Photosynthesis is a redox process. In this process, in the presence of light, water is oxidized to produce O2, and in the absence of light, CO2 is oxidized to form sugars. In 1905, Blackman divided the photosynthesis process into two stages. Light phase and light neutral phase.
Light Phase
The phase of photosynthesis that takes place in the presence of light is called the photophase. In 1957, Arnon and his colleagues described two types of photophosphorylation in the light phase. Cyclic photophosphorylation and acyclic photophosphorylation. Both processes can run simultaneously if necessary.

When photons of sunlight fall into chloroplasts, chlorophyll molecules are absorbed and activated. Active chlorophyll
An energized electron from the double bond at the head of the molecule moves from the lower energy ring of the atom to the higher energy ring. The primary electron acceptor accepts electrons from the high energy ring and photosynthesis begins.
There are three types of transitions of electrons in the high energy ring. These are-
1. Electrons from the higher energy band lose energy and return to the lower energy band. In this case the absorbed energy is released as heat or emitted as fluorescence. This energy is not used in photosynthesis.
2. The absorbed energy gives an electron to a neighboring pigment and the electron from the higher energy ring returns to the lower energy ring. In this case energy is transferred but electrons are not transferred. Thus the absorbed energy of the antenna complex is transferred to the reaction center chlorophyll-a.
3. The high energy electron from the high energy ring is accepted by the primary acceptor. In this case the high energy electron is transferred and the process of photosynthesis starts.

The system through which electrons are transferred from one place to another is called electron transport system or ETS. It has 6 types of carriers.
1. Ferredoxin-Fd: Ferredoxin is an iron-sulfur protein. It accepts and transfers electrons.
2. Cytochrome-Cyt. : It is a protein with ferrous heme group. It accepts and transfers electrons.
3. Plastoquinone-PQ: It is a small mobile lipid molecule. It can move freely across the thylakoid membrane.
4. Pheophytin-Ph: It is converted to chlorophyll-a. It binds to the next carrier plastoquinone.
5. Plastocynin-PC: It is a small motile protein molecule containing copper. It circulates freely in thylakoid cells.
6. NADP/FAD-reductase: It is a flavoprotein. Its flavin part accepts electrons.

A photosystem consists of one reaction center. The reaction center is located in the bilayer of the thylakoid membrane. Reaction centers contain a small number of proteins. The protein has chlorophyll-a on one side and an electron acceptor on the other. Electrons move from electron acceptors through the electron transport chain (ETC). Absorbed light energy can be converted into chemical energy only if the chlorophyll in the reaction center donates electrons to the electron acceptor.

An electron in a pigment molecule being energized by absorbing light energy is called photoactivation.

The photosynthetic unit on the thylakoid of chloroplast is called photosynthetic unit. Each photosynthetic unit contains 300-400 molecules of chlorophyll-a, chlorophyll-b, carotene and xanthophyll. It contains various enzymes, phospholipids, sulfolipids, metal ions, quinones, etc. Photosynthetic units absorb light energy and cause photophosphorylation. Earlier the photosynthetic unit was called quantosome. Quantum comes from quantozome. Quantum means indivisible unit of energy.

Photosystem-II consists of three parts. Light absorption organs, reaction centers (trapping centers) and electron transport systems. It consists of chlorophyll-ÔaÕ 673, chlorophyll-b and P-680. P-680 is a reaction dye. It absorbs red light the most at a wavelength of 680 nm. Hence it is called P-680. Photosystem-II resides mainly at the edges of the grana lamellae. Photosystem-II evolved in cyanobacteria only 3 billion years ago.

Photosystem-I consists of three parts. Light absorption organs, reaction centers (trapping centers) and electron transport systems. It contains chlorophyll- ÔaÕ 683, carotene, xanthophyll and P-700. P-700 is a reaction dye. It absorbs ultraviolet light at a wavelength of 700 nm the most. Hence it is called P-700.

Photosystems are of two types. Photosystem-I and Photosystem-II.
1. Photosystem-I: Photosystem-I consists of three parts. Light absorption organs, reaction centers (trapping centers) and electron transport systems. It contains chlorophyll- ÔaÕ 683, carotene, xanthophyll and P-700. P-700 is a reaction dye. It absorbs ultraviolet light at a wavelength of 700 nm the most. Hence it is called P-700.
2. Photosystem-II: Photosystem-II consists of three parts. Light absorption organs, reaction centers (trapping centers) and electron transport systems. It consists of chlorophyll-ÔaÕ 673, chlorophyll-b and P-680. P-680 is a reaction dye. It absorbs red light the most at a wavelength of 680 nm. Hence it is called P-680. Photosystem-II resides mainly at the edges of the grana lamellae. Photosystem-II evolved in cyanobacteria only 3 billion years ago.

Chlorophyll molecules and electron acceptors together form a unit. This unit is called photosystem. The photosystem is located in the thylakoids and contains 400 chlorophylls. The chlorophyll and other pigments of the photosystem are collectively called the Antenna complex (AC).