What are ATP and NADPH and how are they related to the production of energy in plants, i.e., how the synthesis of ATP and NADPH in the light reaction is helpful in the production of energy (basically what are their functions)?

Asked by Himadri Sarkar | 17th Dec, 2015, 06:05: PM

Expert Answer:

  • The light reaction of photosynthesis involves absorption of sunlight by the pigments of photosystems and the conversion of their energy into usable chemical energy in the form of Adenosine Triphosphate (ATP) and Nicotinamide Adenine Diphosphate (NADPH) + H+.
  • ATP and NADPH act as energy driving factors in the biosynthetic phase (Calvin cycle) of photosynthesis. The Calvin cycle uses ATP and NADPH2 for the fixation and reduction of CO2 to form carbohydrates.
  • Each Calvin cycle is completed in three phases—carboxylation, reduction and regeneration.
  • During reduction or the glycolytic reversal phase, carbohydrate is formed at the expense of the photochemically made ATP and NADPH. Here, two molecules each of ATP and NADPH are required for fixing one molecule of CO2.
  • During regeneration phase, RuBP regenerates to enable the cycle to continue uninterrupted. Here, 1 ATP molecule is required. For the formation of one molecule of glucose, six molecules of CO2 need to be fixed; hence, six cycles are required.
  • Total ATP required: 
    For fixing 1 molecule of CO2− 3 (2 for reduction and 1 for regeneration)
    For fixing 6 molecules of CO2 − 3 × 6 = 18 ATP
  • Total NADPH required:
    For fixing 1 molecule of CO2 − 2 (for reduction)
    For fixing 6 molecules of CO2 − 2 × 6 = 12 NADPH
  • Thus, the synthesis of 1 molecule of glucose requires 18 ATP and 12 NADPH.

Answered by Sheetal Kolte | 18th Dec, 2015, 12:12: PM