Z-SCHEME OR LIGHT REACTION:

"The reaction which depends upon light are called light reactions or Z-Scheme."

It is also called an "ENERGY CONVERSION PHASE".

That phase is completed by following the steps.

(1) Photosystems:

Photosynthetic pigments are organized into clusters called photosystems in the thylakoid membrane. The photosystems absorb and utilize solar energy.

There are the following parts of photosystems.

(a) Antenna Complex:

In the photosystem, the part which absorbs light energy is called the antenna complex.

The antenna complex contains carotenoid chlorophyll b and chlorophyll a.

(b) Reaction Centre:

The area which takes electrons (energy)k from the antenna complex and starts the light reaction is called the reaction center. The reaction center contains.

(i) One or more molecules of chlorophyll a.

(ii) Primary electron acceptor.

(iii) Electron-associated protein.

(c) Electron Transport Chain :

Photosystems contain a series of electron transport systems and chains such systems contain PQ, cytochrome, and PC.

There are two photo systems:

• Photosystem I

• Photosystem II

Reaction in Photosystem II:

PS II absorbs light of 680 nm wavelength it contains chlorophyll which is the best light absorber of 680 nm. PS II chlorophyll antenna complex transfer this energy to the reaction center and then from the reaction center to the primary electron acceptor.

This acceptor traps the high-energy electrons and then pushes these electrons to the series of electron carriers' electron transport chains.

During this flow of electrons following reactions take place.

Non-Cycle Phosphorylation:

That process is completed by following the steps.

(i) PS II absorbs light and an electron is excited.

(ii) P680 As Oxidizing Agent:

The excited electron is shifted to the reaction center and then to electron (primary) acceptors of PS II.

That chlorophyll is oxidizing and acts as a strong oxidizing agent. It has an electron "hole" that must be filled.

(iii) Photolysis:

The breakage of water during this light reaction is called photolysis.

This reaction breaks water into two Hᐩ, 2eー, and one oxygen atom. Oxygen atoms combine with another atom to form oxygen molecules O2  ─ to remove in the air.

(iv) Electron Transport Chain: (First):

Photoexcited electrons are shifted from PS II to PS I by the electron transport chain. This chain consists of :

(a) PQ          Plastoquinone

(b) Cyt         Cytochromes - two complex.

(c) PC           Plastocyanin - Copper-containing protein.

(v) Photophosphorylation:

The formation of ATP in the presence of light is called photophosphorylation.

Energy flow provides energy for the synthesis of ATP-ATP formation during non-cyclic electron flow is called non-cyclic photophosphorylation.

This ATP provides chemical energy for dark reactions as the electrons move down the chain their energy is decreased.

(vi) Reactions in Photosystem I:

The electrons reach the bottom of the electron transport chain and fill the electron hole in PS I.

PS I has chlorophyll a and absorb light of 700 nm. It is also called P700.

When PS I absorb light and its electrons move towards the primary acceptor of PS I. So the hole is produced which is filled by the electron of PS II.

(vii) Second Electron Transport Chain:

From the primary acceptor of PS I the electrons are shifted to a second electron transport chain. This chain consist of

(a) Fd - ferredoxin - ion-containing protein.

(b) NADP - nicotinamide adenine dinucleotide phosphate.

The electrons are from the primary acceptor to Fd and from Fd to NADP. In the presence of NADP reductase enzyme, the NADP changes into NADPH.

NADPH molecule provides reducing power for the formation of sugar during Calvin Cycle.

(viii) Cyclic Phosphorylation:

The process in PS-I is in which electrons are shifted from Fd to cytochrome, then to PC, and again to PS I. During this reaction, no NADPH and oxygen is produced. It does not take place by PS-II. It takes place by PS II. It takes place in PS I when the chloroplast produces low ATP for the Calvin Cycle. When more NADPH is collected in the chloroplast and the demand for ATP is fully supplied that cyclic phosphorylation is down.

The cyclic flow is a short circuit it is started due to the rise of NADPH. It is a temporary shifting of electrons from non-cyclic to cyclic electron flow.