The Light Reaction
Created by Caitlynn Lied 2008
H
ATP
H
O
NADP+
Sunlight filled with
photons shines onto the
plant
H
Photosystem I
Photosystem II
The photons excite the electrons in the chlorophyll
molecules
The electrons move to the outer energy levels
As the electron moves from an excited state (high
energy) to ground state (low energy), the difference in
energy is passed to a neighboring chlorophyll molecule.
H
The energy continues to move to every
chlorophyll molecule in the photosystem
H
Photosystem I
Photosystem II
The last chlorophyll molecule that the energy
reaches is called the reaction center (also
called P680 chlorophyll)
The molecule becomes so energized that it throws
off its electron, which is accepted by the Primary
Electron Acceptor (PEA)
H
The PEA moves along a pathway known
as an Electron Transport Chain to
Photosystem I. Passing along a H –pump.
While energy was being passed in
Photosystem II, the same was
happening in Photosystem I
The electron from the first PEA then
replaces the one lost in Photosystem I
H
Photosystem I
Photosystem II
NOTE: The reaction center in Photosystem II is called P680,
but its called P700 in Photosystem I- the difference is in
what type of wavelength the molecule absorbs
H
As the PEA traveled along the E.T.C, energy from
the electron was used to actively transport a
hydrogen ion from the stroma into the thylakoid
space
H
Photosystem I
Photosystem II
2H2O molecules split to produce 4 hydrogen
ions, 1 oxygen gas molecule, and 4 electrons.
These electrons replaces the
ones lost in the reaction center
in Photosystem II
The O2 molecule leaves the plant, and the
H+ moves off to a latter part of the cycle
H
H
O
H
At this point, the reaction no longer
needs Photosystem II
Photosystem I
Photosystem II
H
Now, NADP+ enters the NADP Reductase from the dark reaction
The electron from the PEA and a Hydrogen Ion
H
from the stroma combine with the NADP+ to
form one NADPH
Then the NADPH leaves to go back to the dark reaction
osystem I
NADPH
H
O
As
H+ATP
passes
through
the
membrane,
enoughfrom
The
After
that,
Synthase
ADP and
pumps
a phosphate
the H+ (originally
energy
is supplied
to through
combine
an
ADPtransport
+ P to
H(also
from
thethe
stroma
dark
reaction)passive
enters
2O) to
form
molecule.
the one
ATPATP
Synthase
osystem I
H
H
O
Then the ATP moves back to the dark reaction
ATP