Light reaction Questions in English

(A) Pigment system-$II$ ($PS$-$II$) is a protein complex involved in the light-dependent reactions of photosynthesis.
It contains the oxygen-evolving complex $(OEC)$ which is responsible for the photolysis of water $(2H_2O \rightarrow 4H^+ + 4e^- + O_2)$.
This process releases electrons to replace those lost by the reaction center chlorophyll $P_{680}$ and releases oxygen as a byproduct.
Therefore,the correct option is $A$.

(A) Photophosphorylation is the process by which light energy is captured by chlorophyll and used to convert $ADP$ and inorganic phosphate $(Pi)$ into $ATP$.
This process occurs in the thylakoid membranes of chloroplasts during the light-dependent reactions of photosynthesis.
Therefore,light energy is converted into chemical energy stored in the high-energy phosphate bonds of $ATP$.

(B) . It is the first stage of photosynthesis which consists of light-driven splitting of water or photolysis. Photolysis requires light energy.

(D) During the light-dependent reactions of photosynthesis,the electron transport chain involves the movement of electrons from photosystem-$I$ $(PS-I)$.
After $PS-I$ is excited by light,it transfers electrons to a primary electron acceptor,which then passes them to an iron-sulfur protein called Ferredoxin $(Fd)$.
Finally,the enzyme $FNR$ (Ferredoxin-$NADP^+$ reductase) transfers these electrons from reduced Ferredoxin to $NADP^+$,reducing it to $NADPH + H^+$.
Therefore,Ferredoxin is the substance that acts as the electron carrier between $PS-I$ and $NADP^+$.
Thus,the correct option is $D$.

(C) The photochemical phase (light reaction) of photosynthesis occurs in the thylakoid membranes of the chloroplasts.
During this process,light energy is captured to drive the photolysis of water,which results in the liberation of $O_2$ as a byproduct.
Simultaneously,the electron transport chain facilitates the synthesis of $ATP$ (photophosphorylation) and the reduction of $NADP^+$ to $NADPH$.
Therefore,both the release of oxygen and the production of assimilatory power ($ATP$ and $NADPH$) occur during the light-dependent reactions.

(B) Hill's reaction refers to the light-dependent stage of photosynthesis where water is photolyzed to release oxygen in the presence of light and an electron acceptor.
Since this process requires light energy to excite electrons in chlorophyll,it occurs only in the presence of light.

(C) In the process of photosynthesis,when a chlorophyll $a$ molecule absorbs light energy,it becomes excited and loses an electron.
Since an electron carries a negative charge,the loss of an electron results in the molecule becoming electron-deficient.
Consequently,the chlorophyll $a$ molecule acquires a positive charge,becoming an oxidized form $(Chl^+)$.

(A) In $PS-I$,the reaction centre chlorophyll $a$ has an absorption peak at $700 \ nm$,known as $P_{700}$.
In $PS-II$,the reaction centre chlorophyll $a$ has an absorption peak at $680 \ nm$,known as $P_{680}$.
Therefore,the reaction centres for $PS-I$ and $PS-II$ are $P_{700}$ and $P_{680}$ respectively.

(C) The light reaction of photosynthesis occurs in the thylakoid membranes of the chloroplast.
During this process,solar energy is captured by chlorophyll pigments and converted into chemical energy.
The primary products of the light-dependent reactions are $ATP$ (Adenosine Triphosphate) and $NADPH$ (Nicotinamide Adenine Dinucleotide Phosphate),which are then utilized in the biosynthetic phase (Calvin cycle) to fix carbon dioxide.

(B) The process of splitting water molecules into hydrogen ions,electrons,and oxygen is known as photolysis of water.
This process occurs in the thylakoid lumen and is essential for providing electrons to Photosystem-$II$.
Photolysis of water is catalyzed by the oxygen-evolving complex,which requires $Mn^{2+}$ (manganese) and $Cl^-$ (chloride) ions as essential cofactors.
Therefore,$Mn$ and chloride ions aid in the separation of water.

(B) Chlorophyll '$a$' exists in different forms in the photosystems. The reaction center of $PS-II$ is $P_{680}$,which absorbs light at wavelengths shorter than or equal to $680 \ nm$. This specific form of chlorophyll '$a$' is known to be fluorescent. Therefore,it is associated with $PS-II$.

(B) In the light-dependent reactions of photosynthesis,the absorption of light energy by the chlorophyll molecules in $PS-II$ (Photosystem $II$) leads to the excitation of electrons.
To replace these lost electrons,$PS-II$ facilitates the splitting of water molecules,a process known as photolysis.
This reaction releases electrons,protons $(H^+)$,and oxygen $(O_2)$ as a byproduct.
Therefore,the absorption of light by $PS-II$ specifically causes the photolysis of $H_2O$.

(D) The process described is the Hill reaction,where light energy is used to split water (photolysis) to produce $O_2$ and reduce an electron acceptor (ferric to ferrous).
When an isolated chloroplast is washed in running water,the soluble components of the electron transport chain,such as plastocyanin $(PC)$,are removed or leached out.
Since these components are essential for the flow of electrons from water to the electron acceptor,the entire process of light-dependent reactions is disrupted.
Therefore,in the absence of these essential components,neither the photolysis of water (production of $O_2$) nor the reduction of ferric to ferrous can occur.

(D) In the process of non-cyclic photophosphorylation ($Z$-scheme),light energy is absorbed by both Photosystem-$II$ $(PSII)$ and Photosystem-$I$ $(PSI)$.
When $PSII$ absorbs light,it becomes excited and loses electrons,which are then transferred through an electron transport chain.
The oxidation of water (photolysis) occurs at $PSII$,which generates a strong oxidant $(P680^+)$ capable of splitting water molecules.
Simultaneously,the electrons are transferred to $NADP^+$,which is reduced to $NADPH$. $NADPH$ acts as a weak reductant in the Calvin cycle.
Therefore,non-cyclic photophosphorylation is the process that generates both a strong oxidant and a weak reductant.

(A) In the light-dependent reactions of photosynthesis,the two photosystems ($PSII$ and $PSI$) work in tandem to drive the flow of electrons.
When $PSII$ absorbs light,it becomes excited and loses electrons,which are then passed through an electron transport chain.
The redox potential of the primary electron acceptor of $PSII$ is approximately $-0.8 \ V$ to $-1.1 \ V$,while the final electron donor,water,has a redox potential of approximately $+0.8 \ V$.
Therefore,the light energy is used to drive electrons against an electrochemical gradient from a potential of approximately $-1.1 \ V$ to $+0.8 \ V$.

(B) In cyclic photophosphorylation,only $PS-I$ is involved. The electrons are circulated within the $PS-I$ system and the phosphorylation occurs due to cyclic electron flow. $PS-II$ is not involved in this process,and consequently,there is no photolysis of water and no production of $NADPH + H^+$. Therefore,$PS-II$ does not participate in the cyclic flow of electrons.

(C) In the light-dependent reactions of photosynthesis,specifically in Photosystem $I$ $(PSI)$,the reaction center is $P_{700}$.
When $P_{700}$ absorbs light,it gets excited and releases electrons.
These high-energy electrons are first accepted by an iron-sulfur $(FeS)$ protein,which is known as ferredoxin.
From ferredoxin,the electrons are transferred to $NADP^+$ reductase to reduce $NADP^+$ to $NADPH$.

(D) During the light-dependent reactions of photosynthesis,light energy is captured and converted into chemical energy.
This chemical energy is stored in the form of $ATP$ (Adenosine Triphosphate) and $NADPH$ (Nicotinamide Adenine Dinucleotide Phosphate).
However,$ATP$ is the primary energy currency of the cell used to transfer energy for various metabolic processes,including the Calvin cycle.
Therefore,the energy formed is primarily transferred in the form of $ATP$.

(C) The photolysis of water,also known as the water-splitting complex,occurs during the light-dependent reactions of photosynthesis.
It takes place in the thylakoid lumen of the chloroplast.
During this process,water molecules $(H_2O)$ are split into oxygen $(O_2)$,protons $(H^+)$,and electrons $(e^-)$ by the oxygen-evolving complex associated with Photosystem $II$ $(PSII)$.
This process is essential for providing electrons to the electron transport chain and releasing oxygen as a byproduct.

(A) Light is directly necessary in the light-dependent reactions of photosynthesis.
Specifically,light energy is absorbed by the pigments in the photosystems,which leads to the excitation of electrons.
This process of electron excitation is the primary event that initiates the electron transport chain,leading to the production of $ATP$ and $NADPH$.

(A) The '$Z$' scheme of photosynthesis refers to the non-cyclic electron transport chain in the light-dependent reactions.
It is called the '$Z$' scheme because the redox potential profile of the electron flow resembles the letter '$Z$'.
This concept was proposed by Hill and Bendall in $1960$ to explain the cooperation between Photosystem $II$ $(PSII)$ and Photosystem $I$ $(PSI)$.

(C) The splitting of water molecules during the light-dependent reactions of photosynthesis is known as photolysis.
This process occurs in the thylakoid lumen and is associated with Photosystem-$II$ $(PS-II)$.
During photolysis,water is split into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$,which is released as a byproduct.

(C) The correct answer is $C$. Manganese $(Mn)$ plays a critical role in the water-splitting complex (also known as the oxygen-evolving complex) of Photosystem $II$.
Light energy induces changes in the oxidation states of Manganese $(Mn^{2+}, Mn^{3+}, Mn^{4+})$,which facilitates the removal of electrons from the $OH^-$ component of water molecules.
This process leads to the photolysis of water,resulting in the release of oxygen $(O_2)$ as a byproduct.

(C) During the light-dependent reactions of photosynthesis,light energy is captured by chlorophyll and used to drive the synthesis of $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ through a process known as photophosphorylation.
This $ATP$ then provides the necessary energy for the subsequent light-independent reactions (Calvin cycle) where $CO_2$ is fixed into carbohydrates.

(D) During the light reaction of photosynthesis,the primary products formed are $ATP$ and $NADPH$,which are known as assimilatory powers. Additionally,water molecules undergo photolysis (splitting of water),which results in the release of $O_2$ as a byproduct. Therefore,all of these are formed during the light reaction.

(C) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light energy.
This process occurs in the chloroplasts of plants during the light-dependent reactions of photosynthesis.
The chemical equation for this reaction is: $ADP + Pi \xrightarrow{\text{Light energy}} ATP$.

(B) Photosystems are protein complexes involved in the light-dependent reactions of photosynthesis.
$PS-I$ and $PS-II$ are located within the thylakoid membranes of the chloroplast.
Since the grana are stacks of thylakoids,these photosystems are found in the grana of the chloroplast.
The stroma is the site of the dark reaction (Calvin cycle),while mitochondria are involved in cellular respiration.

(B) The correct answer is $B$.
Photosystem-$II$ $(PS-II)$,with its reaction center $P-680$,is responsible for the photolysis of water.
During the light-dependent reactions,$PS-II$ extracts electrons from water molecules to replace the electrons lost by the reaction center upon excitation.
This process,known as the water-splitting complex or oxygen-evolving complex,results in the release of $O_2$ as a byproduct,along with the production of protons $(H^+)$ and electrons $(e^-)$ according to the equation: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$.

(B) The correct answer is $B$. During the light reaction of photosynthesis,not only is reduced $NADP$ $(NADPH)$ formed and $O_2$ evolved,but $ATP$ is also synthesized.
This process of synthesizing high-energy phosphate molecules $(ATP)$ using light energy is known as photophosphorylation.

(D) The Hill reaction,discovered by Robert Hill in $1937$,refers to the light-dependent reaction of photosynthesis where water is photolyzed to release oxygen in the presence of an artificial electron acceptor.
This process occurs in the light and requires the presence of an electron acceptor (like ferricyanide or $DCPIP$) to accept electrons released from the photolysis of water.
Therefore,it is a light-dependent process that demonstrates the evolution of oxygen from water during photosynthesis.

(A) Non-cyclic photophosphorylation,also known as the $Z$-scheme,involves both photosystems $PS-II$ and $PS-I$ working in series.
$1$. Electrons are released from $PS-II$ upon excitation by light.
$2$. These electrons pass through an electron transport chain to $PS-I$.
$3$. $PS-I$ is also excited by light and transfers electrons to $NADP^+$ to form $NADPH$.
$4$. Since the electrons lost by $PS-II$ are replaced by the photolysis of water,the flow of electrons is non-cyclic.

(C) Cyclic photophosphorylation involves only Photosystem $I$ $(PS-I)$.
In this process,the excited electrons are cycled back to the reaction center $(P700)$ through an electron transport chain.
As the electrons move through the transport chain,the energy released is used to pump protons across the membrane,creating a proton gradient that drives the synthesis of $ATP$ via $ATP$ synthase.
Unlike non-cyclic photophosphorylation,cyclic photophosphorylation does not involve the photolysis of water (so no $O_2$ is released) and does not involve the reduction of $NADP^+$ to $NADPH$.
Therefore,the only product of cyclic photophosphorylation is $ATP$.

(A) In noncyclic photophosphorylation,both $PS-II$ and $PS-I$ are involved.
$PS-II$ contains a reaction center chlorophyll-$a$ molecule known as $P_{680}$,which absorbs light at $680 \ nm$.
When light energy is absorbed by the antenna pigments,it is transferred to the reaction center.
The $P_{680}$ molecule is the first to be excited,losing an electron to the primary electron acceptor.
Therefore,$P_{680}$ is the pigment molecule that is first excited in the noncyclic electron transport chain.

(A) In non-cyclic photophosphorylation,the photolysis of water occurs at the oxygen-evolving complex $(OEC)$ associated with Photosystem $II$.
The reaction for the photolysis of $2$ molecules of $H_2O$ is: $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$.
This process releases $4$ protons $(H^+)$ directly into the thylakoid lumen.
Additionally,the electron transport chain contributes to the proton gradient,but the specific protons released due to the splitting of $2$ water molecules is $4 H^+$.

(B) Light energy travels in the form of small discrete packets of energy known as $Photons$. According to the quantum theory of light,these $Photons$ are the fundamental units of light energy that interact with chlorophyll molecules during the light-dependent reactions of photosynthesis.

(A) In the light reaction of photosynthesis,the process begins when the chlorophyll molecule in Photosystem-$II$ $(PS-II)$ absorbs light energy (photons).
This absorption of light energy leads to the excitation of electrons in the chlorophyll molecule,causing it to lose electrons and become oxidized.
This oxidation of chlorophyll is the primary event that triggers the subsequent electron transport chain and the photolysis of water to replace the lost electrons.

(C) During the process of photosynthesis,light energy (radiation energy) is captured by chlorophyll pigments.
This energy is used to drive the synthesis of $ATP$ (Adenosine Triphosphate) and $NADPH$ through the light-dependent reactions (photophosphorylation).
$ATP$ acts as the primary energy currency of the cell,storing chemical energy in its high-energy phosphate bonds.
Therefore,the radiation energy of light is converted into chemical energy and stored in the form of $ATP$.

(B) The light-dependent reactions of photosynthesis,also known as the light reaction,require solar energy to function.
These reactions take place in the thylakoid membranes of the chloroplasts.
During this process,light energy is captured by chlorophyll and other pigments to produce $ATP$ and $NADPH$,which are essential for the subsequent dark reactions (Calvin cycle).
Therefore,the light reaction occurs only in the presence of light.

(C) Bacterial photosynthesis,such as that found in purple and green sulfur bacteria,is anoxygenic.
Unlike oxygenic photosynthesis in plants and cyanobacteria,which requires both $PS-I$ and $PS-II$ to split water,bacterial photosynthesis relies on a single photosystem.
Most photosynthetic bacteria possess a photosystem that is structurally and functionally similar to $PS-I$ of higher plants.
Therefore,bacterial photosynthesis involves only $PS-I$.

(D) Photosynthesis is most efficient in the blue and red regions of the visible light spectrum. Chlorophyll $a$ and $b$ show maximum absorption in these regions. While blue light carries more energy,red light is highly effective for the photochemical reactions of photosynthesis. Among the given options,the red region is recognized as one of the primary peaks for photosynthetic activity.

(C) The phenomenon of $Red \, Drop$ was observed by $R. \, Emerson$ and $C.M. \, Lewis$ in $1943$. They found that the quantum yield of photosynthesis significantly decreases when plants are exposed to monochromatic light at wavelengths greater than $680 \, nm$ (far-red region of the spectrum). This drop occurs because light at these wavelengths is insufficient to excite both Photosystem $I$ and Photosystem $II$ simultaneously,which is necessary for efficient photosynthesis.

(A) The phenomenon described is known as the Emerson Enhancement Effect.
When plants are exposed to red light alone,the rate of photosynthesis is lower than the sum of the rates observed when exposed to shorter wavelengths.
However,when red light is supplemented with far-red light,the rate of photosynthesis increases significantly,exceeding the sum of the rates of the two individual light exposures.
This synergistic effect demonstrates the cooperation between the two photosystems,$PS-I$ and $PS-II$,in the light-dependent reactions of photosynthesis.

(B) The phenomenon where the rate of photosynthesis is enhanced when plants are exposed to both red and far-red light simultaneously (or when red light is supplemented with light of shorter wavelengths like blue light) compared to the sum of the rates when exposed to each wavelength separately is known as the $Emerson$ enhancement effect. This discovery provided evidence for the existence of two distinct photosystems,$PS-I$ and $PS-II$,operating in the light-dependent reactions of photosynthesis.

(D) The 'red drop' effect refers to the sharp decrease in the quantum yield of photosynthesis when plants are exposed to light with wavelengths greater than $680 \ nm$ (far-red light).
This occurs because light at these wavelengths is primarily absorbed by Photosystem-$I$ $(PS-I)$ only,failing to excite Photosystem-$II$ $(PS-II)$,which is necessary for the complete electron transport chain.
Robert Emerson observed that the rate of photosynthesis drops significantly in the far-red region of the spectrum.

(C) Cyclic photophosphorylation involves only $Photosystem-I$ $(PSI)$.
It occurs when the $NADP^+$ concentration is low,meaning there is a shortage of $NADP^+$ to accept electrons from the electron transport chain.
Under conditions of low light intensity or when the demand for $ATP$ is high relative to $NADPH$,the electrons are cycled back to $PSI$ through the cytochrome complex.
Therefore,aerobic conditions (presence of oxygen) and low light intensity are generally considered favourable for this process to maintain the $ATP/NADPH$ ratio.

(C) During the light reaction of photosynthesis,$O_2$ is released as a result of the photolysis of water.
$DCMU$ ($3$-($3$,$4$-dichlorophenyl)$-1,1-$dimethylurea) is a potent photosynthetic inhibitor.
It acts by binding to the $Q_B$ binding site of $PS-II$,thereby blocking electron transport from $PS-II$ to plastoquinone.
This inactivation of $PS-II$ effectively inhibits the Hill reaction and prevents the photolysis of water,thus inhibiting $O_2$ release.

(B) The electron transport chain $(ETC)$ in photosynthesis consists of protein complexes such as Photosystem $I$ $(PS I)$,Photosystem $II$ $(PS II)$,Cytochrome $b_6f$ complex,and mobile electron carriers like plastoquinone and plastocyanin.
These components are embedded within the thylakoid membranes of the chloroplast.
This arrangement allows for the generation of a proton gradient across the thylakoid membrane,which is essential for $ATP$ synthesis via chemiosmosis.

(B) In the light-dependent reactions of photosynthesis,the two photosystems,$PS-II$ and $PS-I$,work in series.
$PS-II$ (Photosystem $II$) absorbs light at $680 \ nm$ and initiates the electron transport chain.
Electrons are excited in $PS-II$ and passed through a series of electron carriers to $PS-I$ (Photosystem $I$).
Therefore,the flow of energy and electrons occurs from $PS-II$ to $PS-I$ in the non-cyclic photophosphorylation process ($Z$-scheme).

(A) During the light-dependent reactions of photosynthesis,the photolysis (splitting) of water occurs in the thylakoid lumen.
This process releases electrons,protons $(H^+)$,and oxygen $(O_2)$.
The electrons released from water are used to replace the electrons lost by Photosystem $II$ $(PSII)$.
These electrons eventually travel through the electron transport chain to reduce $NADP^+$ into $NADPH$.

(C) The mechanism of $ATP$ synthesis in both chloroplasts (photophosphorylation) and mitochondria (oxidative phosphorylation) is explained by the Chemiosmotic theory,proposed by Peter Mitchell.
This theory states that $ATP$ synthesis is linked to the development of a proton gradient across the membrane (thylakoid membrane in chloroplasts and inner mitochondrial membrane in mitochondria).
The movement of protons $(H^+)$ from the intermembrane space or thylakoid lumen back into the matrix or stroma through the $F_0-F_1$ $ATP$ synthase complex provides the energy required for the phosphorylation of $ADP$ to $ATP$.