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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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