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Biology LibreTexts

Photosynthesis

 

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)