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Biology LibreTexts

Photosynthesis

  • Page ID
    1342
  • Compare photosynthesis to respiration:
    Respiration
    C6H12O6 + 6O2 → 6CO2 + 6H2O
    ΔGo’ = -2840 kJ/mol
    Photosynthesis in green plants
    6 CO2 + 6H2O → C6H12O6 + 6O2
    ΔGo’ = +2840 kJ/mol (Unfavorable ΔGo’ explains the need for energy input.)
    Compare photosynthesis in plants with bacteria
    Plants (divided by 6, H2O added to each side)
    CO2 + 2 H2O → C(H2O) + O2 + H2O
    Photosynthesis in green sulfur bacteria
    CO2 + 2H2S → C(H2O) + 2S + H2O
    Photosynthesis in purple non-sulfur bacteria
    CO2 + 2CH3CHOHCH3 → C(H2O) + 2CH3COCH3+ H2O
    isopropanol acetone
    C O2 + 2 CH3CHOHCOOH → C(H2O) + 2 CH3COHCOOH + H2O
    lactate pyruvate
    Generalize
    CO2 + 2H2D → C(H2O) + 2D + H2O
    D = H/electron donor
    photosyn.png
    So the light reactions must produce NADPH and ATP to provide energy and H
    The light reactions occur on the thylakoid membranes of chloroplasts (or the plasma membrane or internal membranes of bacteria).
    Picture 4.png
    (Note the “lumen” of thylakoids)
    The light reactions occur on the thylakoid membranes of chloroplasts (or the plasma membrane or internal membranes of bacteria).

    Light reactions

    • 2 photosystems
    • transfer of e- from H2O (in the lumen) to NADP+ (in the stroma)
    • transfer of H+ from stroma to lumen (and back through ATP synthase)