Unit 8.4

• Photosynthetic Organisms


  • Algae
  • Plants
  • Cyanobacteria

  
• Photosynthesis


  A process that captures solar energy and transforms it into chemical energy stored in carbohydrates

  
• Photosynthesizers produce food energy to feed themselves and heterotrophs
• Photosynthetic organisms


  • Photosynthesis takes place in the green portions of plants
  • The raw materials for photosynthesis are carbon dioxide and water

  
• The Process of Photosynthesis

  1. Light Reactions

  2. Calvin Cycle Reactions

  
• NADP+


  A redox coenzyme active during photosynthesis, when reduced accepts 2 e- and 1 H, and when oxidized, gives up e-

  
• Light Reactions


  • Take place only in the presence of light
  • Energy-capturing reactions
  • Chlorophyll absorbs solar energy
  • Energizes electrons
  • Electrons move down an electron transport chain
  • Pumps H+ into thylakoids
  • Used to make ATP out of ADP and NADPH out of NADP
• Calvin Cycle Reactions


  • Take place in the stroma
  • CO2 is reduced to a carbohydrate
  • Use ATP and NADPH to produce carbohydrate

  
• Pigments


  Chemicals that absorb certain wavelengths of light, wavelengths that are not absorbed are reflected/transmitted

  
• Light Reactions


  • Consist of two alternate electron pathways: Noncyclic pathway and Cyclic pathway
  • Both pathways produce ATP
  • The noncyclic pathway also produces NADPH
• Noncyclic Pathway


  1. Takes place in the thylakoid membrane

  2. Uses two photosystems, PS I and PS II

  3. PS II captures light energy and causes an electron to be ejected

  4. Electron travels down electron transport chain to PS I

  5. Replaced with an electron from water, which is split to form O2 and H+

  6. H+ accumulates in thylakoid chambers

  7. H+ gradient is used to produce ATP

  8. PS I captures light energy and ejects an electron, which is transferred permanently to a molecule of NADP+, causing NADPH production

  
• Thylakoid Space


  • Acts as a reservoir for hydrogen ions (H+)
  • Each time water is oxidized, two H+ remain in the thylakoid space
  • Transfer of electrons in the electron transport chain yields energy used to pump H+ across the thylakoid membrane
  • Flow of H+ back across the thylakoid membrane energizes ATP synthase, which enzymatically produces ATP from ADP + Pi
• Chemiosmosis


  The method of producing ATP where the flow of H+ back across the thylakoid membrane energizes ATP synthase
• The Calvin Cycle


  1. A cyclical series of reactions

  2. Utilizes atmospheric carbon dioxide to produce carbohydrates

  3. Involves three stages: Carbon dioxide fixation, Carbon dioxide reduction, RuBP regeneration

  
• The Calvin Cycle


  • One CO2 is attached to a 5-carbon ribulose-1,5-biphosphate (RuBP) resulting in a 6-carbon molecule that splits into two 3-carbon molecules, 3-phosphoglycerate (3PG)
  • ATP used to convert 3PG into glyceraldehyde-3-phosphate (G3P)
  • RuBP carboxylase (Rubisco) protein that speeds up this reaction
  • For every 3 turns of the Calvin Cycle, 5 molecules of G3P are used to reform 3 molecules of RuBP, allowing the cycle to continue

  
• Photorespiration


  A wasteful process where O2 starts combining with RuBP, leading to the production of CO2, which is not part of the Calvin Cycle

  
• C4 Plants


  • Fix CO2 to a C3 molecule phosphoenolpyruvate (PEP) to form oxaloacetate, a C4 molecule
  • Avoid photorespiration
  • Have a net productivity about 2-3 times greater than C3 plants
  • Include sugarcane, corn, Bermuda grass, crab grass

  
• CAM Photosynthesis


  • Crassulacean-Acid Metabolism
  • CAM plants partition carbon fixation by time: During the night they fix CO2 and form C4 molecules which are stored, and during daylight they release the CO2 to the Calvin cycle

  
• Advantages and Disadvantages of Photosynthesis Methods

  • C4 plants are most adapted to high light intensities, high temperatures, and limited rainfall
  • C3 plants are better adapted to cold (below 25°C) and high moisture
  • CAM plants are better adapted to extreme aridity