Light reaction Questions in English

(D) The chemiosmotic hypothesis for $ATP$ synthesis was proposed by Peter Mitchell in $1961$.
This mechanism explains how a proton gradient across the membrane (mitochondrial inner membrane or thylakoid membrane) drives the synthesis of $ATP$ through the enzyme $ATP$ synthase.
Therefore,the correct option is $D$.

(A) The chemiosmotic theory,proposed by Peter Mitchell,explains how $ATP$ is synthesized in chloroplasts and mitochondria.
It states that the energy for $ATP$ synthesis is derived from a proton gradient ($H^+$ gradient) across the membrane (thylakoid membrane in chloroplasts and inner mitochondrial membrane in mitochondria).
As protons move down their electrochemical gradient through the $ATP$ synthase enzyme,the energy released is used to phosphorylate $ADP$ into $ATP$.

(A) Ferredoxin is an iron-sulfur protein that mediates electron transfer in a wide range of metabolic reactions.
It plays a crucial role in photosynthesis and other biological processes by acting as an electron carrier.
Therefore,it is classified as a protein.

(B) Most herbicides are designed to inhibit the light-dependent reactions of photosynthesis. Specifically,many common herbicides (such as diuron or atrazine) target the $D1$ protein in the reaction center of Photosystem $II$. By binding to this site,they block the electron flow from the primary electron acceptor to the plastoquinone pool,thereby inhibiting the photolysis of water and oxygen evolution,which ultimately leads to the death of the plant.

(B) Many herbicides,such as $DCMU$ $(3-(3,4-dichlorophenyl)-1,1-dimethylurea)$,act by blocking the electron transport chain in photosynthesis. Specifically,they inhibit the process of photolysis of water by interfering with the $PS-II$ complex. This prevents the splitting of water molecules,thereby stopping the production of oxygen and the flow of electrons required for the light-dependent reactions.

(D) The Hill reaction,discovered by Robert Hill in $1937$,refers to the light-driven splitting of water (photolysis) by isolated chloroplasts in the presence of an artificial electron acceptor.
In the absence of $CO_2$,the Hill reaction occurs when an artificial electron acceptor,such as ferricyanide $(Fe(CN)_6^{3-})$,is added to the chloroplast suspension.
This acceptor replaces $NADP^+$ and gets reduced to ferrocyanide $(Fe(CN)_6^{4-})$ while oxygen is evolved as a byproduct.
Therefore,the presence of an artificial electron acceptor like ferricyanide is essential for the Hill reaction to proceed.

(C) In the light-dependent reactions of photosynthesis,the process begins at $PS-II$ (Photosystem $II$).
When the reaction center chlorophyll $a$ $(P_{680})$ absorbs light energy,it becomes excited and releases an electron.
This high-energy electron is immediately captured by the primary electron acceptor,which is a molecule of pheophytin (a chlorophyll molecule lacking the central $Mg^{2+}$ ion).
Following this,the electron is passed to a plastoquinone $(PQ)$,which is a quinone-based electron carrier.
Among the given options,quinone is the correct category of the primary electron acceptor system involved in the electron transport chain following the initial excitation.

(C) The red drop phenomenon refers to the sharp decrease in the rate of photosynthesis when plants are exposed to monochromatic light with wavelengths greater than $680 \ nm$.
This occurs because $PS-II$ (Photosystem $II$) is primarily excited by light wavelengths up to $680 \ nm$.
When the wavelength exceeds $680 \ nm$,$PS-II$ becomes inactive,leading to a drop in the photosynthetic rate as the electron transport chain is disrupted.

(A) The photolysis of water (splitting of water molecules) occurs during the light-dependent reactions of photosynthesis. This process requires the presence of $Mn^{2+}$ (Manganese) and $Cl^-$ (Chloride) ions. Manganese acts as a cofactor for the oxygen-evolving complex in Photosystem $II$,which facilitates the oxidation of water into oxygen,protons,and electrons. Therefore,Manganese is essential for this process.

(B) In the chloroplast,the light-dependent reactions occur in the thylakoid membranes.
$PS-II$ (Photosystem-$II$) is primarily located in the appressed regions of the grana thylakoids.
$PS-I$ is found in both the grana thylakoids and the non-appressed stroma lamellae.
However,$PS-II$ is absent from the stroma lamellae because the stroma lamellae lack the necessary components for water splitting and the oxygen-evolving complex associated with $PS-II$.
Therefore,$PS-II$ is found only in the grana thylakoids.

(C) Plastocyanin is a copper-containing protein involved in electron transport during photosynthesis.
It acts as an electron carrier between the cytochrome $b_6f$ complex and Photosystem $I$ $(PSI)$.
The copper ion in plastocyanin cycles between the oxidized $(Cu^{2+})$ and reduced $(Cu^+)$ states to facilitate electron transfer.

(D) $DCMU$ $(3-(3,4-dichlorophenyl)-1,1-dimethylurea)$ is a specific inhibitor of photosynthesis.
It acts by binding to the $Q_B$ binding site of the $D1$ protein in $PS-II$.
This binding prevents the transfer of electrons from the primary quinone acceptor $(Q_A)$ to the secondary quinone acceptor $(Q_B)$.
Consequently,the electron flow from $PS-II$ to $PQ$ (plastoquinone) is blocked,thereby inhibiting non-cyclic photophosphorylation.

(A) Bacterial photosynthesis is anoxygenic,meaning it does not produce oxygen as a byproduct. Unlike green plants and cyanobacteria,which utilize both $PS-I$ and $PS-II$ for non-cyclic photophosphorylation,photosynthetic bacteria (such as purple and green sulfur bacteria) possess only one type of photosystem,which is similar to $PS-I$. This system allows for cyclic photophosphorylation to generate $ATP$ without the photolysis of water.

(A) The light reaction of photosynthesis occurs in the thylakoid membranes of the chloroplast.
During this process,solar energy is trapped and converted into chemical energy.
The primary products formed at the end of the light-dependent reactions are $ATP$ (Adenosine Triphosphate) and $NADPH$ (Nicotinamide Adenine Dinucleotide Phosphate).
These products are essential for the subsequent dark reaction (Calvin cycle) to synthesize sugars.
Therefore,the correct answer is $ATP$ and $NADPH_2$.

(D) The light reaction of photosynthesis occurs in the thylakoid membranes of the chloroplasts.
During this process,light energy is converted into chemical energy.
The primary products formed during the light-dependent reactions are $ATP$ (Adenosine triphosphate),$NADPH$ (Nicotinamide adenine dinucleotide phosphate),and $O_2$ (Oxygen) is released as a byproduct.
$ATP$ and $NADPH$ are then utilized in the dark reaction (Calvin cycle) to synthesize carbohydrates.

(B) During the light-dependent reactions of photosynthesis,water molecules $(H_2O)$ are split into oxygen $(O_2)$,protons $(H^+)$,and electrons $(e^-)$. This process is known as photolysis of water or the Hill reaction. It occurs in the lumen of the thylakoid membrane and is essential for providing electrons to Photosystem $II$ $(PSII)$.

(C) The $Z$-scheme of electron transport,which describes the light-dependent reactions of photosynthesis,was proposed by $Hill$ and $Bendall$ in $1960$.
This scheme illustrates the flow of electrons from water to $NADP^+$ through two photosystems,$PSII$ and $PSI$,arranged in a series.

(A) The Emerson effect,discovered by Robert Emerson,showed that the rate of photosynthesis is enhanced when plants are exposed to light of two different wavelengths simultaneously (far-red and red light) compared to the sum of the rates when exposed to them separately. This phenomenon provided the first experimental evidence for the existence of two distinct photosystems,$PS-I$ and $PS-II$,working in tandem to drive the light-dependent reactions of photosynthesis.

(A) In the light-dependent reactions of photosynthesis,when the reaction center of Photosystem-$II$ $(P_{680})$ absorbs light,it becomes excited and releases an electron.
This high-energy electron is immediately captured by the primary electron acceptor,which is Pheophytin.
However,among the given options,the electron is subsequently transferred to a plastoquinone $(PQ)$,which acts as the primary stable electron acceptor in the electron transport chain.
Therefore,in the context of standard textbook options provided,Quinone (specifically Plastoquinone) is the correct answer.

(A) In the light-dependent reactions of photosynthesis,$PS-II$ (Photosystem $II$) plays a crucial role in the photolysis of water.
When $PS-II$ absorbs light,it becomes excited and loses electrons.
To replace these lost electrons,the oxygen-evolving complex associated with $PS-II$ splits water molecules $(H_2O)$ into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$.
This process is known as the photolysis of water,which provides the necessary electrons to maintain the electron transport chain.

(C) Robert Emerson and his colleagues observed that the quantum yield of photosynthesis drops significantly when the wavelength of light is increased beyond $680 \ nm$ (in the far-red region). This phenomenon is known as the $Red \ Drop$ effect. The reason for this decrease is that at wavelengths greater than $680 \ nm$,only $Photosystem \ I$ $(PSI)$ is functional,while $Photosystem \ II$ $(PSII)$ becomes inactive,leading to a lack of electrons for the electron transport chain and reduced production of $NADPH$ and $ATP$.

(B) Photophosphorylation is the process of synthesis of $ATP$ from $ADP$ and inorganic phosphate $(P_i)$ in the presence of light energy in chloroplasts.
The reaction is represented as: $ADP + P_i \xrightarrow{\text{Light energy}} ATP$.
Therefore,option $B$ correctly represents the process of photophosphorylation.

(C) During the light-dependent reactions of photosynthesis,the photolysis or photo-oxidation of water occurs at the oxygen-evolving complex $(OEC)$ located on the lumenal side of the thylakoid membrane.
This process is specifically associated with Photosystem-$II$ $(PS-II)$.
When $PS-II$ absorbs light,it becomes excited and loses electrons,creating a strong oxidizing potential that splits water molecules into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$.

(B) In the light-dependent reactions of photosynthesis,the light-harvesting complexes are organized into two photosystems: Photosystem-$I$ $(PS-I)$ and Photosystem-$II$ $(PS-II)$.
Each photosystem has a reaction center composed of chlorophyll-$a$ molecules.
In Photosystem-$II$ $(PS-II)$,the reaction center is a special chlorophyll-$a$ molecule that absorbs light at a wavelength of $680 \ nm$,and is therefore referred to as $P-680$.
In contrast,the reaction center of Photosystem-$I$ $(PS-I)$ absorbs light at $700 \ nm$ and is known as $P-700$.
Therefore,the correct answer is $P-680$.

(C) The photochemical phase (or light-dependent reaction) of photosynthesis occurs in the thylakoid membranes of the chloroplasts.
During this phase,light energy is captured by chlorophyll pigments.
This energy is used for the photolysis of water,which results in the release of $O_2$ as a byproduct.
Additionally,the electron transport chain facilitates the synthesis of $ATP$ (photophosphorylation) and the reduction of $NADP^+$ to $NADPH_2$ (or $NADPH + H^+$).
Therefore,both $O_2$ release and the formation of $ATP$ and $NADPH_2$ occur during this phase.

(C) Photophosphorylation is the process in which light energy is used to phosphorylate $ADP$ to $ATP$.
This process occurs in the chloroplasts of plants during the light-dependent reactions of photosynthesis.
In this reaction, an inorganic phosphate group $(Pi)$ is added to $ADP$ in the presence of light and the enzyme $ATP$ synthase to form $ATP$ $(ADP + Pi \xrightarrow{\text{light}} ATP)$.
Therefore, it is the synthesis of $ATP$ from $ADP$.

(D) During the light-dependent reactions of photosynthesis,water molecules undergo photolysis,releasing electrons and protons ($H^+$ ions).
These protons and electrons are accepted by the coenzyme $NADP^+$ (Nicotinamide Adenine Dinucleotide Phosphate) to form $NADPH$.
$NADPH$ acts as a reducing agent and carries the hydrogen (protons and electrons) from the light reaction to the dark reaction (Calvin cycle),where it is used to reduce $CO_2$ into glucose.
Therefore,$NADP$ is the carrier responsible for the transfer of hydrogen.

(C) In the $Z$-scheme of light reaction,$P_{700}$ (Photosystem $I$) absorbs light and gets excited,releasing electrons.
These electrons are first accepted by an $FeS$ (Iron-Sulfur) protein complex,which acts as the primary electron acceptor.
From the $FeS$ complex,the electrons are transferred to Ferredoxin $(Fd)$,and subsequently to $NADP^+$ reductase to reduce $NADP^+$ to $NADPH + H^+$.
Therefore,the first electron acceptor molecule in the electron transport chain following $P_{700}$ is the $FeS$ protein/Ferredoxin complex.

(A) Photosystem-$II$ $(PS-II)$ is a protein complex involved in the light-dependent reactions of photosynthesis.
It is located in the thylakoid membranes of the chloroplasts.
The thylakoid membranes are stacked to form structures called grana.
Therefore,$PS-II$ is found in the grana of the chloroplasts.

(A) During the light-dependent reactions of photosynthesis,the electron transport chain $(ETC)$ facilitates the movement of electrons through various complexes.
As electrons move through these complexes,they release energy,which is used to pump protons $(H^+)$ across the thylakoid membrane into the lumen,creating a proton gradient.
The dissipation of this proton gradient through the enzyme $ATP$ synthase provides the energy required to phosphorylate $ADP$ and inorganic phosphate $(P_i)$ to synthesize $ATP$.
This process is known as photophosphorylation.

(A) In the light-dependent reactions of photosynthesis,when the reaction center of Photosystem-$II$ $(P_{680})$ absorbs light,it becomes excited and loses an electron.
This high-energy electron is immediately captured by the primary electron acceptor,which is Pheophytin.
Among the given options,the electron is subsequently transferred to the plastoquinone $(PQ)$,which is a quinone molecule.
Since Pheophytin is not listed,the first acceptor in the provided options that participates in the electron transport chain following the primary event is the quinone $(PQ)$.

(D) The process of photosynthesis begins with the light-dependent reaction.
When a photon of light strikes the chlorophyll molecule in the photosystem,it transfers energy to the electrons.
This causes the excitation of an electron of chlorophyll to a higher energy state,which is the primary photochemical event that initiates the entire process of photosynthesis.

(C) Photosynthesis is a process driven by light energy absorbed by pigments like chlorophyll $a$ and $b$.
According to the action spectrum of photosynthesis,the rate of photosynthesis is highest in the red and blue regions of the visible light spectrum.
Among the given options,red light is the most effective for photosynthesis,while green light is least effective because it is mostly reflected by the leaves.

(B) During the light-dependent reactions of photosynthesis,water molecules undergo photolysis (splitting of water) in the lumen of the thylakoid.
This process releases electrons,protons $(H^+)$,and oxygen $(O_2)$.
The electrons move through the electron transport chain,while the protons accumulate in the thylakoid lumen.
Finally,these $H^+$ ions are used to reduce $NADP^+$ to $NADPH$ with the help of the enzyme $NADP$ reductase,which is located on the outer side of the thylakoid membrane.

(B) The overall equation for photosynthesis is: $6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$.
In this process,$12$ molecules of water are consumed to provide the necessary hydrogen atoms for the reduction of $CO_2$ into hexose sugar $(C_6H_{12}O_6)$.
Although $6$ molecules of water are produced as a byproduct,the net requirement for the reduction of $6$ molecules of $CO_2$ involves the splitting of $12$ water molecules during the light-dependent reactions.

(B) In Photosystem-$I$ $(PS-I)$,the reaction center is $P_{700}$.
When $P_{700}$ absorbs light,it gets excited and releases an electron.
This excited electron is first captured by an iron-sulfur protein (also known as $Fe-S$ center or $Fd$ complex).
Therefore,the first electron acceptor in $PS-I$ is an iron-sulfur protein.

(D) The Hill reaction,also known as the light-dependent reaction of photosynthesis,occurs in the thylakoid membranes of the chloroplasts.
During this process,light energy is captured to split water molecules (photolysis),which releases oxygen $(O_2)$ as a byproduct.
The electrons released from water are used to reduce $NADP^+$ to $NADPH$ (or $NADPH_2$) and to generate $ATP$ through photophosphorylation.
Therefore,the products of the Hill reaction are $ATP$,$NADPH$,and $O_2$.

(A) The light-dependent reaction (light reaction) of photosynthesis occurs in the thylakoids of the chloroplast,which are stacked to form the $Grana$.
Robert Hill demonstrated that isolated chloroplasts could produce oxygen in the presence of light and an electron acceptor,a process known as the $Hill$ reaction.
Daniel Arnon further elucidated the mechanism of photophosphorylation in the $Grana$ of chloroplasts.
Therefore,Arnon and Hill are the scientists associated with the light reaction and its site,the $Grana$.

(A) During the light-dependent reaction of photosynthesis,the process of photolysis of water occurs.
Water molecules $(H_2O)$ are split into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$ in the presence of light and the oxygen-evolving complex associated with Photosystem $II$.
This oxygen is released as a byproduct into the atmosphere.
Therefore,the oxygen released during photosynthesis originates from water,not from $CO_2$.

(B) The Hill reaction,also known as the light-dependent reaction of photosynthesis,involves the photolysis of water and the reduction of an electron acceptor in the presence of light. Since this process requires light energy to drive the splitting of water molecules $(H_2O)$ into oxygen $(O_2)$,protons $(H^+)$,and electrons $(e^-)$,it can only occur when light is available. Therefore,the Hill reaction is observed only during the daytime.

(A) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light. This process occurs in the thylakoid membranes of the chloroplast during the light-dependent reactions of photosynthesis. Therefore,the correct site for photophosphorylation is the chloroplast.

(B) The light reaction of photosynthesis takes place in the thylakoid membranes of the chloroplast.
During this process,light energy is captured by chlorophyll and used to split water molecules,a process known as photolysis.
The reaction is represented as: $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$.
As a result,oxygen $(O_2)$ is released as a byproduct,and protons $(H^+)$ and electrons $(e^-)$ are used to generate $ATP$ and $NADPH$.

(B) The process of $ATP$ synthesis from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light in chloroplasts is called photophosphorylation.
This process occurs during the light-dependent reactions of photosynthesis.
Phosphorylation is a general term for $ATP$ synthesis,while oxidative phosphorylation occurs in mitochondria during cellular respiration.

(B) Photosynthesis is a process in which green plants use sunlight to synthesize nutrients from $CO_2$ and water.
During this process,water molecules are split (photolysis) to release electrons,protons,and oxygen.
Oxygen is released as a byproduct into the atmosphere.
Therefore,the correct answer is Oxygen.

(D) The photolysis of water (splitting of water) occurs during the light-dependent reactions of photosynthesis.
This process takes place in the lumen of the thylakoid.
The water-splitting complex,which is associated with Photosystem $II$ $(PSII)$,is located on the inner side of the thylakoid membrane,facing the lumen.
As water molecules are split into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$,the protons accumulate within the thylakoid lumen,contributing to the proton gradient required for $ATP$ synthesis.

(A) The chemiosmotic hypothesis,proposed by Peter Mitchell,explains how $ATP$ is synthesized in chloroplasts and mitochondria.
It states that the energy for $ATP$ synthesis is derived from a proton gradient ($H^+$ gradient) across the membrane (thylakoid membrane in chloroplasts and inner mitochondrial membrane in mitochondria).
When protons accumulate in the lumen (in chloroplasts) or intermembrane space (in mitochondria),a concentration gradient is created.
The movement of these protons back across the membrane through the $ATP$ synthase enzyme complex provides the energy required to phosphorylate $ADP$ into $ATP$.

(B) During the light-dependent reactions of photosynthesis,the electron transport system $(ETS)$ is located in the thylakoid membrane.
As electrons move through the electron transport chain,they pass through various carriers,including plastoquinone,the cytochrome $b_6f$ complex,and plastocyanin.
This movement of electrons releases energy,which is used to pump protons $(H^+)$ into the thylakoid lumen,creating a proton gradient.
The dissipation of this gradient through $ATP$ synthase drives the synthesis of $ATP$.
Therefore,cytochromes are essential components of the electron transport chain during $ATP$ synthesis.

(B) According to the quantum requirement of photosynthesis,it has been experimentally determined that $8$ quanta of light are required to evolve one molecule of $O_2$ during the light-dependent reactions of photosynthesis. This is based on the fact that $4$ electrons are transferred through the electron transport chain for the photolysis of $2$ molecules of $H_2O$,resulting in the release of $1$ molecule of $O_2$ and the absorption of $8$ photons (quanta) of light.

(B) Photosynthetic bacteria,such as purple and green sulfur bacteria,perform anoxygenic photosynthesis.
They possess a single photosystem,$PS-I$,which is used for cyclic photophosphorylation to generate $ATP$.
They lack $PS-II$,which is required for the photolysis of water and the evolution of oxygen.
Therefore,they cannot perform non-cyclic photophosphorylation and do not produce oxygen as a byproduct.

(D) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light.
This process occurs in the thylakoid membranes of chloroplasts during the light-dependent reactions of photosynthesis.
During this process,light energy is captured by chlorophyll pigments and converted into chemical energy stored in the form of $ATP$ molecules.
Therefore,the correct description is that light energy is converted into chemical energy and $ATP$ is produced.