Photosynthesis

Overview

• Carbon dioxide plus Water plus solar energy produces sugar plus oxygen
  • 8 - 12% efficiency solar energy to plant biomass
• Metabolic pathways
  • light reactions
    • cyclic phosphorylation
      
    • non-cyclic phosphorylation
  • Calvin cycle (dark reactions)
    
  • electron transport system

cyclic phosphorylation

• solar energy
• chlorophyll a
  • releases e^-
• Electron Transport System
  • ADP → ATP
  • returns e^- to chlorophyll a
• Allows plants to utilize solar energy without making sugar
  • store ATP for other metabolic functions
  • may heat leaf during early morning light

non-cyclic phosphorylation

• solar energy
  
• photosystem II (chlorophyll a)
  • releases e^-
  • H₂O → 2 H⁺ + ½ O₂ + e^-
  • returns e^-
• Electron Transport System
  • ADP → ATP
    
  • delivers e^- to
• photosystem I (pigment complex)
  • releases e^- to
  • NADP⁺ → NADPH
• NADPH and ATP utilized as energy source for Calvin Cycle
  • may store ATP for other metabolic functions

Calvin Cycle

• 3 CO₂ + 3 RuBP (5-C)
• 3 6-C intermediate
• 6 PGA (3-C)
  • 6 ATP → 6 ADP
• 6 PGAP (3-C)
  • 6 NADPH → 6 NADP⁺
• 6 PGAL (3-C)
• 5 PGAL (3-C)
  • 3 ATP → 3 ADP
  • releases 1 PGAL
  • 2 PGAL → glucose-P, or fructose-P
    
  • glucose-P + fructose-P → sucrose (disaccharide)
• 3 RuBP (5-C)
  
  
• outside chloroplast:
  • sucrose → energy source for other structures in the plant
    
  • glucose-P → starch
  • glucose-P → cellulose
  • PGAL → fatty acid synthesis
  • PGAL → amino acid synthesis
• Only some known photosynthetic organisms (C₃ plants)
  use Calvin Cycle as described
• others utilize different pathways
  • C₄ [some grasses]
  • CAM [crassulacea, orchids, bromeliads, cacti, other succulents] plants
  • photosynthetic bacteria (Bluegreens and others)

electron transport system (in thylakoid of grana)

• NADH
• NADH reductase
  • 2 ADP → 2 ATP
• coenzyme Q
• cytochrome b
  • 2 ADP → 2 ATP
• cytochrome c
• cytochrome oxidase
  • 2 ADP → 2 ATP
• water
  
  
• All known organisms use this electron transport system
  

other important reactions

• sulfates → sulfites
  • incorporated into sulfur-contain amino acids
• nitrates → nitrites [in roots]
• nitrites → ammonia [in chloroplasts]
  • converted to amino groups (H₂N-)
  • incorporated into amino acids

excess energy from photosynthesis

• stored in woody tissues
  • converted to peat
  • converted to lignite [soft coal]
  • converted to bituminous coal
  • converted to anthracite [hard coal]
• also sequesters excess CO₂ from atmosphere
• CO₂ also sequestered by animals as calcium carbonate
  • as shells
  • converted to limestone

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