7.8 In light
reactions,
electron chains
generate ATP
and NADPH
Overview of light reactions
 During
light reactions, light energy is
transformed to chemical energy (ATP and
NADPH)
 In this reaction, electrons that are
removed from water molecules pass from
photosystem II to photosystem I to NADP+.
 Between these 2 photosystems, electrons
move down an electron transport chain,
providing energy to make ATP.
Flow of electrons
Steps 1-3

1: Photon is absorbed by a
pigment and then passed
down to other pigments until
reaching the reaction center
of Photosystem II, exciting
an electron of chlorophyll
P680 to a higher energy
level.

2: Electron is captured by
primary electron acceptor.

3: Water is split, and its
electrons are supplied one
by one to P680, replacing
those lost to primary
electron acceptor. Then the
oxygen atom combines with
another oxygen from a split
water molecule, forming O2.
Step 4
 4:
Each
photoexcited
electron moves
from Photosystem II
to Photosystem I by
an electron
transport chain. The
“fall” of electrons
provides energy for
the creation of ATP.
Steps 5-6

5: Meanwhile, light
energy excites an
electron of chlorophyll
P700 in the reaction
center of photosystem I.
The primary electron
acceptor captures the
excited electron and an
electron from the bottom
of the electron transport
chain replaces the lost
electron in P700.

6: Excited electron of
photosystem I goes
through short electron
transport chain to NADP+,
reducing it to NADPH.
Mechanical analogy
This analogy can help
you understand this
process. The yellow
mallets represent the
input of light energy
which pushes electrons
in the photosystems to
excited states. These
electrons are then
caught by the primary
electron acceptor (the
person on top of each
platform). Photosystem
II then passes the
electrons through an
ATP mill, and
Photosystem I hands its
electrons off, reducing
NADP+ to NADPH.