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Chapter 7 Notes

by: Min-Young Kim

Chapter 7 Notes BIOL 3040

Min-Young Kim
GPA 4.0

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About this Document

These notes cover Chapter 7 of our textbook regarding Photosynthesis.
Biology of Plants
Christina Wells
Class Notes
biology of plants
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This 3 page Class Notes was uploaded by Min-Young Kim on Tuesday April 5, 2016. The Class Notes belongs to BIOL 3040 at Clemson University taught by Christina Wells in Spring 2016. Since its upload, it has received 15 views. For similar materials see Biology of Plants in Biology at Clemson University.


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Date Created: 04/05/16
Chapter  7  Notes     -­‐ In  photosynthesis,  light  energy  is  converted  to  chemical  energy  and  carbon  is   “fixed”  into  organic  compounds   3CO 2+  6H2O  light  à 3  C6H 3  + 2 3O  2  3H O   o First  step:  absorption  of  light  energy  by  pigment  molecules   § Eukaryotic  photosynthesis:  chlorophylls  and  carotenoids,   packed  in  thylakoids  of  chloroplasts  as  photosynthetic  units   called  photosystems   § Light  absorbed  by  pigment  molecules  boosts  electrons  to   higher  energy  level   § Arrangement  of  pigment  molecules  allows  transfer  of  energy  to   a  pair  of  special  chlorophyll  a  molecules  at  reaction  centers   o Photosystem  I  &  Photosystem  II  work  together  simultaneously  and   continuously   § Photosystem  I:  can  carry  out  photosynthesis  independently  of   Photosystem  II   • No  external  donor  (water)  and  thus  no  production  of   NADPH   o Cyclic  phosphorylation  only  results  in  proton  gradients  used  for  ATP   production   o Major  reactions  of  photosynthesis  divided  into  light  reactions  and   carbon-­‐fixation  reactions   -­‐ In  light  reactions,  electrons  flow  from  water  to  Photosystem  II,  down   Electron  Transport  Chain  to  Photosystem  I,  and  finally  to  NADP+   o Light  energy  enters  Photosystem  II,  trapped  by  pigment  molecules,   passed  to  P680  chlorophyll  molecules  of  reaction  center   o Energized  electrons  transferred  from  P 680  to  electron  acceptor   o As  electrons  removed  from  P 680,  replaced  by  low-­‐energy  electrons   from  water  molecules  and  oxygen  is  produced  (water  photolysis)   o Pairs  of  electrons  pass  downhill  to  Photosystem  I  along  ETC;  passage   generates  proton  gradient  that  drives  synthesis  of  ATP  from  ADP  and   phosphate  (photophosphorylation)     o Light  energy  absorbed  in  Photosystem  I  is  passed  to  P 700  chlorophyll   molecules  of  Photosystem  I  reaction  center   o Energized  electrons  ultimately  accepted  by  coenzyme  molecules   NADP+,  and  electrons  removed  from  P 700  are  replaced  by  electrons   from  Photosystem  II   o Energy  yield  from  light-­‐dependent  reactions  stored  in  molecules  of   NADPH  and  in  ATP  formed  by  photophosphorylation   o Photophosphorylation  also  occurs  in  cyclic  electron  flow;  does  not   require  Photosystem  II   o Only  product  of  cyclic  electron  flow  is  ATP   o Extra  ATP  is  required  by  Calvin  cycle,  which  uses  ATP  and  NADPH  in  a   3:2  ratio   -­‐ In  electron  transport  chain,  electron  flow  is  coupled  to  proton  pumping  and   ATP  synthesis  by  a  chemiosmotic  mechanism   o Like  oxidative  phosphorylation  in  mitochondria,  photosphorylation  in   chloroplasts  is  a  chemiosmotic  process.   o As  electrons  flow  down  ETC  from  Photosystem  II  to  Photosystem  I,   protons  pumped  from  stroma  into  thylakoid  lumen,  creating  gradient   of  potential  energy   o As  protons  flow  down  gradient  from  thylakoid  lumen  back  into   stroma,  pass  through  ATP  synthase,  generating  ATP   -­‐ In  Calvin  cycle,  C2  is  fixed  via  a  3-­‐Carbon  pathway   o In  carbon-­‐fixation  reactions,  which  take  place  in  stroma  of   chloroplast,  NADPH  and  ATP  produced  in  light  reactions  are  used  to   reduce  carbon  dioxide  to  organic  carbon   o Calvin  cycle  responsible  for  initial  fixation  of  CO 2 and  subsequent   reduction  of  newly  fixed  carbon   o In  Calvin  cycle,  molecule  of  CO 2 combines  with  starting  compound:  5-­‐ carbon  sugar  called  ribulose  1,5-­‐biphosphate  (RuBP),  to  form  two   molecules  of  3-­‐carbon  compound  3-­‐phosphoglycerate  (PGA)   o PGA  reduced  to  3-­‐carbon  molecule  glyceraldehyde  3-­‐phosphate   (PGAL),  with  electrons  provided  by  NADPH  and  energy  provided  by   ATP  hydrolysis   o At  each  turn  of  Calvin  cycle,  one  carbon  atom  enters  cycle.  Three  turns   of  cycle  produce  one  molecule  of  glyceraldehyde  3-­‐phosphate.     o At  each  turn  of  cycle,  RuBP  is  generated.  Most  of  fixed  carbon  is   converted  to  either  sucrose  or  starch   -­‐ Carbon-­‐fixation  pathway  in  C  4lants  is  a  solution  to  problem  of   photorespiration   o C  3lants:  Plants  in  which  Calvin  cycle  is  only  carbon-­‐fixation  pathway,   and  in  which  first  detectable  product  of  C2  fixation  is  3-­‐carbon   compound  3-­‐phosphoglycerate  (PGA)   o C  4lants:  CO 2initially  fixed  to  phosphoenolpyruvate  (PEP)  to  yield   oxaloacetate,  a  four  carbon  compound   § Reaction  occurs  in  mesophyll  cells  of  leaf   § Oxaloacetate  rapidly  converted  to  malate,  which  moves  from   mesophyll  cells  to  bundle-­‐sheath  cells   § Malate  decarboxylated  and  CO  ent2rs  Calvin  cycle  by  reacting   with  ribulose  1,5-­‐biphosphate  (RuBP)  to  form  PGA   § C 4  pathway  takes  place  in  mesophyll  cells,  but  Calvin  cycle   occurs  in  bundle-­‐sheath  cells   o C  4lants  more  efficient  utilizers  of  CO 2  than  C3  plants   § PEP  carboxylase  is  not  inhibited  by  O2.     § C 4  plants  can  attain  same  photosynthetic  rate  as  C3  plants,  but   with  smaller  stomatal  openings  and  less  water  loss   § C  plants  more  competitive  than  C  plants  at  high  temperatures   4 3 -­‐ CAM  plants  can  fix  CO2  in  the  dark   o Crassulacean  acid  metabolism  (CAM)  occurs  in  many  succulent  plants   o Fixation  of  CO 2 to  phosphoenolpyruvate  (PEP)  to  form  oxaloacetate   occurs  at  night  when  stomata  open   o Oxaloacetate  rapidly  converted  to  malate,  which  is  stored  overnight  in   vacuole  as  malic  acid   o During  daytime,  when  stomata  are  closed,  malic  acid  recovered  from   vacuole  and  fixed  CO2  is  transferred  to  ribulose  1,5-­‐biphosphate   (RuBP)  of  Calvin  cycle   o C 4pathway  and  Calvin  cycle  occur  within  same  cells  in  CAM  plants;   hence,  two  pathways  which  are  spatially  separated  in  C4  plants,  are   temporally  separated  in  CAM  plants  


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