed, electrons end up in the "quinone pool"
• Photosynthetic electron flow in other anoxygenic phototrophs, such as filamentous anoxygenic phototrophs and green sulfur bacteria, employ structurally similar type reaction centers, but the excited state of the reaction center bacteriochlorophylls is significantly more electronegative than in purple bacteria and actual chlorophyll a
• Composition of reaction centers

  • Two bacteriochlorophyll a (special pair)
  • Two additional bacterio chlorophyll a
  • Two bacteriopheophytina
  • Two Quinones
  • One cannonoid molecules
• Acceptors in green sulfur bacteria and Heliobacteria are FeS-proteins that have a much more electronegative redox potential than does NAD+, hence reverse electron flow is unnecessary in green sulfur bacteria and Heliobacteria
• Electron excitation in reaction centers

  Reduces a molecule of bactenochlorophyll a
• Electron transfer in reaction centers

  Bacteriochlorophyll a proceeds to reduce bacteriopheophytin a
• Cytochrome ca

  Periplasmic cytochrome that functions as an election shuttle between the membrane bound bel complex and the reaction center
• Photophosphorylation


  Generation of ATP through photosynthetic electron flow
• Cyclic Photophosphorylation


  Electrons move within a closed loop with no net input or consumption of electrons
• Generation of Reducing power

  Cyclic photophosphorylation only produces ATP but not NADH
• Reducing power for purple bacteria can come from many sources, in particular reduced sulfur compounds such as H2S
• When H2S is formed, electrons end up in the "quinone pool"
• 50% of quinone is insufficiently electronegative to reduce NAD+, hence they must use "reverse electron transport" which is driven by the proton motive force
• Both filamentous anoxygenic phototrophs and purple bacteria employ structurally similar type reaction centers
• Excited state of the reaction center bacteriochlorophylls is significantly more electronegative than in Purple bacteria and that actual chlorophyll a
• Acceptors in green sulfur bacteria and Heliobacteria are Fe-S proteins that have a much more electronegative than NAD+, hence reverse electron flow is unnecessary in green sulfur bacteria or Heliobacteria
• Oxygenic Photosynthesis


  • Contains both types of reaction centers (PS I and PS II)
  • Has two distinct phototypes
• Photosystem I (PSI)


  Has an Fe-S type reaction center
• Photosystem II (PSII)


  Has a Mn4Ca cluster type reaction center
• Electron Flow of oxygenic photosynthesis


  1. P680 chlorophyll a molecule in PSII is excited to a very electronegative state, donating electron to pheophytin a

  2. Oxidation of water by PSII occurs at the water-oxidizing complex and is catalyzed by a "Mn4Ca cluster", which binds 2 molecules of H2O (Generates 4H+)

  3. QA and QB transfer the electrons to PQ (Plastoquinone) which allows for the generation of proton motive force (Generates 4H+ per electron pairs)

  4. Plastocyanin transfers its electron to PSI reaction center

  5. Total of 12 protons are pumped (from H2O and 8 at the PQ)
• Photosystems I and II normally function in tandem in oxygenic photosynthesis
• Calvin cycle


  1. 3PGA will form glucose/fructose

  2. 10 PGA will be rearranged to synthesize 6 ribulose biphosphate

  3. Phosphoribulokinase regenerates 6 molecules of the acceptor molecule, ribulose bisphosphate
• Rubisco can react with oxygen leading to photorespiration