Photosynthesis

• Photosynthetic pigment

  A pigment involved in photosynthesis
• The significance of photosynthesis

  • Increases atmospheric oxygen
  • Photosynthesis and the global carbon carbon cycle, reduces carbon in the atmosphere
  • Photosynthesis helps plants and producers, some organisms to survive by generating food (glucose)
• Photosynthesis

  Converting light to chemical energy
• Life on earth ultimately depends on energy derived from the sun
• Photosynthesis is the only biological process that can harvest this energy
• Chloroplast

  Where photosynthesis happens
• The key components of the light reactions

  • Pigments
  • Photosystems
  • Electron transport chain
• Photosynthetic pigment function

  Absorbs the light that powers photosynthesis
• The electromagnetic spectrum and the segment most important to life (380 nm - 750 nm)
• The atmosphere is selective and only allows visible light to pass through
• Chlorophyll a


  Green pigment found in plants, algae and cyanobacteria that absorbs light to power photosynthesis
• Chlorophyll b

  Chlorophyll with a different structure that absorbs 500-640 nm
• Carotenoids

  Accessory pigments that absorb blue-violet light and provide photoprotection
• The photosynthetic pigments

  • Chlorophyll a
  • Chlorophyll b
  • Carotenoids
• The light reactions

  1. Resonance energy transfer

  2. Charge separation

  3. Electron transfer

  4. Photolysis
• Exergonic


  Releasing energy
• Endergonic


  Requiring energy
• ATP

  Most important and versatile energy carrier in cells
• ATP

  1. Phosphorylation

  2. Hydrolysis
• NADP+


  Carrier of energy in the form of electrons and protons, used in anabolic reactions
• The electron transport chain

  1. Light energy drives ATP and NADPH synthesis

  2. Oxygen evolving complex catalyzes water splitting

  3. Proton gradient drives ATP synthase
• The Z-scheme

  Coupling of PSII and PSI to boost electron energy for NADPH production
• Cyclic electron flow


  Generates more ATP without NADPH
• Non-cyclic vs cyclic photophosphorylation

  • Non-cyclic: Involves PSI and PSII, produces O2, NADPH and ATP

  Cyclic: Involves only PSI, produces ATP
• The light reactions


  • The splitting of water molecules

  The production of oxygen

  The excitation and transport of electrons

  The generation of an electrochemical gradient

  The production of NADPH and ATP
• The 'dark' reactions are the light-independent reactions, also called the carbon reactions
• The Calvin-Benson-Bassham cycle


  The light-independent reactions that fix CO2 into organic compounds
• The Calvin-Benson-Bassham cycle


  1. Regeneration

  2. Reduction

  3. Carboxylation

  4. Output
• RuBisCO


  Ribulose bisphosphate carboxylase/oxygenase, central to the Calvin cycle
• RuBisCO is large, slow and confused!
• Photorespiration


  Occurs when RuBisCO fixes oxygen instead of CO2, an energetically costly salvage pathway
• Carbon concentrating mechanisms


  • C4 plants

  CAM plants

  Algae
• Crassulacean acid metabolism (CAM)


  Night: CO2 stored as malate

  Day: CO2 released and fixed by RuBisCO
• Law of limiting factors

  The factor which is in the shortest supply is likely to be the one which is determining the rate of photosynthesis
• Limiting factors

  • Light

  Carbon dioxide

  Temperature
• Bright light


  Increases production of ATP, NADPH and O2
• Dim light

  Increases 3-phosphoglycerate (GP) but not enough to convert to triose phosphate (TP)
• High CO2

  More carbon is fixed by RuBisCO, increased GP and TP
• Low CO2

  RuBP accumulates as carbon fixation is limited, GP and TP not formed
• Increasing temperature

  Increases carboxylation rates but decreases RuBisCO's affinity for CO2, reducing CO2 uptake