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Medicinal plants (PG 101) Level 1 Clinical Pharmacy-PharmD
• Stages of Photosynthesis :
• Photosynthesis is a two-stage process. In the light-dependent reactions, which
require the presence of light, energy from sunlight is absorbed by photosynthetic
pigments (chiefly the green pigment chlorophyll) and used to bring about the
photolysis of water:
+
−
H2O → 2 +2e +½O2.
• The electrons released by this reaction pass along a series of electron carriers
(electron transport chain); as they do so they lose their energy, which is used to
convert ADP to ATP in the process of photophosphorylation. The electrons and
protons produced by the photolysis of water are used to reduce NADP:
−
+
+
+
2H +2e +NADP → NADP +
• The ATP and NADPH produced during the light-dependent reactions provide
energy and reducing power, respectively, for the ensuing light-independent
reactions (formerly called the 'dark reaction'), which nevertheless cannot be
sustained without the ATP generated by the light-dependent reactions. During these
reactions carbon dioxide is reduced to carbohydrate in a metabolic pathway known
as the Calvin cycle. Photosynthesis can be summarized by the equation:
CO2+2H2O → [ 2O] +H2O+O2
• Since virtually all other forms of life are directly or indirectly dependent on plants
for food, photosynthesis is the basis for all life on earth. Furthermore, virtually all
the atmospheric oxygen has originated from oxygen released during
photosynthesis.
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