Big Picture 06
Photosynthesis and Respiration



PHOTOSYNTHESIS:
  • Photosynthesis only occurs in autotrophs such as plants, algae, and cyanobacteria.
  • Photosynthesis is a two-step process:
    • 1. The "light reactions" use solar energy and chlorophyll to split water molecules into hydrogen and oxygen (the "Hill reaction").   Oxygen gas is released as a waste product. The hydrogen combines with NADP to make NADPH.
      Energy is used to combine ADP + phosphate and synthesize ATP; this energy is used to drive the dark reactions.
    • 2. The "dark reactions" (Calvin cycle): In a series of reactions, atmospheric CO2 combines with NADPH and is "fixed" (initially) into carbohydrate compounds such as glucose. Water is released.
    • OVERALL REACTION:
      12 H2O     +     6 CO2       ——>       C6H12O6     +     6 H2O     +     6 O2
              water       carbon dioxide                 glucose           new water       oxygen gas



    CELLULAR RESPIRATION:
    • Respiration occurs in all organisms, both autotrophs (in the dark) and heterotrophs (all the time).
    • Chemically, respiration is the opposite of photosynthesis:
      C6H12O6     +     6 O2      
        glucose           oxygen      
         respiration
         ————>
         <————
      photosynthesis
        6 H2O     +     6 CO2
          water       carbon dioxide


    GLOBALLY, Photosynthesis takes CO2 from the atmosphere and releases oxygen;
          respiration takes oxygen from the atmosphere and releases CO2.