Reaction centre in Photosystems of higher plants

Reaction centre in Photosystems of higher plants

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In photosystems of higher plants, there are about 250-400 pigments (number wise) in a particular photosystem. Out of which,approx 170-180 pigments are of chlorophyll a molecules. And any one of them behaves as a Reaction centre in a photosystem.

My question is:

  1. What are the factors and laws which decide which chlorophyll a molecule will work as a reaction centre?
  2. What's the difference in chlorophyll a molecules present in reaction centres of PS I and PS II since they have different threshold energy. Is there any structural difference?

Your first question…

What are the factors and laws which decide which chlorophyll a molecule will work as a reaction centre?

… has no answer because it's based on a wrong premise: namely that, regarding the chlorophyll molecules, "any one of them behaves as a Reaction centre in a photosystem" (sic).

On the contrary, the special pair is bound to the reaction center's proteins (called D1 and D2). According to Berg (2002):

The photochemistry of photosystem II begins with excitation of a special pair of chlorophyll molecules that are bound by the D1 and D2 subunits. (emphasis mine).

Regarding your second question, it was already answered here.

Source: Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Available from:

Someone else too had asked your question no. 2. here Photosystem 1 and 2; P680/P700; Chlorophyll a/b

The answer basically quotes Molecular Biology 4 ed. by Lodish as follows.

"The chlorophylls in the two reaction centers differ in their light-absorption maxima because of differences in their protein environment."

[I could've just commented this but I can't locate add comment option on your post. Am new here.]

Light-dependent phosphorylation of D1 reaction centre protein of photosystem II: hypothesis for the functional role in vivo

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

E. Rintamäki, R. Kettunen, E. Tyystjärvi and E.-M. Aro (corresponding author), Dept of Biology, Univ. of Turku, FIN-20500 Turku, Finland.

This paper is part of the contributions to a Plant Cell Biology Workshop on Thylakoid Protein Phosphorylation, held at Lund University, Sweden, 27–29 March, 1994.


Reversible phosphorylation of the D1 reaction centre protein of photosystem II (PSII) occurs in thylakoid membranes of higher plants. The significance of D1 protein phosphorylation in the function of PSII is not yet clear. This paper summarizes the data implying that phosphorylation of D1 protein in higher plants is involved in the regulation of the repair cycle of photoinhibited PSII centres. Photoinhibition of PSII, D1 protein phosphorylation and degradation have been studied in vivo in higher plant leaves acclimated to different growth irradiances. It is shown that photoinhibitory illumination induces maximal phosphorylation of the D1 protein. Under these conditions D1 turnover is also saturated. We postulate that phosphorylation retards the degradation of damaged D1 protein under conditions where rapid replacement by a new D1 copy is not possible. This would protect PSII from total disassembly and degradation of all PSII subunits. We conclude that the phosphorylation of D1 protein and the regulation of D1 protein degradation may have evolved together. Furthermore, these characteristics seem to be related to the highly organized structure of higher-plant type thylakoid membranes, since the capability to phosphorylate D1 protein is restricted to seed plants.

Each photosystem has all the pigments except one molecule of chlorophyll A forming a light harvesting system called antenna. Please explain this statement.

The photosystems comprise of all types of pigments, majorly chlorophyll b, xanthophylls and carotenoids. They also consist of chlorophyll a in small quantities. These combinations of pigments enables the photosystem to absorb light in various wavelengths of the absorption spectra and thus increase the overall rate of photosynthesis.

The statement says "Except one molecule of chlorophyll a" as it means that the photosystems contain all the pigments which are same in both of them but only one particular molecule of chlorophyll is different in both the photosystems P-I and P-II. This one molecule acts as a reaction centre in both the photosystems. The 'chlorophyll a' which acts as the reaction centre in Photosystem (P-I) absorbs light at wavelength 700 nm (and is thus called P700) and the one which acts as the reaction centre in Photosystem (P-II) absorbs light at wavelength 680 nm (and is thus called P680).

The statement thus sums up the information that the pigments together form the Light harvesting complex, which is also called Antennae in each photosystem. These Light harvesting complexes absorb the light and pass the photons to the Reaction centre (The single molecule of chlorophyll a, P700 or P680) which further gets excited and releases electrons to the electron transport system carrying forward the process of light reaction.

Hope this information will clear your doubts about topic.

If you have any more doubts just ask here on the forum and our experts will try to help you out as soon as possible.

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Research output : Contribution to journal › Review article › peer-review

T1 - Photoreduction of NADP+ in photosystem II

AU - Allakhverdiev, Suleyman I.

N2 - Aim of this minireview is to integrate possible evidences concerning NADP+ reduction in photosystem II (PS II). Investigations of this photoreaction show that under conditions when the first quinone acceptor, QA, is in its reduced state (QA-), electrons are transferred from reduced pheophytin (Pheo-) to NADP+, indicating that PS II can reduce NADP+ without the participation of PS I. On the basis of own experimental material and data, an alternate pathway for electron transfer in PS II reaction centres of higher plants is suggested. Some problems concerning Z-scheme are discussed.

AB - Aim of this minireview is to integrate possible evidences concerning NADP+ reduction in photosystem II (PS II). Investigations of this photoreaction show that under conditions when the first quinone acceptor, QA, is in its reduced state (QA-), electrons are transferred from reduced pheophytin (Pheo-) to NADP+, indicating that PS II can reduce NADP+ without the participation of PS I. On the basis of own experimental material and data, an alternate pathway for electron transfer in PS II reaction centres of higher plants is suggested. Some problems concerning Z-scheme are discussed.

JO - Journal of scientific & industrial research. C. Biological sciences

JF - Journal of scientific & industrial research. C. Biological sciences

NEET Biology Chapter 13 Photosynthesis in Higher Plants

Nearly all of the members (with some exceptions) categorized under the kingdom Plantae are autotrophic in nature. Being autotrophic gives the plants the ability to synthesize their own food for their own nutrition and well being as well as providing a source of nutrition for heterotrophic organisms which are incapable of synthesizing their own food and hence are dependent on autotrophs. Plants fix gaseous carbon dioxide from the atmosphere and water transported from the roots in order to synthesize their food in the form of complex, organic substances, majorly sugars and starches while releasing free, gaseous oxygen which returns to the environment and is used by other organisms apart from the plant itself during respiration process. This process, which requires the presence of electromagnetic radiations (light) or solar energy (sun light) is termed as Photosynthesis.

Q.1. During photosynthesis, oxygen is evolved from
(A) H2S (B) H2O (C) CO2 (D) HCO3

Q.2. Bacteriochlorophyll differs from chlorophyll ‘a’ in having
(A) One pyrrole nucleus with one hydrogen
(B) One pyrrole nucleus with two hydrogen
(C) One pyrrole nucleus with three hydrogen
(D) One pyrrole nucleus with four hydrogen

Q.3. In chlorophyll molecule ‘’Mg’’ in situated in
(A) Centre of porphyrin ring (B) Corner of porphyrin
(C) In phytol tail (D) In isocyclic ring

Q.4. Which one of the following concerns with photophosphorylation
(A) ADP+ AMP (Light Energy)→ATP
(B) ADP+ Inorganic PO4 (Light Energy)→ATP
(C) ADP+ Inorganic PO4 →ATP
(D) AMP +Inorganic PO4→ATP

Q.5. Hill reaction occurs in
(A) High altitude plants (B) Total darkness
(C) Presence of ferricyanide (D) Absence of water

Q.6. Which of the following absorb light energy for photosynthesis
(A) Chlorophyll (B) Water molecule (C) O2 (D) RUBP

Q.7. The enzyme that fixes atmospheric CO2 in C4 plants is
(A) PEP carboxylase (B) Hexokinase
(C) RUBP oxygenase (D) Hydrogenase

Q.8. Bundle sheath chloroplast of C4 plant are
(A) Large and agranal (B) Large and granal
(C) Small and agranal (D) Small and granal

Q.9. Photorespiration in C3 plants starts from
(A) Phosphoglycerate (B) Glycerate
(C) Glycine (D) Phosphoglycolate

Q.10. Photorespiration is favored by
(A) Low light intensity (B) Low O2 and high CO2
(C) Low temperature (D) High O2 and low CO2

Q.11. The substrate of photorespiration is
(A) Glycolate (B) Glucose (C) Pyruvic acid (D) Acetyl CO-A

Q.12. Tracer elements are
(A) Micro-elements (B) Macro-elements (C) Radio-isotopes (D) Vitamins

Q.13. Choose the correct match
Bladderwort, sundew, Venus flytrap
(A) Nepenthese, dionea, Drosera (B) Nepenthese, Utricularia, Vanda
(C) Utricularia, Drosera, Dionea (D) Dionea, Trapa, Vanda

Q.14. Which one of the following is wrong in relation to photorespiration
(A) It occurs in chloroplasts (B) It occurs in daytime only
(C) It is a characteristic of C4 plants (D) It is a characteristic of C3 plants

Q.15. Plants adapted to low light intensity have
(A) Leaves modified to spines
(B) Large photosynthetic unit size than the sun plants
(C) Higher rate of CO2 fixation than the sun plants
(D) More extended root system

Q.16. In chloroplasts, chlorophyll is present in the
(A) Stroma (B) Outer membrane
(C) Inner membrane (D) Thylakoids

Q.17. Which one of the following categories of organisms do not evolve oxygen during photosynthesis
(A) Red algae (B) Photosynthetic bacteria
(C) C4- plants with Kranz anatomy (D) Blue green algae

Q.18. Which pair is wrong
(A) C3 plant-maize (B) Calvin cycle PGA
(C) Hatch slack cycle (D) C4 plant Kranz anatomy

Q.19. Chlorophyll in chloroplasts is located in
(A) Grana (B) Pyrenoid (C) Stroma (D) Both grana and stroma

Q.20. As compared to a C3 plant how many additional molecules of ATP are needed for net production of one molecule of hexose sugar by C4 plants
(A) Two (B) Six (C) Zero (D) Twelve

Q.21. Carbohydrates, the most abundant biomolecules on earth, are produced by
(A) All bacteria, fungi and algae
(B) Fungi, algae and green plant cells
(C) Some bacteria, algae and green plants cells
(D) Viruses, fungi and bacteria

Q.22. Photosynthetic Active Radiation (PAR) has the following range of wavelengths
(A) 400-700 nm (B) 450-920 nm (C) 340-450 nm (D) 500-600 nm

Q.23. In light reaction of photosynthesis oxygen comes from
(A) Water (B) CO2 (C) Soil (D) Atmosphere

Q.24. Product of light reaction of photosynthesis is
(A) Carbohydrate (B) ATP
(C) NADP and O2 (D) NADPH2, ATP and O2

Q.25. During photorespiration, the oxygen consuming reaction(s) occur in
(A) Grana of chloroplasts and peroxisomes
(B) Stroma of chloroplasts
(C) Stroma of chloroplasts and mitochondria
(D) Stroma of chloroplasts and peroxisomes

Q.26. The first acceptor of electrons from an excited chlorophyll molecule of photosystem II is
(A) Quinone (B) Cytochrome
(C) Iron-sulphur protein (D) Ferredoxin

Q.27. In the leaves of C4 plants, malic acid formation during CO2 fixation occurs in the cells of
(A) Epidermis (B) Mesophyll (C) Bundle sheath (D) Phloem

Q.28. In leaves of C4 plants malic acid synthesis during CO2 fixation occurs in
(A) Bundle sheath (B) Guard cells
(C) Epidermal cells (D) Mesophyll cells

Q.29. The C4 plants are photosynthetically more efficient than C3 plants because
(A) The CO2 efflux is not prevented
(B) They have more chloroplasts
(C) 2CO2 compensation point is more
(D) CO2 generated during photorespiration is trapped and recycled through PEP carboxylase

Q.30. Electron from excited chlorophyll molecule of photosystem II are accepted first by
(A) Quinone (B) Ferredoxin
(C) Cytochrome-b (D) Cytochrome-f

Q.31. Oxygenic photosynthesis occurs in
(A) Oscillatoria (B) Rhodospirillum
(C) Chlorobium (D) Chromatium

Q.32. Cyclic photophosphorylation results in the formation of
(A) ATP and NADPH (B) ATP, NADPH and O2

Q.33. PGA as the first CO2 fixation product was discovered in photosynthesis of
(A) Bryophyte (B) Gymnosperm (C) Angiosperm (D) Alga

Q.34. C4 plants are more efficient in photosynthesis than C3 plants due to
(A) Higher leaf area
(B) Presence of larger number of chloroplasts in the leaf cells
(C) Presence of thin cuticle
(D) Lower rate of photorespiration

Q.35. Read the following four statements, A, B, C and D and select the right option having both correct statements Statements
(1) Z scheme of light reaction takes place in present of PSI only
(2) Only PSI is functional in cyclic photophosphorylation
(3) Cyclic photophosphorylation results into synthesis of ATP and NADPH2
(4) Stroma lamellae lack PSII as well as NADP
(A) 1 and 2 (B) 2 and 3 (C) 3 and 4 (D) 2 and 4

Q.36. CAM helps the plants in
(A) Conserving water (B) Secondary growth
(C) Disease resistance (D) Reproduction

Q.37. In Kranz anatomy, the bundle sheath cells have
(A) Thick walls, many intercellular spaces and few chloroplasts
(B) Thin walls, many intercellular spaces and no chloroplasts
(C) Thick walls, no intercellular spaces and large number of chloroplasts
(D) Thin walls, no intercellular spaces and several chloroplasts

Q.38. A process that makes important difference between C3 and C4 plants is
(A) Photosynthesis (B) Photorespiration
(C) Transpiration (D) Glycolysis

Q.39. The correct sequence of cell organelles during photorespiration is
(A) Chloroplast-mitochondria-peroxisome
(B) Chloroplast-vacuole-peroxisome
(C) Chloroplast-Golgi bodies-mitochondria
(D) Chloroplast-Rough endoplasmic reticulum-Dictyosomes.

Q.40. In C3 plants, the first stable product of photosynthesis during the dark reaction is
(A) Phosphoglyceraldehyde (B) Malic acid
(C) Oxaloacetic acid (D) 3-phosphoglyceric acid

Q.41. What is common between chloroplasts, chromoplasts and leucoplasts:
(A) Presence of pigments (B) Possession of thylakoids and grana
(C) Storage of starch, proteins and lipids (D) Ability to multiply by a fission like process

Q.42. Structurally chlorophyll a and b are different as:
(A) Chl a has a methyl group and Chl b has an aldehyde group
(B) Chl a has a carboxyl group and Chl b has an aldehyde group
(C) Chl a has an aldehyde group and Chl b has a methyl group
(D) Chl a has an ethyl group and Chl b has an aldehyde group

Q.43. Which one does not occur in cyclic photophosphorylation?
(A) Oxygen is not given off (B) Water is not consumed
(C) Only photosystem-I is involved (D) NADPH formation

Q.44. In higher plants, the shape of the chloroplasts is:
(A) Discoid (B) Cup shaped (C) Girdle shaped (D) Reticulate

Q.45. In C4 plants, the bundle sheath cells:
(A) Have thin walls to facilitate gaseous exchange (B) Have large intercellular spaces
(C) Are rich in PEP carboxylase (D) Have a high density of chloroplasts

Q.46. Kranz anatomy is observed in:
(A) C2 plants (B) C3 plants (C) C4 plants (D) CAM plants

Q.47. Primary CO2 acceptor of CAM plant:
(A) OAA (B) PGA (C) PEP and RuBP (D) Citric add

Q.48. First stable compound in C3 plant is:
(A) PGA (B) OAA (C) RuBP (D) PEP

Q.49. Stomata of CAM plants:
(A) Are always open (B) Open during the day and close at night
(C) Open during the night and close during the day (D) Never open

Q.50. Stomata in the chloroplasts of higher plants contain:
(A) Light independent reaction enzymes (B) Light dependent reaction enzymes
(C) Ribosomes (D) Chlorophyll

Q.51. Oxygenic photosynthesis occurs in:
(A) Chromatium (B) Oscillatoria (C) Rhodospirillum (D) None of above

Q.52. The active component of photosystem-I is composed of:
(A) Chlorophyll a with absorption peak at 680 nm
(B) Chlorophyll a with absorption peak at 700 nm
(C) Chlorophyll h with absorption peak at 680 nm
(D) Chlorophyll a and h with absorption peak at 700 nm

Q.53. In photorespiration, the cell organelles involved are:
(A) Chloroplast and mitochondrion (B) Chloroplast only
(C) Chloroplast, mitochondrion and ribosome (D) Chloroplast, mitochondrion and peroxisome

Q.54. The conversion of phosphoglyceric acid to phosphoglyceraldehyde during photosynthesis can be described as:
(A) Oxidation (B) Hydrolysis (C) Electrolysis (D) Reduction

Q.55. ATP can be formed in the photosynthesizing plant cells by:
(A) Photophosphorylation (B) Oxidative phosphorylation
(C) Substrate level phosphorylation (D) All of the above

Q.56. Energy released during movement of electrons through the photosystems in photosynthesis is used to drive protons across the membrane against concentration gradient. As a result the protons accumulate in:
(A) Thylakoid lumen (B) Stroma
(C) Intrathylakoid space (D) Stromal lamella

Q.57. The first event in photosynthesis is:
(A) Photoexcitation of chlorophyll and electron emission
(B) Photolysis of water
(C) Release of oxygen
(D) Synthesis of ATP

Q.58. Rubisco is the most abundant enzyme in the world and present in very high concentration in chloroplasts. It is required in very high concentration for photosynthesis because it:
(A) Is a very slow acting enzyme (B) Also acts as an oxygenase
(C) Catalyzes a reversible reaction (D) Is degraded very rapidly

Q.59. The enzyme, sucrose synthase, catalyzes the synthesis of sucrose from:
(A) UDPG + fructose (B) UDPF + glucose
(C) UDPG + glucose-6-phosphate (D) UDPG + fructose-6-phosphate

Q.60. Light reaction of photosynthesis occurs inside:
(A) Stroma (B) Grana
(C) Endoplasmic reticulum (D) Cytoplasm

Q.61. A reduction in the quantity of oxygen evolution during photosynthesis may be observed at:
(A) Light having wave length more than 680 nm
(B) Light having wave length less than 680 nm
(C) Light having wave length 560 nm
(D) Light having wave length less than 360 nm

Q.62. Plants requiring low light intensity for optimum photosynthesis is called:
(A) Heliophytes (B) Pteridophytes (C) Sciophytes (D) Bryophytes

Q.63. Sunken stomata are usually found in:
(A) C3 plants (B) CAM plants (C) Insectivorous plants (D) Phanerogams

Q.64. Select the incorrect matched pair with regard to C4 cycle:
(A) Primary CO2 fixation product – PGA
(B) Site of initial carboxylation – mesophyll cells
(C) Primary CO2 acceptor – PEP
(D) C4 plant – maize
(e) Location of enzyme Rubisco – Bundle sheath cells

Q.65. In C3 cycle for the fixation of every CO2 molecule, the reduction and regeneration steps require:
(A) 3 ATP and 2 NADPH2 (B) 2 ATP and 2 NADPH2
(C) 2 ATP and 3 NADPH2 (D) 3 ATP and 3 NADPH2
(e) 3 ATP and 1 NADPH2

Q.66. Which of the following is formed during photorespiration?
(A) Sugar (B) Phosphoglycolate (C) NADPH
(D) ATP (E) Oxaloacetate

Q.67. Which of the following statements is true with regard to the light reaction of photosynthesis?
(A) In PSII the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P 700
(B) In PSI the reaction centre chlorophyll a has an absorption maxima at 680 nm and is called P 680
(C) The splitting of water molecule is associated with PS I
(D) Photosystems I and II are involved in Z scheme
(e) Lamellae of the grana have PS I and PS II and stroma lamellae membranes have PS II only.

Q.68. Read the following four statements (1 – 4):
(1) Both photophosphorylation and oxidative phosphorylation involve uphill transport of protons across the membrane
(2) In dicot stems, a new cambium originates from cells of pericycle at the time of secondary growth
(3) Stamens in flowers of Gloriosa and Petunia are polyandrous
(4) Symbiotic nitrogen fixers occur in free-living state also in soil
How many of the above statements are right?
(A) Two (B) Three (C) Four (D) One

Q.69. In the overall process of photosynthesis, the number of CO2, water, sugar and O2 molecules utilized and produced is:
(A) 12 (B) 13 (C) 19 (D) 31

Q.70. During Calvin cycle the total number of C02, ATP and NADPH molecules utilized and glucose, ADP and NADP molecules generated is:
(A) 31 (B) 36 (C) 61 (D) 67

Q.71. Melvin Calvin was professor of:
(A) Botany (B) Plant physiology (C) Chemistry (D) Biochemistry

Q.72. The essential element needed for water splitting in photosynthesis leading to O2 evolution is:
(A) Mo (B) Mn (C) Mg (D) K

Q.73. Non-cyclic photophosphorylation results in the production of:

Q.74. Photosynthetically active radiation (PAR) represents which of the following range of wavelength?
(A) 500-600 nm (B) 450-950 nm (C) 340-450 nm (D) 400-700 nm

Q.75. Which elements are essential for the photophosphorylation?
(A) Mg and P (B) Zn and I (C) K and Cl (D) Mn and Cl

Q.76. Kranz anatomy is usually associated with:
(A) C3 plants (B) C4 plants (C) CAM plants (D) C3 – C4 intermediate plants

Q.77. Which of the following statements regarding C4 pathway is false?
(A) The primary CO2 acceptor is phosphoenol pyruvate
(B) The enzyme responsible for CO2 fixation is PEP case
(C) The mesophyll cells lack RUBISCO enzyme
(D) The bundle sheath cells contain the enzyme PEP case.

Q.78. Consider the following statements with respect to photosynthesis:

For Students & Teachers

For Teachers Only

The highly complex organization of living systems requires constant input of energy and the exchange of macromolecules.

Describe the photosynthetic processes that allow organisms to capture and store energy.

Explain how cells capture energy from light and transfer it to biological molecules for storage and use.

Organisms capture and store energy for use in biological processes–

  1. Photosynthesis captures energy from the sun and produces sugars.
    1. Photosynthesis first evolved in prokaryotic organisms.
    2. Scientific evidence supports the claim that prokaryotic (cyanobacterial) photosynthesis was responsible for the production of an oxygenated atmosphere.
    3. Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.

    The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture energy present in light to yield ATP and NADPH, which power the production of organic molecules.

    During photosynthesis, chlorophylls absorb energy from light, boosting electrons to a higher energy level in photosystems I and II.

    Photosystems I and II are embedded in the internal membranes of chloroplasts and are connected by the transfer of higher energy electrons through an electron transport chain (ETC).

    When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of protons (hydrogen ions) is established across the internal membrane.

    The formation of the proton gradient is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthase.

    The energy captured in the light reactions and transferred to ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle, which occurs in the stroma of the chloroplast.


    Memorization of the steps in the Calvin cycle, the structure of the molecules, and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam.

    What is Photosystem 2

    PS II is the collection of pigments of chlorophyll, absorbing mostly the wavelength of light at 680 nm. The first stage of the light reaction is catalyzed by PS II. The reaction center of PS II consists of chlorophyll A-680. PS II is an integral membrane protein, which consists of a core made up of D1 and D2 subunits. PS II consists of a lot of other proteins and pigments arranged in the photosystem. The pigments are chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B and phycobilins and xanthophylls. PS II achieves energy from absorbing photons or associated accessory pigments in the antenna complex. High energy electrons are generated from the energy of the absorbed photons. These electrons are passed through an electron transport chain. During the electron transport chain, PS II passes electrons to plastoquinone (PQ), which carries the electrons to cytochrome bf complex. In PS II, photolysis of water occurs in order to replace the released electrons from PS II. For each water molecule, that is hydrolyzed, two molecules of PQH2 are formed. The overall reaction in PS II is shown below.

    2PQ (Plastoquinone) + 2H2O → O2 + 2PQH2 (Plastoquinol)

    Figure 2: Photosystem 2


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    Regulation of photosynthesis: Lys acetylation and FNRs

    Photosynthetic reactions capture the energy of sunlight into chemical form. We aim at resolving how photosynthetic reactions are regulated, and how the reducing power produced by photosystems is distributed towards the stromal reactions. We have specifically focused (i) on the physiological specificities of different FNR (ferredoxin-NADP+ oxidoreductase) isoforms, and (ii) on the effects of Lys acetylation on photosynthetic processes.

    The chloroplasts of higher plants perform oxygenic photosynthesis, which captures light energy into chemical form and thus is the basis of life on Earth. Photosynthesis can be dived into two sets of reactions: light reactions taking place in the thylakoid membranes, and carbon fixation reactions in soluble stroma. To ensure immaculate primary production under a wide spectrum of environmental conditions, the structure and function of photosynthetic machinery must be extremely dynamic. The molecular mechanisms behind these dynamic changes remain largely uncharacterized, although detailed knowledge of these reactions is of utmost importance for understanding the adaptation on organism level.

    Ferredoxin-NADP+-oxidoreductase (FNR) enzymes catalyze the transfer of electrons between ferredoxin and NADPH, thus playing a major role in distribution of reducing power in the plastids. We have focused on the specific physiological roles of the distinct leaf- and root-type FNR isoforms in Arabidopsis, resolved the binding partners of FNR, and studied the effects of post-translational modifications on the function of FNR and other chloroplast proteins. Recently, we have focused on the mechanisms of lysine (Lys) acetylation of chloroplast proteins, and we are currently studying the impact of Lys acetylation on photosynthetic reactions. The ultimate aim of our project is to resolve how the reducing power produced by the photosystems is distributed towards various stromal reactions, and to reveal the significance of Lys acetylation on the regulation of photosynthesis.

    Oxygenic photosynthesis, producing carbohydrates and oxygen, is the basis of life on Earth. The success to apply photosynthesis in production of sustainable and carbon neutral energy as well as in production of maximal yields for food and feed supplies depends on the thorough understanding of photosynthetic reactions, especially the mechanisms regulating photosynthetic efficiency under changing environmental conditions.

    Conclusions and Perspectives

    Photosynthesis plays very important roles in molecular oxygen production, atmospheric carbon dioxide control and global food supply. Structural information of the photosystems is invaluable for our understanding of photosynthesis, probably the most important process on earth. The information will also help design artificial photosynthetic system for the improvement of bioenergy production and the enhancement of agricultural productivity. Most recently, the structure of the largest light-harvesting complex, the phycobilisome (PBS) from Griffithsia pacifica was also reported (Zhang et al., 2017). As the main light-harvesting antenna in cyanobacteria and red algae, it exhibits a very fast energy transfer rate with a high quantum yield (Glazer, 1989). The structural information of the PBS will provide a firm basis for understanding its energy transfer pathways and further applications in the designs of artificial light-harvesting machineries.

    Recent advances in single-particle cryo-EM have provided unprecedented structural information about these huge membrane complexes. However, there are also several open questions to be answered. First, the exact reaction mechanism underlying water oxidation and possible structural rearrangements during the S-state transitions still await the structures of PSII in more intermediate S states. Second, it is still not well understood why in PSII only one electron transfer chain is functional (as in bacterial RC), whereas in PSI both are functional (Santabarbara et al., 2010). Since static structures solved thus far has provided no conclusive clues in this respect, new studies investigating the dynamic nature of PSII might shed more light on this, which is very relevant to make PSII not only a proton pump but also the site of O2 evolution. Third, more structural information is needed to figure out the localizations and functions of PsbR and PsbS, PSII subunits that are essential for oxygen-evolving activity (Allahverdiyeva et al., 2007) and photoprotection of plants (Fan et al., 2015), respectively. Last, new high-resolution structures of the photosystems from cyanobacteria, algae, and plants will provide more insights into the evolution of oxygenic photosynthesis, based on which better artificial photosynthetic machineries could be developed.

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