photosynthesis

• Photosynthesis

  Anabolic, endergonic, carbon dioxide requiring process that uses light energy an water to produce organic macromolecules
• Photosynthesis

  6CO2 + 12H2O + Light energy → C6H12O6 + 6O2 + 6H2O
• Oxidation


  Adding oxygen, Removing hydrogen, Loss of electrons, Releases energy
• Reduction


  Removing oxygen, Adding hydrogen, Gain of electrons, Stores energy
• Photosynthesis is a redox process (water is oxidized, carbon dioxide is reduced)
• Where does photosynthesis take place


  • In all photoautotrophic organisms
  • All organs with photosynthetic pigments
  • In true plants, the leaves are the major sites of photosynthesis
• Photosynthesis

  1. Light reactions - conversion of solar energy to chemical (NADPH and ATP) energy

  2. Dark reactions
• Light


  Consists of photons; each photon has fixed quantity of energy<|>Is a form of electromagnetic energy (travels in waves)<|>Distance between wave crests determines electromagnetic energy
• Electromagnetic spectrum


  • Range of electromagnetic energy/radiation
  • Visible light spectrum - includes colours of light we can see and includes wavelengths that drive photosynthesis
• Photosynthetic pigments

  Absorb visible spectrum
• Photosynthetic pigments

  • Chlorophyll a - main photosynthetic pigment; fond in all autotrophic organisms
  • Accessory chlorophyll pigments - absorb different wavelengths of light and pass energy to chlorophyll a
  • Chlorophyll b - in all true plants
  • Chlorophyll c - in all golden brown and black algae
  • Chlorophyll d - in red algae
  • Carotenoids - absorb excessive light that would damage chlorophyll
• Absorption spectrum


  Graph plotting pigment's light absorption vs wavelength
• Absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
• Thylakoids


  Where photosynthetic pigments are located
• Thylakoids have organized complexes called photosystems
• Photosystem components

  • Major reaction center (chl a)
  • Accessory pigments
  • Primary electron acceptor
• Thylakoid membrane has 2 types of photosystems


  • Photosystem I (PS I) : P700
  • Photosystem II (PS II) : P680
• Two types of electron flow

  • Cyclic electron flow - electrons cycle back from Fd to PS I reaction center
  • Non-cyclic electron flow - Involves photosystems I and II
• Linear electron flow

  1. Photon hits pigment and energy is passed among pigment molecules until it makes P680 excited

  2. Excited electron is transferred to primary electron acceptor (turns into P680+)

  3. P680+ is the strongest known biological oxidizing agent

  4. H2O is split by enzymes and electrons are transferred from hydrogen atoms to P680+, reducing it to P680

  5. In PS I, transferred light energy excites P700, which loses an electron to electron acceptor

  6. P700+ accepts an electron passed down from PSI via transport chain

  7. Each electron falls down electron transport chain from primary electron acceptor of PSI to ferredoxin (fd)

  8. Electrons are then transferred to NADP+ and reduces it to NADPH
• Light-dependent reaction


  • H2O, Light
• Carbon-fixation reaction


  • ATP, NADPH, CO2
• Photosynthesis produces

  • ATP
  • NADPH
  • O2 (by-product)
  • Carbohydrate
• Some organisms such as purple sulfur bacteria have PS I but not PS II
• Cyclic electron flow is thought to have evolved before linear electron flow
• Cyclic electron flow may protect cells from light-induced damage
• Chemiosmosis


  Ions move by diffusion across a semi-permeable membrane, such as the membrane inside mitochondria
• During chemiosmosis, ions move down electrochemical gradient from areas of high concentration to areas of low concentration to balance out electric charge across membrane
• Calvin Cycle


  1. Carbon fixation = dark reaction

  2. Occurs in stroma

  3. C3 plants (80% of plants on earth)

  4. Uses ATP and NADPH from light reaction

  5. Uses CO2

  6. 6 turns, 18 ATP, 1 NADPH to produce glucose
• Carbon-fixation reactions

  • Large nitrogen requirement for Rubisco and other photosynthetic enzymes
  • Rubisco accounts for abt 25% of nitrogen in photosynthetic cells
  • Dependence on ATP and NADPH
  • Frequent limitation by CO2 supply to chloroplast
  • Rubisco is both a carboxylase and an oxygenase
• Photorespiration


  Initiates breakdown of sugars to CO2<|>Consumes O2 while producing CO2<|>Reduces photosynthetic efficiency of calvin cycle by as much as 50%
• Alternative mechanisms of carbon fixation

  • Hatch-slack / C4 pathway
  • Crassulacean Acid Metabolism (CAM)
• Photosynthesis is responsible for presence of O2 in atmosphere
• Organic compounds produced by photosynthesis provides energy and building materials for all ecosystems