The following complexes are found in the photosynthesis electron transport chain:
and the complex that makes ATP
In addition to the complexes,
three mobile carriers are also involved:
Other key components include:
and the electrons to form NADPH
which combine to form ATP
Photosynthesis occurs in the chloroplasts
of plants and algae.
The process is also found in single-cell organisms
such as cyanobacteria
that do not have chloroplasts.
Like its mitochondrial counterpart,
the chloroplast electron transport chain
consists of several protein complexes and mobile electron carriers.
First, a photon of light hits a chlorophyll molecule
surrounding the Photosystem II complex.
This creates resonance energy that is transferred through neighboring chlorophyll molecules.
When this energy reaches the reaction center embedded in photosystem II,
an electron is released.
The reaction center chlorophyll contains electrons that can be transferred when excited.
One photon is needed to excite each of the electrons in this chlorophyll.
Once excited, two electrons are transferred to plastoquinone Qb, the first mobile carrier.
In addition to the two electrons,
Qb also picks up two protons from the stroma.
The two electrons lost from photosystem II
are replaced by the splitting of water molecules.
Water splitting also releases hydrogen ions into the lumen.
This contributes to a hydrogen ion gradient
similar to the one created by mitochondrial electron transport.
After two water molecules have been split, one molecule of molecular oxygen is created.
Plastoquinone Qb then transfers the two electrons
to the cytochrome b6-f complex.
The two protons it picked up
are released into the lumen.
These transfers are coupled with the pumping of two more hydrogen ions into the lumen space
by cytochrome b6-f.
The electrons are next transferred to plastocyanin, another mobile carrier.
Next,
the electrons are transferred from plastocyanin
to the Photosystem I complex.
It is here that photons again energize each electron and propel their transfer to ferredoxin.
Ferredoxin then transfers the electrons
to the ferredoxin-NADP-reductase,
also known as FNR.
After two electrons are transfered to FNR,
NADPH is made by adding the two electrons
and a hydrogen ion to NADP+.
The gradient created by the electron transport chain
is utilized by ATP synthase
to create ATP from ADP and Pi.
This is similar to the way ATP is synthesized in the mitochondria.
ATP, NADPH, and molecular oxygen are the final, vital, products of photosynthesis.