Photosystems Iand II:

Algae  and green  plants  use  two  kinds  of  photosystem  known as  photosystem  I (PSI) and PSII (photosystem  II).  The chlorophyll in the reaction middle of PSI has an absorption maximum at 700 nm and so is known as P for pigment (P700) and in which the reaction center of PSII has an absorption maximum at 680 nm and so is known as P680. The two photosystems are linked through other electron carriers. When it arranged according to their redox potentials the several parts from the so-called Z scheme because the whole shape of the redox diagram looks such as a Z.

Figure:  The Z scheme of noncyclic photophosphorylation in green plants. Note that the arrows in this diagram represent the sequence of events during photosynthesis, as described in the text, and are not metabolic interconversions.

The series of reactions during light absorption shown in the figure is as follows:

1.  Light  is  harvested  through  the  antenna  complex  chlorophylls   of  PSII  and  the energy is channeled towards the reaction center at that P680 is situated.

2.  The excited P680 (P680*) emits a high-energy electron which passes to PQ (plastoquinone), a mobile quinone in the thylakoid membrane.  This leaves P680 as the P680⁺ cation. The Plastoquinone accepts a total of two electrons and two H⁺ ions to form PQH₂.

3.  P680⁺ extracts an electron from water, frequent to its unexcited state.  A removal of four electrons from two molecules of water needs four quanta of light to fall on PSII and leads to the production of four H⁺ ions and one molecule of O₂:

4 photons

2H₂O→4e^-+4H⁺+O₂

This reaction is mediated through a cluster of four manganese ions (Mn²⁺) in PSII.

4.  The  electrons  are  now  passed  from  PQH₂    through  the  cytochrome  bf  complex (also known as cytochrome  b₆f complex)  to plastocyanin  (PC). PC is a copper- containing protein which accepts electrons through the copper cycling among Cu²⁺ and Cu⁺states:

Cytochrome b₆f complex

PQH₂+2 PC (Cu²⁺) →PQ+2 PC (Cu⁺) +2H⁺

5.  Light energy falling onto the antenna complex of PSI is funneled to the reaction center. Now P700 is excited to P700* and emits a high-energy electron to ferredoxin and a protein which contains at least one FeS cluster, becoming the P700⁺ cation. The P700⁺receives the electron from PC and so returns to the unexcited state.

6.  Two  high-energy  electrons  from  two  molecules  of  decrease  ferredoxin  are now transferred  to NADP⁺ to build NADPH.  The reaction is carried out through NADP reductase.

NADP reductase

NADP⁺+2e^-+H⁺→ NADPH

Taking account of the entire series of electron transport and the reaction can be written as:

Light

2H₂O+2NADP⁺→2 NADPH+2H⁺+O₂

showing which electrons flow from H₂O to NADP⁺, reducing it to NADPH.