photosynthesis

photosynthesis
    n 1: synthesis of compounds with the aid of radiant energy
         (especially in plants)
    

Photosynthesis \Pho`to*syn"the*sis\, n. (Plant Physiol.)
   The process of constructive metabolism in which green plants
   utilize the energy of sunlight to manufacture carbohydrates
   from carbon dioxide and water in the presence of chlorophyll.
   It was formerly called {assimilation}, but this is now
   commonly used as in animal physiology. --
   {Pho`to*syn*thet"ic}, a. -- {Pho`to*syn*thet"ic*al*ly}, adv.
   [Webster 1913 Suppl.]

   Note: In green plants water is absorbed by the roots and
         carried to the leaves by the xylem, and carbon dioxide
         is obtained from air that enters the leaves through the
         stomata and diffuses to the cells containing
         chlorophyll. The green pigment chlorophyll is uniquely
         capable of converting the active energy of light into a
         latent form that can be stored (in food) and used when
         needed.
         The initial process in photosynthesis is the
         decomposition of water (H2O) into oxygen, which is
         released, and hydrogen; direct light is required for
         this process. The hydrogen and the carbon and oxygen of
         carbon dioxide (CO2) are then converted into a series
         of increasingly complex compounds that result finally
         in a stable organic compound, glucose (C6H12O6 ), and
         water. This phase of photosynthesis utilizes stored
         energy and therefore can proceed in the dark. The
         simplified equation used to represent this overall
         process is 6CO2+12H2O+energy=C6H12O6+6O2+6H2 O. In
         general, the results of this process are the reverse of
         those in respiration, in which carbohydrates are
         oxidized to release energy, with the production of
         carbon dioxide and water.
         The intermediary reactions before glucose is formed
         involve several enzymes, which react with the coenzyme
         ATP (see adenosine triphosphate ) to produce various
         molecules. Studies using radioactive carbon have
         indicated that among the intermediate products are
         three-carbon molecules from which acids and amino
         acids, as well as glucose, are derived.
         --http://www.infoplease.com/ce6/sci/A0860378.html

   Note: The role of chlorophyll
         Chlorophyll contains a hydrophyllic head group and a
         hydrophobic tail region. A magnesium atom is held in
         the center of a cyclic, conjugated double bond
         porphyrin ring which is responsible for absorbing red
         light. (There also is an absorption band in the blue.
         Thus red and blue are absorbed and green passes
         through, giving plants a characteristic green color.)
         Light is absorbed by antenna chlorophyll molecules,
         then transferred to the reaction center chlorophylls.
         Some hundreds of antenna chlorophyll molecules transfer
         energy to a reaction center, with transfer times of
         about 10-10 sec from the edge of the unit to the
         center.
         The energy from light is used to pump H+ ions from the
         stroma into the thylakoid space and to reduce NADP+ to
         NADPH. Flow of H+ back into the stroma releases energy
         which is used to phosphorylate ADP to ATP. The
         chemiosmotic coupling is working here in a similar way
         to the mechanism of ATP generation used in
         mitochondria.
         Carbon Fixation Carbon fixation is catalyzed by
         ribulose bisphosphate carboxylase (RuBP carboxylase),
         the world's most abundent enzyme.
         The

   { Calvin cycle} combines three carbon dioxide molecules into
      one molecule of three carbon glyceraldehyde 3-phosphate.
      Some plants, particularly many which live in hot, dry
      climates, have a mechanism for storing carbon dioxide by
      combining it with a three carbon molecule to form a four
      carbon molecule. This pathway is known as the C4 or
      Hatch-Slack pathway.

      --http://fig.cox.miami.edu/Faculty/Tom/bil255/bil255sum98/17_photo.html
      [PJC] Phototaxis