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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