Download During the light reactions, there are two possible routes of electron

During the light reactions, there are two possible routes of electron flow: cyclic and noncyclic.
NONCYCLIC ELECTRON FLOW
 predominates during the light reactions of photosynthesis
 electrons ejected from chlorophyll molecules do not cycle back to the ground state in
chlorophyll
Two products:
 NADP+ is reduced to NADPH + H+ (needed for the Calvin cycle)
 ATP is synthesized (needed for the Calvin cycle)
Note: ATP synthesis occurs through photophosphorylation (light-dependent
formation of ATP by chemiosmosis)
The steps:
1. A photon strikes photosystem II and excites an electron of P680 (chlorophyll a of
photosystem II). The excited electron is then captured by the primary electron
acceptor. This process occurs twice, therefore 2 excited electrons are passed on.
2. The two photoexcited electrons pass from the primary electron acceptor of
photosystem II to photosystem I via an electron transport chain.
3. As the electrons are passed down the ETC, energy is given off. This energy is used to
pump hydrogen ions (protons) from the stroma into the thylakoid lumen, creating a H+
gradient for chemiosmosis. Four protons are translocated into the thylakoid lumen for
each pair of electrons that passes through the transport chain. Protons that
accumulate in the thylakoid lumen create an electrochemical gradient. As protons
move through the ATPase complex, ATP is formed. This process is called noncyclic
photophosphorylation.
4. The two electrons reach the bottom of the ETC and are passed to P700 (chlorophyll a
of photosystem I). These electrons replace the electrons that are lost by P700 when it
is struck by photons and loses its electrons to the primary electron acceptor of
photosystem I.
5. The primary electron acceptor of photosystem I then passes the electrons through
another ETC containing ferredoxin (Fd). An enzyme called NADP+ reductase then
transfers the two electrons and H+ from the stroma to reduce NADP+ to NADPH.
Note: An enzyme splits water into an oxygen atom, 2H+, and 2 electrons. The 2 electrons are
given to P680, replacing the electrons that the chlorophyll molecule lost when it absorbed light
energy. The oxygen atom combines with another one, releasing O 2. The 2 H+ are used by
the ATPase complex.