Light reaction Questions in English

(A) The correct answer is $A$.
Photolysis of water is the process of splitting water molecules into hydrogen ions,electrons,and oxygen in the presence of light during the light-dependent reactions of photosynthesis.
Manganese $(Mn^{2+})$ acts as an essential cofactor for the oxygen-evolving complex $(OEC)$ in Photosystem $II$,which catalyzes this reaction.
Without manganese,the water-splitting mechanism cannot function,thereby halting the supply of electrons to the electron transport chain.

(A) Statement $(i)$ is incorrect because the $Z$ scheme involves both $PSII$ and $PSI$ working in series.
Statement $(ii)$ is correct because cyclic photophosphorylation involves only $PSI$ where electrons circulate within the photosystem.
Statement $(iii)$ is incorrect because cyclic photophosphorylation produces only $ATP$,not $NADPH_2$.
Statement $(iv)$ is correct because the stroma lamellae lack both $PSII$ and the enzyme $NADP$ reductase,which is why they only perform cyclic photophosphorylation.
Therefore,statements $(ii)$ and $(iv)$ are correct.

(C) In cyclic photophosphorylation,only $ATP$ molecules are synthesized.
This process involves the cyclic flow of electrons through the photosystem $I$ $(PS-I)$ complex.
Cyclic photophosphorylation is not associated with the photolysis of water,therefore $O_2$ is not evolved.
Additionally,$NADP^+$ is not reduced to $NADPH$ because the electrons are cycled back to the reaction center of $PS-I$ rather than being transferred to $NADP^+$.

(C) The correct answer is $C$. In the light-dependent reactions of photosynthesis,the reaction center of photosystem $II$ $(P680)$ gets excited by absorbing light energy and releases electrons. These high-energy electrons are immediately captured by the primary electron acceptor,which is a molecule of pheophytin (a chlorophyll derivative),followed by plastoquinone $(PQ)$. Among the given options,quinone acts as the primary electron acceptor in the electron transport chain of photosystem $II$.

(A) : In the light reaction of photosynthesis,two types of photosystems are involved. $PS-I$ consists of a large amount of chlorophyll-$a$ and a very small quantity of chlorophyll-$b$. These pigments absorb light energy and transfer it to the reaction centre,$P_{700}$.
After absorbing an adequate amount of light energy,an electron gets excited from the $P_{700}$ molecule and moves to an iron-sulphur protein complex,designated as $A$ $(Fe-S)$.
It gets reduced after accepting electrons. It later transfers these electrons to ferredoxin and gets oxidized again.

(C) Non-cyclic photophosphorylation,also known as the $Z$-scheme,is a process in photosynthesis where electrons move in a non-cyclic manner.
This process involves both photosystems,$PS-I$ and $PS-II$,working in series.
$PS-II$ absorbs light energy,leading to the photolysis of water and the release of electrons.
These electrons are then transferred through an electron transport chain to $PS-I$,which subsequently reduces $NADP^+$ to $NADPH$.
Since both photosystems are required for the flow of electrons from water to $NADP^+$,the correct answer is both $A$ and $B$.

(B) In photosystem $II$ $(PS \ II)$,the reaction center is $P680$,which means it consists of chlorophyll $a$ molecules that have an absorption peak at $680 \, nm$.
This wavelength of $680 \, nm$ falls within the red region of the visible light spectrum.
When these chlorophyll $a$ molecules absorb light at this specific wavelength,the electrons in the pigment molecules become excited and jump into a higher energy orbit,initiating the electron transport chain.

(D) Chemiosmosis in chloroplast requires a membrane,a proton pump,a proton gradient,and $ATP$ase enzyme to synthesize $ATP$.
$CO_2$ is not involved in the light-dependent reactions of photosynthesis where chemiosmosis occurs; rather,it is used in the Calvin cycle (dark reactions) for carbon fixation.
Therefore,$CO_2$ is not required for the process of chemiosmosis.

(A) During the light reaction of photosynthesis,the $NADP$ reductase enzyme is essential for the reduction of $NADP^+$ to $NADPH + H^+$.
This enzyme is found associated with the thylakoid membrane.
Specifically,it is located on the stroma side of the thylakoid membrane,where it can easily access the $NADP^+$ and protons present in the stroma to facilitate the reaction.

(D) In $PS-I$ (Photosystem $I$),the reaction center chlorophyll $a$ has an absorption peak at $700\, nm$,which is why it is also known as $P700$.
In $PS-II$ (Photosystem $II$),the reaction center chlorophyll $a$ has an absorption peak at $680\, nm$,which is why it is also known as $P680$.
Therefore,statement $X$ is incorrect because $PS-I$ absorbs at $700\, nm$,and statement $Y$ is incorrect because $PS-II$ absorbs at $680\, nm$.
Thus,both statements $X$ and $Y$ are incorrect.

(C) Photosystem $II$ $(PSII)$ is a protein complex involved in the light-dependent reactions of photosynthesis.
It is primarily located in the thylakoid membranes of the grana (stacked thylakoids) within the chloroplast.
$PSII$ contains chlorophyll $a$ and other pigments that absorb light energy to initiate the photolysis of water.
In contrast,Photosystem $I$ $(PSI)$ is found in both the grana and the stromal lamellae,whereas $PSII$ is absent from the stromal lamellae.

(D) Chemiosmosis is the movement of ions across a semipermeable membrane down their electrochemical gradient.
In biological systems like chloroplasts and mitochondria,it requires:
$1$. $A$ membrane: To separate the two compartments and maintain the gradient.
$2$. $A$ proton pump: To create the proton gradient by pumping $H^+$ ions across the membrane using energy (e.g.,from electron transport).
$3$. $A$ proton gradient: The difference in $H^+$ concentration across the membrane,which provides the potential energy for $ATP$ synthesis via $ATP$ synthase.
Since all three components are essential for the process,the correct answer is $I, II,$ and $III$.

(A) In non-cyclic photophosphorylation ($Z$-scheme),electrons are released from $PS-I$ after excitation by light.
These high-energy electrons are transferred to the protein ferredoxin.
From ferredoxin,the electrons are passed to the enzyme $NADP^+$ reductase.
Finally,$NADP^+$ accepts these electrons along with protons $(H^+)$ from the stroma to form $NADPH + H^+$.
Therefore,the correct acceptor of electrons released by $PS-I$ in this pathway is $NADP^+$.

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

(D) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light.
During the light-dependent reactions of photosynthesis,light energy is captured by chlorophyll pigments.
This energy is used to drive the electron transport chain,which creates a proton gradient across the thylakoid membrane.
The flow of protons back into the stroma through $ATP$ synthase enzyme provides the energy required to convert $ADP$ into $ATP$.
Thus,light energy is converted into chemical energy stored in $ATP$ molecules.

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

(C) The process of synthesis of $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light in the chloroplast is known as photophosphorylation.
Since this process occurs in the grana (thylakoids) of the chloroplast during the light-dependent reactions of photosynthesis,it is specifically termed photophosphorylation.
Therefore,the correct option is $C$.

(A) Chemiosmosis is a mechanism that explains the synthesis of $ATP$ in both mitochondria and chloroplasts.
In the chloroplast,the process involves the development of a proton gradient across the thylakoid membrane.
This gradient is created due to the accumulation of protons ($H^+$ ions) in the lumen of the thylakoid,resulting from the photolysis of water and the activity of the electron transport chain.
The movement of these protons back into the stroma through the $CF_0-CF_1$ $ATP$ synthase enzyme complex provides the energy required for the phosphorylation of $ADP$ to form $ATP$.

(C) In the light reaction of photosynthesis,a proton gradient is established across the thylakoid membrane,with a higher concentration of $H^+$ inside the lumen compared to the stroma.
When this proton gradient breaks down,protons move from the thylakoid lumen to the stroma through the $CF_0-CF_1$ $ATP$ synthase enzyme.
This movement of protons provides the energy required for the conformational changes in the $ATP$ synthase enzyme,which facilitates the synthesis of $ATP$ from $ADP$ and inorganic phosphate $(Pi)$.
Therefore,the breakdown of the proton gradient leads to the production of more energy in the form of $ATP$.

(B) During the light-dependent reactions of photosynthesis,electrons are transported through an electron transport chain located in the thylakoid membrane.
This chain includes various protein complexes,most notably the cytochrome $b_6f$ complex.
As electrons pass through the cytochrome complex,energy is released and used to pump protons $(H^+)$ from the stroma into the thylakoid lumen,creating a proton gradient.
This proton gradient drives the synthesis of $ATP$ via the enzyme $ATP$ synthase.

(B) The photolysis of water,also known as the water-splitting complex,occurs in the thylakoid lumen of the chloroplast during the light-dependent reactions of photosynthesis. This process requires specific mineral ions as cofactors. Manganese $(Mn^{2+})$,along with Calcium $(Ca^{2+})$ and Chloride $(Cl^-)$ ions,is essential for the functioning of the Oxygen Evolving Complex $(OEC)$,which catalyzes the splitting of water molecules into protons,electrons,and oxygen gas. Therefore,$Mn$ is the correct element required for this process.

(C) The process of water splitting,also known as photolysis of water,occurs in the thylakoid lumen during the light-dependent reactions of photosynthesis.
This process is essential to provide electrons to Photosystem-$II$ $(PS-II)$ to replace those lost during excitation.
The elements $Mn^{+2}$ (Manganese) and $Cl^-$ (Chloride ions) are critical cofactors for the Oxygen Evolving Complex $(OEC)$ associated with $PS-II$.
Therefore,$Mn^{+2}$ and $Cl^-$ are the elements helpful in the splitting of water.

(D) In the process of photosynthesis,the light-harvesting complex consists of hundreds of pigment molecules bound to proteins.
Each photosystem has all the pigments except one molecule of chlorophyll $-a$ forming a light-harvesting system also called antennae.
These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light.
The single chlorophyll $-a$ molecule forms the reaction center.
In Photosystem $I$ $(PS I)$,the reaction center chlorophyll $-a$ has an absorption peak at $700 \ nm$ $(P700)$,while in Photosystem $II$ $(PS II)$,it has an absorption peak at $680 \ nm$ $(P680)$.

(D) In non-cyclic photophosphorylation ($Z$-scheme):
$(1)$ When all the carriers are placed in a sequence on a redox potential scale,a $Z$ shape is formed. This statement is correct.
$(2)$ The electrons released from $PS-II$ are accepted by electron acceptors and move to $PS-I$. They do not return to $PS-II$. The electrons released from $PS-I$ are accepted by $NADP^+$ to form $NADPH$. The statement saying they enter $PS-II$ instead of $PS-I$ is incorrect.
$(3)$ Both $PS-I$ and $PS-II$ are involved in this process. This statement is correct.
$(4)$ Since the electrons do not return to their original donors,it is called non-cyclic electron transport. This statement is correct.
Therefore,only statement $(2)$ is incorrect.

(A) 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.
Light energy is used to create a proton gradient across the thylakoid membrane,which drives the enzyme $ATP$ synthase to produce $ATP$.

(D) Chemiosmosis requires a membrane,a proton pump,a proton gradient,and $ATP$ synthase.
$1$. $A$ membrane is required to separate the two compartments.
$2$. $A$ proton pump is needed to create a concentration gradient of protons ($H^+$ ions) across the membrane.
$3$. $A$ proton gradient (difference in $H^+$ concentration) provides the potential energy necessary for $ATP$ synthesis.
$4$. $ATP$ synthase is the enzyme that facilitates the movement of protons back across the membrane,using the energy released to synthesize $ATP$ from $ADP$ and inorganic phosphate.
Therefore,all the listed components are essential for the process.

(C) The process of $ATP$ synthesis in both chloroplasts and mitochondria is explained by the $Chemiosmotic$ $hypothesis$.
This hypothesis was proposed by $Peter$ $Mitchell$ in $1961$.
It 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 back across the membrane through the $ATP$ $synthase$ enzyme complex provides the energy required for the phosphorylation of $ADP$ to $ATP$.

(D) During the light-dependent reactions of photosynthesis,specifically in non-cyclic photophosphorylation (also known as the $Z$-scheme),electrons are transferred from water to $PS-II$,then to $PS-I$,and finally to $NADP^+$. The enzyme $FNR$ (Ferredoxin-$NADP^+$ reductase) facilitates the reduction of $NADP^+$ to $NADPH$ using electrons derived from $PS-I$. Therefore,$NADPH$ is produced during non-cyclic photophosphorylation.

(A) Ferredoxin is an iron-sulfur protein that acts as an electron carrier in the photosynthetic electron transport chain.
In the light-dependent reactions of photosynthesis,electrons are excited in $PS-I$ (Photosystem $I$) and are transferred to ferredoxin.
Therefore,ferredoxin is functionally and structurally associated with the $PS-I$ complex.

(C) 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 known as grana.
Therefore,Photosystem-$II$ is found in the grana of the chloroplasts.

(B) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate in the presence of light.
In non-cyclic photophosphorylation,both $PS-I$ and $PS-II$ are involved,leading to the production of $ATP$,$NADPH$,and the release of $O_2$.
In cyclic photophosphorylation,only $PS-I$ is involved,and it results in the production of $ATP$ only,without the release of $O_2$ or the formation of $NADPH$.
Therefore,the common product produced in both processes is $ATP$.

(C) In the light-harvesting complex of photosynthesis,the reaction center is composed of a special pair of chlorophyll-$a$ molecules.
In Photosystem-$I$ $(PS-I)$,the reaction center is known as $P_{700}$ because it absorbs light at a wavelength of $700 \ nm$.
In Photosystem-$II$ $(PS-II)$,the reaction center is known as $P_{680}$ because it absorbs light at a wavelength of $680 \ nm$.
Among the given options,$P_{700}$ is the pigment that acts as the reaction center for $PS-I$.

(A) In the light-dependent reactions of photosynthesis,the $Z$-scheme involves both photosystems.
$Photosystem-II$ $(PS-II)$ absorbs light and initiates the electron transport chain,leading to the photolysis of water.
The electrons are then transferred to $Photosystem-I$ $(PS-I)$.
$Photosystem-I$ $(PS-I)$ is responsible for the final reduction of $NADP^+$ to $NADPH$ using the electrons received from the electron transport chain and protons from the stroma.
Therefore,$NADPH$ is produced by the activity of $Photosystem-I$.

(D) The light-dependent reaction (light reaction) of photosynthesis occurs in the thylakoid membranes of the chloroplast.
During this process,light energy is captured by chlorophyll pigments.
This energy is used for two primary events:
$1$. Photolysis of water $(H_2O \rightarrow 2H^+ + 2e^- + 1/2 O_2)$,which releases oxygen as a byproduct.
$2$. Photophosphorylation,which involves the synthesis of $ATP$ and $NADPH$ using the energy derived from the electron transport chain.
Options $A$,$B$,and $C$ are associated with the light-independent reaction (Calvin cycle),which occurs in the stroma of the chloroplast.

(A) The first step of photosynthesis is the absorption of light energy by chlorophyll molecules.
When a photon of light strikes the chlorophyll molecule,it excites an electron to a higher energy state,causing it to be released or ejected from the chlorophyll molecule.
This process is known as photo-excitation of chlorophyll,which initiates the light-dependent reactions.

(C) The red drop effect refers to the sharp decrease in the quantum yield of photosynthesis when the wavelength of light is increased beyond $680 \ nm$.
At wavelengths greater than $680 \ nm$,only $PS-I$ is functional,while $PS-II$ becomes inactive.
Since $PS-II$ is required for the photolysis of water and the production of $O_2$,its inactivity leads to a significant drop in the rate of photosynthesis,which is known as the red drop effect.

(B) During the light-dependent reactions of photosynthesis,solar energy is converted into chemical energy in the form of $ATP$ and $NADPH$.
These molecules are then released into the stroma of the chloroplast,where they serve as the energy source and reducing power for the dark reaction (Calvin cycle) to fix carbon dioxide into glucose.
Therefore,$ATP$ is the primary energy carrier transferred from the light reaction to the dark reaction.

(C) In $Photosystem-I$ $(PS-I)$,the reaction center is $P_{700}$.
When $P_{700}$ absorbs light,it gets excited and releases an electron.
This electron is first captured by a specialized chlorophyll molecule $(A_0)$,which is then passed to a series of membrane-bound carriers.
The first stable electron acceptor in the electron transport chain of $PS-I$ is an iron-sulfur protein ($Fe-S$ protein).
Therefore,the correct option is $C$.

(C) In the light-dependent reactions of photosynthesis,the process begins at Photosystem-$II$ $(PS-II)$.
When the reaction center chlorophyll-$a$ $(P_{680})$ absorbs light,it gets excited and releases an electron.
This excited electron is immediately captured by the primary electron acceptor,which is a molecule of pheophytin (a chlorophyll molecule lacking magnesium).
Among the given options,the primary electron acceptor in the electron transport chain following $PS-II$ is often associated with the quinone pool (specifically plastoquinone).
However,strictly speaking,the primary acceptor is pheophytin,which then transfers it to plastoquinone $(PQ)$.
Given the standard options provided in many biology curricula,'Quinone' (Plastoquinone) is the correct choice as it acts as the primary mobile electron carrier in the chain.

(D) Cyclic photophosphorylation involves only Photosystem $I$ $(PS-I)$.
In this process,the electron is excited and transferred through an electron transport chain,which leads to the synthesis of $ATP$ from $ADP$ and inorganic phosphate.
Unlike non-cyclic photophosphorylation,cyclic photophosphorylation does not involve the photolysis of water (hence no $O_2$ is released) and does not involve the reduction of $NADP^+$ to $NADPH$.
Therefore,only $ATP$ is produced during this process.

(A) During the light-dependent reactions of photosynthesis,the photolysis of water occurs to release electrons,protons,and oxygen.
This process is catalyzed by the Oxygen Evolving Complex $(OEC)$ associated with Photosystem $II$.
The essential elements required for the photolysis of water are Manganese $(Mn^{2+})$,Chlorine $(Cl^-)$,and Calcium $(Ca^{2+})$.
Therefore,the pair of elements involved in this process from the given options is Manganese and Chlorine.

(A) During the light-dependent reactions of photosynthesis,the photolysis of water occurs in the lumen of the thylakoid,which releases protons ($H^+$ ions) into the lumen. Additionally,the electron transport chain pumps protons from the stroma into the thylakoid lumen. As a result,the concentration of protons becomes significantly higher in the thylakoid lumen compared to the stroma,creating a proton gradient that drives $ATP$ synthesis via $ATP$ synthase. Therefore,the highest concentration of protons is found in the lumen of the thylakoid.

(A) The $Red$ $Drop$ effect refers to the sharp decrease in the quantum yield of photosynthesis when the wavelength of light is increased beyond $680 \ nm$ (far-red region).
Robert Emerson observed that when light of a shorter wavelength (red light) was provided simultaneously with far-red light,the rate of photosynthesis increased significantly,which is known as the $Emerson$ $Enhancement$ $Effect$.
These observations led to the conclusion that two distinct photochemical systems,known as $Photosystem$ $I$ ($PS$ $I$) and $Photosystem$ $II$ ($PS$ $II$),operate simultaneously to drive photosynthesis efficiently.

(C) The mechanism of $ATP$ synthesis in both chloroplasts (during photophosphorylation) and mitochondria (during oxidative phosphorylation) is explained by the Chemiosmotic Hypothesis.
This hypothesis was proposed by Peter Mitchell in $1961$.
It states that $ATP$ synthesis is linked to the development of a proton gradient across the membranes (thylakoid membrane in chloroplasts and inner mitochondrial membrane in mitochondria).
The movement of protons across the membrane through the $F_0-F_1$ ATPase enzyme complex provides the energy required for the phosphorylation of $ADP$ to form $ATP$.

(C) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light energy in chloroplasts.
The chemical reaction can be represented as: $ADP + Pi \rightarrow ATP$ (in the presence of light energy).
Therefore,option $C$ correctly represents the process.

(D) The chemiosmotic hypothesis,proposed by Peter Mitchell,explains how $ATP$ is synthesized in mitochondria and chloroplasts.
It states that the energy for $ATP$ synthesis is derived from a proton gradient ($H^+$ gradient) across the inner mitochondrial membrane or the thylakoid membrane.
As protons move down their electrochemical gradient through the $ATP$ synthase enzyme,the energy released is used to phosphorylate $ADP$ to $ATP$.
Therefore,the establishment and maintenance of a proton gradient are essential for this process.

(A) The light reaction of photosynthesis produces $ATP$,$NADPH$,and $Oxygen$ as end products.
$NADH$ is primarily produced during cellular respiration (e.g.,glycolysis and the Krebs cycle),not during the light-dependent reactions of photosynthesis.

(D) The thylakoid membranes are differentiated into two regions: grana lamellae and stroma lamellae.
The grana lamellae (or grana stacks) are rich in $PS\, II$ and $LHC$ (Light Harvesting Complex),but they contain very little $ATP$ synthetase.
In contrast,the stroma lamellae (stromal thylakoids) are rich in $PS\, I$ and $ATP$ synthetase,but they are poor in $PS\, II$ and $LHC$.
Since the Assertion states that stromal thylakoids are rich in both $PS\, I$ and $PS\, II$,the Assertion is incorrect because they are poor in $PS\, II$.
The Reason states that stroma membranes are rich in $ATP$ synthetase,which is correct.
Therefore,the Assertion is incorrect,but the Reason is correct.

(A) In the process of photophosphorylation,the cytochrome $b_6f$ complex acts as a proton pump.
It facilitates the movement of protons from the stroma into the thylakoid lumen,creating a proton gradient.
This proton gradient is essential for the functioning of $ATP$ synthase,which utilizes the energy of the proton motive force to synthesize $ATP$ from $ADP$ and inorganic phosphate.