Photosystems are crucial units in photosynthesis that absorb light and facilitate energy and electron transfer in chloroplasts. Each photosystem consists of an antenna complex that captures light and a reaction center where light energy is converted to chemical energy. Photosystem I specifically transfers electrons from plastocyanin to ferredoxin, producing NADPH, with its function dependent on various protein complexes and pigment molecules.

1. PLANT
PHYSIOLOGY
PHOTOSYSTEM I

2. PHOTOSYSTEM
 Photosystems are functional and structural units of protein
complexes involved in photosynthesis. Together they carry
out the primary photochemistry of photosynthesis: the
absorption of light and the transfer of energy and
electrons.

3.  In the chloroplast the pigment molecules (chlorophylls a and b and
carotenoids) are embedded in the thylakoids in discrete units of
organization called photosystems.
 Each photosystem includes an assembly of about 250 to 400
pigment molecules and consists of two closely linked components:
an antenna complex and a reaction center.
 The antenna complex consists of pigment molecules that gather
light energy and “funnel” it to the reaction center.

4.  The reaction center is made up of a complex of proteins
and chlorophyll molecules that enable light energy to be
converted into chemical energy.
 Within the photosystems, the chlorophyll molecules are
bound to specific chlorophyll-binding membrane proteins
and held in place to allow efficient capture of light energy.

5.  All of the pigments within a photosystem are capable of absorbing
photons, but only one special pair of chlorophyll a molecules per
reaction center can actually use the energy in the photochemical
reaction.
 This special pair of chlorophyll a molecules is situated at the core
of the reaction center of the photosystem.
 The other pigment molecules, called antenna pigments because
they are part of the light-gathering network, are located in the
antenna complex.

6.  Each photosystem is generally associated with a light harvesting
complex composed of chlorophyll a and b molecules, along with
carotenoids and pigment-binding proteins.
 Like antenna complex of a photosystem, the light-harvesting
complex also collects light energy, but it does not contain a
reaction center. A photosystem and its associated light-harvesting
complexes are collectively referred to as a photosystem
complex.

7.  Light energy absorbed by a pigment molecule anywhere in an
antenna complex or light-harvesting complex is transferred from one
pigment molecule to the next by resonance energy transfer until it
reaches the reaction center, with its special pair of chlorophyll a
molecules.
 When either of the two chlorophyll a molecules of the reaction center
absorbs energy, one of its electrons is boosted to a higher energy
level and is transferred to an electron-acceptor molecule to initiate
electron flow.

9. PHOTOSYSTEM I
 Photosystem I is an integral membrane
protein complex that uses light energy to
catalyze the transfer of electrons across
the thylakoid membrane from
plastocyanin to ferredoxin.
 Ultimately, the electrons that are
transferred by Photosystem I are used to
produce the moderate-energy hydrogen
carrier NADPH.

10. COMPONENTS OF PHOTOSYSTEM I
 LIGHT HARVESTING COMPLEX
 REACTION CENTER

11. 1. LIGHT HARVESTING COMPLEX
 This is the antenna complex formed by hundreds of
pigment molecules that capture photons and transfer the
harvested light energy to the second component named
the reaction center

12. ANTENNA
 The Surface which captures the energy of the photons of
various wavelengths.

13. 2. REACTION CENTER
 At the heart of a photosystem lies the reaction center,
which is an enzyme that uses light to reduce and oxidize
molecules (give off and take up electrons).
 This reaction center is surrounded by light-harvesting
complexes that enhance the absorption of light.

14. PHOTON
 Photoexcitation of the pigment molecules in the antenna
complex induces electron and energy transfer.

15. ANTENNA COMPLEX
 The antenna complex is composed of molecules
of chlorophyll and carotenoids mounted on two proteins.
 These pigment molecules transmit the resonance energy from
photons when they become photoexcited.
 Antenna molecules can absorb all wavelengths of light within
the visible spectrum.
 The number of these pigment molecules varies from organism to
organism.

16. P700 REACTION CENTER
 The P700 reaction center is composed of
modified chlorophyll a that best absorbs light at a wavelength of
700 nm.P700 receives energy from antenna molecules and uses
the energy from each photon to raise an electron to a higher
energy level (P700*).
 These electrons are moved in pairs in
an oxidation/reduction process from P700* to electron acceptors,
leaving behind high-energy P700+.

17.  The pair of P700* - P700+ has an electric potential of
about −1.2 volts.
 The reaction center is made of two chlorophyll molecules
and is therefore referred to as a dimer.
 The dimer is thought to be composed of one
chlorophyll a molecule and one chlorophyll a′ molecule.
However, if P700 forms a complex with other antenna
molecules, it can no longer be a dimer.

18. MODIFIED CHLOROPHYLL A0 AND A1
 The two modified chlorophyll molecules are early electron
acceptors in PSI.
 A0 accepts electrons from P700*, passes it to A1 of the
same side, which then passes the electron to the quinone
on the same side.

19. PHYLLOQUINONE
 A phylloquinone, sometimes called vitamin K1,is the next
early electron acceptor in PSI. It oxidizes A1 in order to
receive the electron and in turn is re-oxidized by Fx, from
which the electron is passed to Fb and Fa.
 The reduction of Fx appears to be the rate-limiting step.

20. IRON–SULFUR COMPLEX
 Three proteinaceous iron–sulfur reaction centers are
found in PSI. Labeled Fx, Fa, and Fb, they serve as
electron relays.
 Fa and Fb are bound to protein subunits of the PSI
complex and Fx is tied to the PSI complex. Fx passes an
electron to Fa, which passes it on to Fb to reach the
ferredoxin.

21. FERREDOXIN
 Ferredoxin (Fd) is a soluble protein that facilitates reduction
of NADP+ to NADPH.
 Fd moves to carry an electron either to a lone thylakoid or to
an enzyme that reduces NADP+ . Thylakoid membranes have one
binding site for each function of Fd.
 The main function of Fd is to carry an electron from the iron-sulfur
complex to the enzyme ferredoxin–NADP+
reductase.

22. FERREDOXIN–NADP+ REDUCTASE (FNR)
 This enzyme transfers the electron from reduced
ferredoxin to NADP+ to complete the reduction to NADPH.
 FNR may also accept an electron from NADPH by binding
to it.

23. PLASTOCYANIN
 Plastocyanin is an electron carrier that transfers the
electron from cytochrome b6f to the P700 cofactor of PSI
in its ionized, high-energy state P700+.

24. YCF4 PROTEIN DOMAIN
 The Ycf4 protein domain found on the thylakoid
membrane is vital to photosystem I.
 This thylakoid transmembrane protein helps assemble the
components of photosystem I.
 Without it, photosynthesis would be inefficient.