Category: Biology Second Paper

The stage of photosynthesis that does not require light is called the dark or light neutral stage. At this stage CO2 is oxidized to produce sugar, so it is called carbon oxidation reaction or carbon oxidation pathway or carbohydrate making process or carbon oxidation stage. In this process, ATP and NADPH+H+ are called energy compounds for carbon assimilation. Modern scientists have discovered three pathways of carbon assimilation in the photoneutral phase using methods such as radioactive isotopes, autoradiography, paper chromatography etc. Calvin-Basham or C3 cycle, Hatch-Slack or C4 cycle and CAM cycle.

In the process of photophosphorylation, the high energy electrons (e-) released from chlorophyll molecules are carried through different carriers, lose energy, become dull and are transferred to another chlorophyll molecule without returning to that chlorophyll molecule is called acyclic photophosphorylation. In 1964 scientists Hill and Bendall explained the reactions of this process by Z diagram. Hence it is called Z scheme reaction. The process of acyclic photophosphorylation is-
(i) Both photosystem-I and photosystem-II also participate in the acyclic photophosphorylation process. Photosystem-II absorbs light energy from the sun. This light energy is transferred to the reaction center P-680.
(ii) Two high energy electrons (2e-) are emitted from P-680. Two electron carriers are accepted by plastoquinin (PQ).
(iii) Two electrons from plastoquinin come to cytochrome-f (Cyt. f). At this time the two electrons lose some energy. Under the influence of this energy, ATP is produced by combining ADP and Pi.
(iv) Electrons then pass from two cytochrome-f to photosystem-I through plastocyanin. Because previously two electrons have been released from photosystem-I. As a result there is a shortage of electrons. In this way photosystem-I regains its lost electrons.
(v) Photosystem-I absorbs light energy from the sun. This light energy is transferred to the reaction center P-700.
(vi) Two high energy electrons (2e-) are emitted from P-700. Electrons are accepted by the two carriers ferridoxine (Pd). Electrons from ferridoxine enter two NADP-reductases.
(vii) During this period photolysis of H2O molecules takes place in the presence of light to produce O2, 2H+ and 2e-. O2 escapes to the atmosphere. 2e- enters photosystem-II. In this way photosystem-II regains its lost electrons. 2H+ combines with NADP-reductase to produce NADPH+H+.
Both cyclic and acyclic processes can occur simultaneously in higher plants. However, in cyanobacteria, algae and green plants cyclic processes occur when the supply of NADP is cut off. When the water supply is stopped, the acyclic process stops and the cyclic process takes place.
[Applying herbicides to the soil stops the photosynthetic electron flow of the weed, so the weed dies]

In the process of photophosphorylation, high energy electrons (e-) released from chlorophyll molecules are carried through various carriers, lose energy and return to the same chlorophyll molecule, it is called cyclic photophosphorylation. Cyclic photophosphorylation process is as follows.
(i) Only photosystem-I participates in the cyclic photophosphorylation process. Photosystem-II absorbs light energy from the sun. This light energy is transferred to the reaction center P-700.
(ii) Two high energy electrons (2e-) are emitted from P-700. Electrons are accepted by the two carriers ferridoxine (Pd).
(iii) Two electrons from ferridoxine come to cytochrome b6 (Cyt. b6). Electrons from cytochrome-b6 pass two very quickly to plastoquinin (PQ).
(iv) Then two electrons come from plastoquinin to cytochrome-f (Cyt. f). At this time the two electrons lose some energy. Under the influence of this energy, ATP is produced by combining ADP and Pi.
(v) Finally electrons pass from cytochrome-f to photosystem-I via plastocyanin. Thus photosystem-I regains its lost electrons.

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.