Transcript The Light-Dependent Reactions of Photosynthesis

The Light-Dependent
Reactions of Photosynthesis
Three main “processes”.
The first two are the ways photosystems I and
II can act:
1. Non-cyclic photophosphorylation
• Photosystems I and II act together to make
the chemiosmotic gradient in order to
reduce NADP+ to NADPH AND establish
(H+) gradient so ATP can be made.
• The energy from a photon is used to excite an
electron in Photosystem II which is passed into
the ETS (and therefore draws H+ into the lumen
of the thylakoid) and onto Photosystem I in order
to replace another electron that has been excited
by a photon that is finally accepted by NADP+ in
order to make NADPH
• This original electron excited in Photosystem II is
replaced by photolysis of water (splitting of water
by light) and oxygen and H+ are formed as byproducts.
2. Cyclic photophosphorylation
• Photosystem I acts alone to make a chemiosmotic
(H+) gradient so ATP can be made instead of
NADPH.
• This occurs because there is plenty of light and too
much NADPH is being made which creates a
shortage of NADP+ that is the final electron
acceptor in Noncyclic photophosphorylation.
• Therefore, the electron passes along carriers
(drawing one H+ into the lumen of the thylakoid)
and back to photosystem I instead of being accepted
by NADP+.
http://bioweb.wku.edu/courses/Biol120/images/Photophosphryl.htm
3. Chemiosmotic Phosphorylation
• is possible because of the chemiosmotic
gradient built up during the cyclic and noncyclic events.
• positive charges (H+) that accumulated in
the lumen and build a steep electrochemical
gradient
• the H+ escape down through channels to
ATP synthase where the free energy of H+
and enzymes join ADP to Pi to make ATP
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http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/harve
stinglight.swf
http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynth
esis.swf
http://faculty.nl.edu/jste/noncyclic_photophosphorylation.htm
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter10/animations.html
http://www.web.virginia.edu/gg_demo/movies/figure18_12b.html
http://www.fw.vt.edu/dendro/forestbiology/photosynthesis.swf