Photosystems - class-XI

Photosystems are functional and structural units of protein complexes involved in photosynthesis, that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. There are 2 kinds of photosystems - photosystem I and II. In photosystem I, the reaction center is P-${ _7}$${ _0}$${ _0}$. In photosystem II, the reaction centers are P-${ _6}$${ _8}$${ _3}$. Thus, the correct answer is option A. 

P$ _{680}$ is the primary donor present in photosystem II. Structurally,  it's chlorophyll pigment dimer is present at the center of photosystem II. 680 suggests that the absorption is maximum at 680nm in red light. P$ _{680}$ or primary donor receives energy either by absorbing light or by excitation of electrons present in nearby chlorophyll. The excited electrons get captured by electron acceptor present in photosystem II, which is pheophytin and oxidized P$ _{680}$ is then reduced by electron generated from water during oxygenic photosynthesis.

Ferredoxin is the iron-containing, soluble compound in chloroplasts that helps in electron transportation and is the constituent of PS I which asses electrons to reductase complex that helps in the reduction of NADP$^+$ to NADPH which is a strong reducing agent.

Cyclic photophosphorylation is carried out by PS I only. This process takes place in stroma lamellae membrane. An external source of electrons is not required. Photolysis of water does not take place. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. It does not involve the formation of NADPH. It operates under low light intensity, anaerobic conditions or when $CO _{2}$ availability is low. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to the cyclic flow of electrons. The membrane and lamella of the grana have both PS I and PS II, the stroma lamella membrane lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $NADP^{+}$ and is cycled back to the PS I complex through the electron transport chain.

The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce organic compound. One of the first steps in this complex process depends on chlorophyll and other pigment molecules. Chlorophyll is the green pigment molecule that make plants appear green. In photosynthetic plant cells, chlorophyll molecules are embedded in stacked membranes (thylakoids) contained in special membrane-bound organelles called chloroplast. The chlorophyll molecules are arranged in discrete units called photosystems. Each one of it contains hundreds of pigment molecules arranged into an "antenna complex" surrounding a reaction center.

Since carotene (alpha and beta), chl A, chl B are all found in both photosystem, the one which is only present in PS II is xanthophyll.

P ${ _7}$${ _0}$${ _0}$ is the reaction center of the PS-I. So, when PS- I receives light energy electron is released by P ${ _7}$${ _0}$${ _0}$.

Photosystem II has P680 reaction center wherein oxygen atoms of two water molecules bind to a cluster of manganese atoms. These Mn atoms are bound to the reaction center and associated with an enzyme that splits water, removes electrons one at a time. These electrons fill the holes which are left in the reaction center due to released to light-energized electrons. Immediately after removal of four electrons from the two water molecules, O2 is released. The absence of PS II results in cyclic photophosphorylation by PSI that does not have water splitting complex and no oxygen is released. Phytochromes are blue-green, proteinaceous pigments that are involved in physiological responses to light. Phycocyanin is the photosynthetic pigment of cyanobacteria. Thus, the correct answer is option B.

Like most other photosynthetic bacteria, purple and green sulphar bacteria do not produce oxygen (anoxygenic), because the reducing agent (electron donor) involved in photosynthesis is not water. They use H$ _{2}$S or Elementary Sulphur as electron donor. In any photosynthetic organism, CO$ _{2}$ is not used as electron donor. Rhodopseudomonas uses isopropanol as electron donor.
So, the correct answer is option B.

There are only two photosystems PS I and PS II. Photosynthetic bacteria have only PS I and absence of PSII makes them carry out cyclic photophosphorylation wherein $H _2S$ serves as an electron donor, rather than water, and there is no release of oxygen. The low light intensity and anaerobic conditions favor cyclic photophosphorylation. The correct answer is B.

Chloroplasts are eukaryotic cell organelles. They are covered by the double membrane and also have an internal membrane system comprised of thylakoid membranes. The thylakoid membranes are of two types. The membranes which make grana are called granal lamellae. The other thylakoid membranes are called stromal membranes . The non-appressed membranes (stromal membranes and edges of granal membranes) are in direct contact with the stroma. The PS- I is abundant in non-appressed membrane whereas PS- II is abundant in granal membranes.

There are two photosystems PS I and PS II. Photosynthetic bacteria have only PS I and absence of PSII makes them carry out cyclic photophosphorylation wherein $H _2S$ serves as an electron donor, rather than water, and there is no release of oxygen. The low light intensity and anaerobic conditions favor cyclic photophosphorylation. The correct answer is A. 

There are two types of photosystems, the photosystem I (PS- I) and the photosystem II (PS II). These are the structural and functional protein components in the photosynthesis. The photosytem I is present in the green and purple bacteria that conduct the bacterial photosynthesis where as the photosystem II is present in the cyanobacteria that conduct the normal plant photosynthesis.

Ferredoxin is an iron-containing protein, associated with photosystem I. The ferredoxin accepts electrons from photosystem I and passes them to oxidized NADP to form reduced $NADPH _2$. The enzyme responsible for reduction of NADP is called as NADP ferredoxin oxidoreductase.

During photosynthesis light energy is converted to chemical energy of glucose. Light energy is absorbed by a pair of chl a molecules, of PS I or PS II, this excited chl a molecule releases one e$^-$, that moves through the ETS of the PSI and PSII releasing energy at each level during the transport, used for the formation of ATP 

PS II (Photosystem II) is associated with the P680 reaction center, not P700. P700 is the reaction center for PS I (Photosystem I).

Z scheme of light reaction is non cyclic fow of electron from PSII through quinone, cyt $b _6$ f complex and plastocyanin to NADPH via PSI; it does not return to PSII. NADP reductase enzyme is located on stromal side of PSI; Z scheme includes both PSI and PSII. Absence of PS-II leads to non cyclic photophosphorylation in which electrons are passed from PS-I, via electron transfer chain, to back to PS-I with formation of ATP. Cyclic photophosphorylation does not pass electrons to NADP; rather electrons are passed back to PSI. Result is formation of ATP but no NADPH2. Stroma lamellae have PSI only but lack PSII and NADP reductse. Statements B an D are correct. Correct answer is A.

Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Reaction centre of it is 700 nm.

Photosystem II is the first link in the chain of photosynthesis. It captures photons and uses the energy to extract electrons from water molecules. First, when electrons are removed water molecules are broken into oxygen gas which bubbles away, and hydrogen ions, which are used to power ATP synthesis. Photosystem II is located in the thylakoids and stacks of thylakoids is known as grana. 

A photosystem is composed of chlorophyll 'a' molecule which can absorb the wavelength of 700 nm. The chlorophyll a molecule acts as a reaction centre. PS-I is involved in a non-cyclic type of photophosphorylation where there is a chain of electron carriers and acceptors that transmits the electron from one to another. The primary electron acceptor is the ferredoxin molecule which is composed of Fe-S protein

The plant needed enough energy to do the biosynthetic reactions. Its reaction centre is a molecule called P680 which absorbs light maximally at 680 nm.

Z scheme of light reaction is non cyclic fow of electron from PSII through quinone, cyt b6b6 f complex and plastocyanin to NADPH via PSI; it does not return to PSII. NADP reductase enzyme is located on stromal side of PSI; Z scheme includes both PSI and PSII. Absence of PS-II leads to cyclic photophosphorylation in which electrons are passed from PS-I, via electron transfer chain, to back to PS-I with formation of ATP. Cyclic photophosphorylation does not pass electrons to NADP; rather electrons are passed back to PSI. PS I is involved in both cyclic and non cyclic photophosphorylation. Correct answer is B.

Two main photosynthetic pigment chlorophyll a and chlorophyll b absorb light in violet blue (400 -500 nm) and red (620-700nm) region of visible spectrum. Neither of these pigments absorbs light between 500 to 600 nm (green light) causing least photosynthesis in this wavelegnth. Thus, the correct answer is option B.

Photosynthetic bacteria are anaerobic and only one type of photosystem is found in them but cyanobacteria or blue-green algae have both systems PSI and PSII.  It has two types of pigments bacteriochlorophyll which differs from chlorophyll a, in having one pyrrole ring with two hydrogens and bactericidin.

PSI passes electron to ferredoxin on stromal side of lumen. Ferredoxin (iron sulfur protein) receives electrons from PSI and reduces to NADP+ to form NADPH. This light reaction is catalyzed by the membrane-bound enzyme NADP reductase. Both B and C are correct answers. 

Photosynthetic bacteria are anaerobic and only one type of photosystem is found in them but cyanobacteria or blue-green algae have both systems PSI and PSII. In PS I contain only one chlorophyll that is chlorophyll a while PS II contains various yellowish carotenoids and phycocyanin that differ in colour in different species.

Photosystems are the functional and structural unit of photosynthesis that contains chlorophyll and various electron carriers that helps in energy liberation.

Photolysis of water is carried out to fulfill the need of electrons that are given by photosystem II to photosystem I in the Z scheme. The splitting of water is associated with PSII in which water is split into H$^+$, [O] and electrons and this also creates oxygen, one of the net product of photosynthesis.

The constituents of PS I is located in the thylakoid membrane of stroma and the outer surface of grana, contain a number of light-harvesting complexes and a reaction centre complex, which is a protein complex with two special chlorophyll a molecules and a primary electron acceptor.

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of co-enzymes and co-factors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen.

Each photosystem is characterized by the wavelength of light at which it's reaction center absorbs maximum light i.e., 700 and 680 nanometers, respectively for PS I and PS II in chloroplasts. thus PS I is denoted by P$ _{700}$. Thus, the correct answer is 'P$ _{700}$'.

A) Electrons from excited chlorphyll molecule of photosystem II are accepted first by Quinone.

Water is split in chloroplasts in the light reaction of photosynthesis. Chlorophyll, acting as a photopigment, captures sunlight and transfers that energy to an electron pair of a water molecule. Water molecule is separated into 2 protons, molecular oxygen and a free electron pair, which then enters to PS II and accepted by plastoquinone and carried by variety of electron acceptors in a series of redox potential. The
excited electrons are used for photolysis of water generating oxygen and
protons which then used in synthesis of ATP and NADPH.

In the electron transport process, the excited electron from P$ _{680}$ is accepted by primary acceptor quinone, which sends them to an electron transport system consisting of plastoquinone, cytochrome complex and plastocyanin.

The excited electron absorbed by P$ _{700}$ reaction centre, then transfer to the ferredoxin (Fe), then the reduced Ferredoxin donates the electron to NADP$^+$.

The NADP$^+$ take an electron from ferredoxin, protons from the medium and get reduced to NADPH in presence of enzyme ferredoxin- NADP- reductase. So, the last electron acceptor of PS I is ferredoxin.

Hence, the correct answer is 'Ferredoxin'.

When the pigment in the reaction center of PS II (P$ _{680}$) absorbs a photon, an electron in this molecule gets excited and first transferred to electron acceptor namely pheophytin then to a series of redox molecules. The electron flow goes from Photosystem II to Photosystem I. It utilizes absorbed light for electron transport from plastocyanin on the lumenal side to ferredoxin on the stromal side of the thylakoid membrane to generate NADPH and ATP. 

In the electron transport chain of photosynthesis, electrons flow from PS2 to PS1. The sequence is Plastoquinone (4), Cytochrome b6 (1), Cytochrome f (2), and Plastocyanin (3).

In photosystem II, photoexcited P680 transfers an electron rapidly to an primary electron acceptor, which is phaeophytin (a chlorophyll lacking a central magnesium ion).

1. The fall in photosynthetic yield beyond the red region of the spectrum (680 runs) is called red drop.
2. Reaction center of PS-2 is 680 while that of PA-1 is 700.
3. So the psII system will be inactive.

Photosystem II or Pigment system II, a multiprotein complex involved in the non cyclic electron, proton translocation and photophosphorylation. It is located in the inner side of the thylakoid membrane.

So, the correct option is ‘Inner
side’.

DCMU (Dichlorophenyl dimethyl urea) is a herbicide which most effectively inhibits PSII in photosynthesis by the stopping CO$ _2$ fixation, which is essential for photosynthesis.

The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2. The reaction center chlorophyll (or the primary electron donor) of photosystem II that is most reactive and best in absorbing light at wavelength of 680 nm. P680 is a group of pigments that are excitonically coupled or that act as if the pigments are a single molecule when they absorb a photon.

Photosystem I (Plastocyanin-ferredoxin oxidoreductase) is located in the membrane of stroma lamellae, while PSII  (water-plastoquinone oxidoreductase) is located in the grana region that is inner side of thylacoid membrane.

Light reactions occur inside thylakoids. It is dependent upon light. It involves photolysis of water (breaking of water into $H _{2}$ and $O _{2}$  and production of assimilation power (NADPH and ATP). Electron released during photolysis of water are  picked up by $P _{680}$ photocentre of photosystem II. From here electrons passes over series of carrier which include PQ, cytochrome b-f complex, and PC. While passing over cytochrome complex, the electron losses sufficient energy for creation of proton gradient and ATP from ADP and inorganic phosphate. From PC electron losses sufficient energy for creation of proton gradient and ATP from ADP and inorganic phosphate. From PC electron is picked by the trap center $P _{700} $of phtotsystem I, which pushes out electron after absorbing light energy. Electron passes over carriers Fes, ferredoxin and NADP reductase which gives electron to $NADP^+$ for combing with $H^+$ to produce NADPH.
$NADP^+ 2e^- + H^{+} \xrightarrow[NADP ,, reductase]{} NADPH $ , So, the correct answer is 'ATP, hydrogen and O2'.

Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis and splitting of water is associated with it. Water is split into H+, [O] and electrons, creating O$ _{2}$ which is one of the net products of photosynthesis. The electrons needed to replace those removed from PS I are provided by PS II.

 The biochemical objective of PSI is to reduce NADP+ to NADPH because NADP+ is able to be reduced in NADPH by gain electron which has been excited from PSI(P700)  and also hydrogen ions from stroma. So electron required to reduce NADP+ is released from PSI so answer is option 4.

A photosynthetic reaction centre is a protein that is the site of the light reactions of photosynthesis. The reaction centre contains pigments such as chlorophyll and phaeophytin. These absorb light, promoting an electron to a higher energy level within the pigment which are located at centre of photosynthetic apparatus. so, 'the correct option is - It is located at centre of photosynthetic apparatus'.

The enzyme cytochrome c oxidase or Complex IV is a large transmembrane protein complex found in bacteria, archaea, and in eukaryotes in their mitochondria. It is the last enzyme in the respiratory electron transport chain of cells located in the membrane. It receives an electron from each of four cytochrome c molecules, and transfers them to one dioxygen molecule, converting the molecular oxygen to two molecules of water. In this process it binds four protons from the inner aqueous phase to make two water molecules, and translocates another four protons across the membrane, increasing the transmembrane difference of proton electrochemical potential which the ATP synthase then uses to synthesize ATP. The complex contains two hemes, a cytochrome a and cytochrome a3, and two copper centers, the CuA and CuB centers. The cytochrome a3 and CuB form a binuclear center that is the site of oxygen reduction.

• A very small part of the electromagnetic spectrum can be seen by the human eye i.e., between the wavelengths 390 and 780 nm.
• This part of the electromagnetic spectrum is called a visible light spectrum.
• Within the spectrum the longer the wavelength of the radiation, the slower the vibration of the photons and the less energy each photon contains. 
  Hence, correct option is B.

Photosystem- II is involved in non-cyclic photophosphorylation with a reaction center P${ _6}$${ _8}$${ _0}$. The electrons excited from P${ _6}$${ _8}$${ _0}$ are accepted by plastoquinoine. 

Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the grana of the thylakoid membrane of plants, algae, and cyanobacteria.

Photosynthetic bacteria posses only one type of photosystem and are mostly anaerobic and contains two type of pigment bacteriochlorophylls and bacterioviridin while cyanobacteria or blue-green algae posses two photosystems  PS I and PS II.

Photosystem I is one of the two membrane-bound photosystems of plants, algae and cyanobacteria that facilitate light-determined electron transport from water to NADPH. It utilizes absorbed light for electron transport from plastocyanin on the lumenal side to ferredoxin on the stromal side of the thylakoid membrane. In plants, this special integral membrane complex consists of more than 15 protein subunits, approximately 175 chlorophyll molecules, 2 phylloquinones and 3 Fe$ _4$S$ _4$ clusters. Whereas Photosystem II (of cyanobacteria and green plants) is as many as 35 chlorophyll a, 12 beta-carotene, two pheophytins, two plastoquinone, two heme, one bicarbonate, 20 lipid molecules and other ionic clusters. Thus Photosystem I contains more chlorophylls and accessory pigments.

DCMU or Dichlorophenyl dimethyl urea inhibits PS II in photosynthetic plants by blocking electron transfer from plastoquinone to cytochrome. DCMU binds to and blocks the site of plastoquinone, thus hinders the path and growth of plants. It is used as herbicide and algicide. It is also used in studying photosynthetic activity.

Being a light reaction, non-cyclic photophosphorylation occurs in the thylakoid membrane. First, a water molecule is broken down into 2H⁺ + 1/2 O₂ + 2e⁻ by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H⁺and 1/2O₂ are left out for further use. Then a photon is absorbed by chlorophyll pigments surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, pheophytin. The deficit of electrons is replenished by taking electrons from another molecule of water. 

A) PS I can funtion at wavelengths of 700 nm or less. So, it will be active at 550 nm. But its maximum activity is at 700 nm.
B) It will be active at 680 nm.
C) It will be active at 690 nm.

Photosystems refer to PSI and PSII. Photosystem II contains chlorophyll a, as well as up to 50% chlorophyll b. It probably evolved later as a supplement to Photo I. It is needed to capture enough energy to do the biosynthetic reactions of the dark reaction. Its reaction centre is a molecule called P680 which absorbs light maximally at 680 nm. Similarly, PSI or P700 absorbs light at 700nm. A reaction centre comprises several (>10 or >11) protein subunits, that provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters and electron acceptors for transduction in the electron transport chain.