Photosystem Notes

Photosystem1(PS 1)

• The photocentre, LHC 1 and electron carriers are present within PS 1, over thylakoid membrane proteins.
• The light absorption centre present within PS 1 is a dimer of chlorophyll a. It absorbs a wavelength of 700 nm, hence called P₇₀₀– Besides this dimer, there are other chlorophyll molecules within PS 1.
• Chlorophyll a is more abundantly found than chlorophyll b in PS 1.
• Both the dimers bind a 4Fe-4S iron-sulfur centre, called FeSx, at an interface region. Between P₇₀₀ and FeS_x, two additional chlorophyll molecules are present.
• Reducing agents A₀, A1 (belongs to a class of cyclic organic compounds called quinones), FeSx, FeSA, FeSB (iron-sulphur centres), Fd (Ferredoxin), cytochrome b6f complex and plastocyanine are present in PS 1.
• Cyclic and non-cyclic photophosphorylation both take place in PS 1. Cyclic phosphorylation can take place in PS 1, independently.
• PS 1 takes an electron from PS 1 and transfers it to NADP⁺. There are two molecules of phylloquinone (vitamin K₁) present per heterodimer, with one molecule bound to each subunit.

“photosystem notes for class 11 biology”

Photosystem 2(PS 2)

1. P₆₈₀ is the reaction centre in PS 2. Its reaction centre contains six molecules of chlorophyll a, two molecules of Pheophytin-a, two molecules of 8-carotene and one cytochrome b559 (a protein that is an important component of PS 2).
2. It contains a complex in the central portion that produces oxygen. It also contains LHC 2 and some electron carriers.
3. The reaction centre within PS 2 contains a dimer of two proteins, that absorbs light of wavelength 680nm. Hence, it is called P₆₈₀
4. Other important components within PS include pheophytin (a chemical compound similar to chlorophyll), plastoquinone (a type of quinone molecule), cytochrome b6f (an iron-containing protein), plastocyanin, etc.
5. It accepts electrons produced by the photolysis of water (splitting of water using light).
6. PS 2 is associated with non-cyclic phosphorylation.

“detailed notes on photosystem 1 and photosystem 2”

Similarities between PS 1 and PS 2

Both photosystems consist of a complex of pigment molecules, proteins and other prosthetic groups (inorganic or organic groups tightly bound to proteins) embedded in the thylakoid membranes of the chloroplast.

photosystem 1 and 2 diagram

Cytochrome B₆f Complex

1. The cytochrome b6f complex is an enzyme found in the thylakoid membrane of chloroplasts. It consists of four major polypeptides—cytochrome f (33 kDa), cytochrome b6 (heme-containing protein, 23 kDa), the ‘Rieske’ iron-sulfur protein (20 kDa) and subunit 4 (a 17 kDa protein made up of four small subunits).
2. The complex catalyses the transfer of electrons between plastoquinol (in PS 2) and plastocyanin (in PS 1). This, in turn, reduces the plastocyanin in the thylakoid lumen.
3. Two protons are transported into the thylakoid lumen for every electron transferred to plastocyanin. This facilitates the formation of a proton gradient that drives ATP synthesis.

“photosystems “

Mechanism Of Photosynthesis

Photosynthesis is divided into two phases

1. Photochemical or light phase and
2. Dark Or light-independent phase.

“difference between photosystem 1 and photosystem 2 notes”

Light Or Photochemical Phase

Definition: The part of photosynthesis that produces a reducing agent (NADPH and H⁺), along with ATP and O₂, in the thylakoid of chloroplast, in the presence of light, is called the light phase.

Site of occurrence: It takes place in the grana of the chloroplast.

Components: The process requires sunlight and chlorophyll as the main components. It also requires ADP and NADP as accessory components.

Significance:

“what is a photosystem “

The significance of the light phase in plants is as follows—

Conversion of energy: The solar energy gets converted into chemical energy (ATP and NADPH) during this phase.

Liberation of oxygen: Due to the oxidation of water, O₂ is released as a byproduct. Oxygen is further required for aerobic respiration.

Production of components required for the dark phase: NADPH and H⁺ ions produced during this phase are required during the dark phase.

Stages of photochemical phases: The photochemical part of photosynthesis begins with light absorption. When an atom or molecule absorbs light, it gains the whole energy of the photon (i.e., quantum), and becomes excited. The stages of the photochemical phase are discussed separately.

“photosystem structure and function notes”

Absorption of solar energy by chlorophyll molecules

1. Absorption of energy in the form of photons causes the chlorophyll molecules to move to their energised state.
2. The electrons remain in this state for a relatively short time, nearly 1 picosecond (= =1×10^-12 second). These can return to the lower excited singlet state by releasing the absorbed energy either as heat or as light.
3. The excited electron may also return to another excited state of lower potential energy but of greater stability, known as metastable triplet state (T).
4. Using the energy of chlorophyll a 680 and chlorophyll a 700, present in a metastable triplet state, a photochemical reaction takes place.

Chemiluminescence

The phenomenon where energy is released by a chemical reaction, in the form of light is called chemiluminescence.

Electron transport chain

Robert Hill was the first to describe the electron in this phase. transport chain. The electron transport may be of two types—non-cydic and cyclic.

“photosystem one and two “

Non-cyclic electron transport: Non-cyclic electron transport involves the following steps

1. Light energy reaches P₆₈₀ through resonance (by vibrations) of accessory pigments. This leads to the excitation (activation) of P₆₈₀ to a metastable triplet state.
2. P₆₈₀ now releases an e“ that is carried through the different electron carriers such as pheophytin (Pheo), quinone (Q), plastoquinone (PQ), cytochrome-f (Cyt f) plastocyanine (PC), etc.
3. Finally, the electron reaches the PS 1 reaction centre, while PS 2 remains positively charged due to the loss of electrons.
4. P_680⁺ (in positively charged PS 2) now oxidizes water to gain the lost electron. Thus water acts as an exogenous electron donor. This oxidation of water is catalyzed by the Mn-protein present in PS 2.
5. on the other hand, the photochemical events that follow the excitation of PS 1 (P₇₀₀) are similar to those of PS photosynthetic pigments in PS 1 absorb light of various wavelengths and transfer it to P₇₀₀ chlorophyll a- p₇₀₀ become excited (P₇₀₀).
6. The excited reaction centre P₇₀₀ loses an electron to an electron carrier protein. Due to the loss of electrons, PS 1 becomes positively charged.
7. This electron is transferred to Fd (ferredoxin, an iron-containing protein) and finally to NADP⁺. NADP collects protons from the medium and forms NADPH in the presence of an enzyme ferredoxin-NADP⁺ oxidoreductase.

⇒ \(2 \mathrm{H}_2 \mathrm{O} \underset{\mathrm{P}_{680}}{\stackrel{\text { chlorophyll a }}{\longrightarrow}} 2 \mathrm{H}^{+}+\mathrm{OH}^{-}\)

⇒ \(2 \mathrm{OH}^{-} \longrightarrow 2 \mathrm{OH}^{-}+2 \mathrm{e}^{-}\)

Cyclic electron transport: It occurs when there is a limited supply of CO₂. Hence, the synthesis of carbohydrates is decreased. As a result, NADPH starts accumulating. So, electron transport must occur, without the formation of more NADPH. So, this transport takes place. It involves the following steps

1. P₇₀₀ absorbs light and releases an electron that is captured by the primary electron acceptor (A).
2. This electron is then transported to Fd.
3. The reduced Fd is unable to reduce NADP+. Therefore, it transfers electrons to cyt b₆, PQ, cyt f and PC.
4. Finally, the electron reaches back to P₇₀₀ in PS 1, thereby completing cyclic electron transport.

Production of assimilatory power: Arnon (1956) used the term assimilatory power to refer to ATP and NADPH. The process of reduction of NADP⁺ to NADPH through the transfer of electrons is called photosynthetic electron transport. The process of formation of ATP from ADP and Pi, utilising light energy is called photophosphorylation.

“photosystem notes for NEET exam”

This also indicates that in photosynthesis a portion of light energy absorbed by the chlorophyll is captured as phosphate bond energy of ATP. The remaining is utilised for the reduction of NADP⁺.

Experiment to prove that oxygen is released during photosynthesis

A Hydrilla plant is placed in a beaker containing water. A small amount of NaHCO₃ is added to water, so as to increase the CO₂ available. Now, an inverted glass funnel is placed above the beaker, with a test tube placed at its open end. This whole set-up is left under sunlight.

When observed after several hours, bubbles are seen within the test tube and a gas is seen to have collected on top. On adding potassium pyrogallate solution to this water, it turns brown and the water level rises. As a result, the tube gets filled again.

“photosystem I and II mechanism explained”

This proves that the gas collected is oxygen, which has been released by Hydrilla during photosynthesis. formation of ATP from ADP and Pi, utilising light energy is called photophosphorylation. This also indicates that in photosynthesis a portion of light energy absorbed by the chlorophyll is captured as phosphate bond energy of ATP. The remaining is utilised for the reduction of NADP⁺