PS II and PS I complex

Most important physio-biochemical process of the world on which existence of life on earth depends

It’s the ability of green plants to utilize the energy of light to produce carbon containing organic matter from stable inorganic matter by photosynthetic process

The oxidation of organic compound release store energy which is utilized by organism to drive essential metabolic process

PHOTOSYNTHESIS

In simple terms photosynthesis can be defined as the formation of carbon containing compounds from carbon dioxide and water by illuminated green cells, water and oxygen being the by-products

Plants use sunlight, carbon dioxide, and water to produce carbohydrate with oxygen as a byproduct.

The overall chemical reaction summarizes the process as:

6 CO2 + 12 H2O + light energy C6H12O6 + 6 H2O + 6 O2

Lightenergy

Light-dependentreactions

H2O O2

Chemical energy

Calvin cycle

ATP, NADPH CO2

Chemicalenergy

Sunlight

Thylakoid Reactions Stroma ReactionsLight reactions Dark reactions

(CH2O)n

Mechanism of photosynthesis can be divided into two phases

Light reaction phase of photosynthesis is a considerably complicated process & can be briefly discussed with the help of following subheadings 1) Red drop, emersion effect & two pigment systems 2) Production of assimilatory powers 3) Energy relationships & efficiency of photosynthesis 4) Interrelationships between light and dark reactions

RED DROP AND EMERSON EFFECT Photosynthesis is considered as two quanta process, i.e. it takes two light quanta energy to drive an electron

•Number of oxygen molecules released can be used to determine the quantum yield of the process.

•Quantum yield is defined as the no. of O2 molecules released per light quanta absorbed

•Emerson & Lewis worked on quantum yield of photosynthesis in monochromatic light of different wavelength. They observed the quantum yield declined sharply at wavelength greater than 680nm in the red zone . This decline is called red drop

•Later Emerson found that the sharp decline in the quantum yield of photosynthesis beyond 680nm can be brought to full efficiency by simultaneously providing short wavelength of light. This photosynthetic enhancement is called Emerson effect

CO2 +4 H+ CH2 O + H2 O

4H2 O 4 ( H+ + e+ ) + 2H2 O + O2

TWO PIGMENT SYSTEMS•Discovery of the red drop & Emerson effect concluded that at least two pigment systems are involved in photosynthesis

•These two pigment system has been referred as pigment system I & pigment system II

•The presence of two such systems has been supported by studies based on chloroplast fractionation process which showed two type of particles within the chloroplast membrane, Smaller & lighter particles of PS I & larger & heavier particle of PS II

•Each photosystem is a network of chlorophyll a molecules, accessory pigments,and associated proteins held within a protein matrix on the surface of the photosynthetic membrane

There are two processes in photosynthesis that capture light and produce energy rich compounds that are used in carbon fixation. These are termed

Photosystem I, andPhotosystem II.

Photosystem:Reaction center surrounded by several light-harvesting complexes

Light-harvesting complex:light-harvesting complexes consist of pigment molecules bound to particular protein. They funnel the energy from photons of light to the reaction center

Photosystems

Reaction center : Protein complex that includes two special chlorophyll a molecules & a primary e- acceptor moleculeWhen a reaction-center chlorophyll a molecule absorbs energy, one of its electrons gets jumped up to a primary electron acceptor

Light reactions occur in the thylakoids (PSII) and stroma lamella (PSI).

Dark reactions in occur in the stroma

Architecture of a PhotosystemEach photosystem is a network of chlorophyll a molecules, accessory pigments,and associated proteins held within a protein matrix on the surface of the photosynthetic membrane.

A photosystem channels the excitation energy gathered by any one of its pigment molecules to a specific molecule, the reaction center chlorophyll.

This molecule then passes the energy out of the photosystem so it can be put to work driving the synthesis of ATP and organic molecules.

A photosystem thus consists of two closely linked components: (1) an antenna complex of hundreds of pigment molecules that gather photons and feed the captured light energy to the reaction center; and (2) a reaction center, consisting of one or more chlorophyll a molecules in a matrix of protein, that passes the energy out of the photosystem.

Basic concept of energy transfer during photosynthesis

How the antenna complex works.

When light of the proper wavelength strikes any pigment molecule within a photosystem,the light is absorbed by that pigment molecule. The excitation energy is then transferred from one molecule to another within the cluster of pigment molecules until it encounters the reaction center chlorophyll a. When excitation energy reaches the reaction center chlorophyll, electron transfer is initiated.

Reactioncenter

Fluorescence

Heat

Photon

Photon

e–

Electronacceptor

Chlorophyll molecules in antenna complex Reaction centerChlorophyll moleculeLower

Higher

Ene

rgy

of

elec

tron

e–

The excited-state energy of pigments increases with distance from the reaction centre. Pigments closer to the reaction centre are lower in energy than those farther from it. This energy gradient ensures that excitation transfer toward the reaction centre is energetically favourable and that transfer back out to the peripheral portions of the antenna is energetically unfavourable.

Chlorophyll donates a light-energized electron to the primaryelectron acceptor, reducing it. The oxidized chlorophyll then fillsits electron “hole” by oxidizing a donor molecule.

Converting light to chemical energy. The reaction center

Photosystem I

PS I complex consist of ˜200 chlorophylls, ˜ 50 carotenoids, a mol of P700, one cyt f, one plastocyanin, two cyt. B 563, FRS (ferredoxin reducing substance), one or two membrane bound ferredoxin molecules etc. It is rich in chl a, iron & copper.PS I controls the process of producing a strong reductant to reduce NADP into NADPH+ H+

Photosystem II

PS II complex consist of ˜ 200 chlorophylls, ˜ 50 carotenoids, a mol of P680, a primary e acceptor Q, a plastoquinone, 4 plastoquinone equivalents, 4 Mn molecules bound to one or more proteins, two cyt. b 559, one cyt. b and chloride. PS II is concerned with the generation of strong reductant and weak reductant coupled with the release of oxygen

Some basic difference between photosystem I & II

Two photosystems work sequentially. First, a photon of light ejects a high-energy electron from photosystem II; that electron is used to pump a proton across the membrane, contributing chemiosmotically to the production of a molecule of ATP. The ejected electron then passes along a chain of cytochromes to photosystem I. When photosystem I absorbs a photon of light, it ejects a high-energy electron used to drive the formation of NADPH.

Z diagram of photosystems I and II

4e–

4 Photons

Ene

rgy

of

elec

tron

2 H+2 NADP+

2 NADPH

Lower

Higher

Photosystem I

Ferredoxin

ETC+

4e–

4 Photons

ETC4e–

Photosystem II

4 H+

PQ

PC

P700ATP

produced viaproton-motive force

Cytochromecomplex

Pheophytin

P680

+ O22 H2O

The Z scheme linking Photosystem II and Photosystem I

When electrons reach the end of the Photosystem II electron chain they are passed to a protein plastocyanin that can diffuse through the lumen of the thylakoid and donate electrons to Photosystem I. Shuttle rate of 1000 electrons per second between photosystems.

ChlorophyllLower

Photon

Ene

rgy

of

elec

tron

Pheophytin

Cytochromecomplex

Higher

PQ

Electron transport chain

1. When an electron in the reaction center chlorophyll is excited energetically the electron binds to pheophytin and the reaction center chlorophyll is oxidized

2. Electrons that reach pheophytin are transferred to plastoquinone (PQ), which is lipid soluble, passed to an electron transport chain (quinones and cytochromes)

In photosystem II, excited electrons feed an electron transport chain.

2H2O O2+ 4H+ + 4e-

Pheophytin has the structure of chlorophyll but without the Mg in the porphyrin-like ring and acts as an electron acceptor.

Photosystem II Feeds an ETC that Pumps Protons

Cytochromecomplex

PQ

PQ

e–

e–

e–

Pheophytin

Antennacomplex

Reaction center

Photosystem IIStroma Photon H+

H+

(low pH) H+ H+

H+

H+

H+

H+ H+

H+

H+

H+

H+

Stroma

Thylakoid Lumen

3. Passage of electrons along the chain involves a series of reduction-oxidation reactions that results in protons being pumped from stroma to thylakoid lumen

Plastoquinone carries protons to the inside of thylakoids, creating a proton-motive force.

An essential component of the reaction is the physical transfer of the electron from the excited chlorophyll. The transfer takes ~200 picoseconds

The ph of the lumen reaches 5 while that of the stroma is around 8 - the concentration of H+ is 1000 times higher in the lumen than the stroma.

+

The oxidized reaction center of the chlorophyll that donated an electron is re-reduced by a secondary donor and the ultimate donor is water and oxygen is produced.

H2OO2

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