Light reaction Questions in English

(C) The splitting of water molecules during photosynthesis is known as photolysis of water.
This process occurs in the thylakoid lumen of the chloroplast.
Both manganese $(Mn^{2+})$ and chloride $(Cl^-)$ ions are essential for this reaction.
$Mn$ acts as a cofactor for the oxygen-evolving complex,while $Cl$ is required for the activation of this complex.
Therefore,both elements are necessary for the photolysis of water.

(D) The correct answer is $D$. Chlorine,in the form of chloride ions $(Cl^-)$,is essential for the photolysis of water during photosynthesis. It facilitates the transfer of electrons from $H_2O$ to the photo-oxidized chlorophyll molecule in Photosystem $II$.

(D) Plastocyanin is a copper-containing protein that acts as an electron carrier in the photosynthetic electron transport chain.
It is located in the lumen of the thylakoid membrane.
It facilitates the transfer of electrons from the cytochrome $b_6f$ complex to Photosystem $I$ $(PSI)$.

(C) The enhancement effect,also known as the $Emerson$ enhancement effect,was discovered by $Robert$ $Emerson$ and his colleagues in $1957$.
They observed that the rate of photosynthesis when plants were exposed to both far-red light and shorter wavelength light simultaneously was significantly higher than the sum of the rates when exposed to each wavelength separately.
This discovery provided crucial evidence for the existence of two distinct photosystems ($PS-I$ and $PS-II$) operating in the light-dependent reactions of photosynthesis.

(B) $Arnon$ et al. $(1954)$ first demonstrated that isolated chloroplasts can produce $ATP$ from $ADP + Pi$. They termed this process of $ATP$ production as photophosphorylation.

(C) The correct answer is $C$. The concept of two photosystems was established through the discovery of the $Emerson$ effect. $Emerson$ observed that the rate of photosynthesis increases significantly when plants are exposed to both shorter and longer wavelengths of light simultaneously,compared to the sum of the rates when exposed to each wavelength separately. This led to the conclusion that there are two distinct groups of pigments,known as photosystems,which operate in the light-dependent reactions of photosynthesis.

(B) During the process of photosynthesis,water $(H_2O)$ undergoes photolysis (splitting of water) in the presence of light.
This process releases electrons,protons ($H^+$ ions),and oxygen $(O_2)$.
The protons $(H^+)$ derived from water are used to reduce $NADP^+$ to $NADPH$,which is then utilized in the Calvin cycle for the reduction of $CO_2$ to form glucose $(C_6H_{12}O_6)$.
Therefore,the hydrogen atoms (as protons and electrons) required for the synthesis of glucose are obtained from water.
The reaction is: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$.

(A) The Emerson effect demonstrated that the rate of photosynthesis increases when plants are exposed to both short-wavelength and long-wavelength light simultaneously,compared to the sum of the rates when exposed to each wavelength separately. This discovery provided evidence for the existence of two distinct photochemical systems (Photosystem $I$ and Photosystem $II$) that operate in synergy to drive photosynthesis.

(B) The correct answer is $B$.
Chloroplasts are the sites of photosynthesis in plants.
During the light-dependent reactions of photosynthesis,solar energy (light energy) is captured by chlorophyll pigments and converted into chemical energy in the form of $ATP$ and $NADPH$.
This process is known as photophosphorylation,which occurs within the thylakoid membranes of the chloroplasts.

(D) Photosynthesis is a redox (oxidation-reduction) process.
In this process,water $(H_2O)$ undergoes photolysis (splitting by light) and is oxidized to release oxygen $(O_2)$.
Simultaneously,carbon dioxide $(CO_2)$ is reduced to form carbohydrates (glucose) using the energy stored in $ATP$ and $NADPH$.

(D) Photosynthesis is a process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll.
In this process, carbon dioxide and water are converted into glucose, and oxygen is released as a byproduct.
The overall chemical equation is: $6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$.
Therefore, $O_2$ (Oxygen) is the byproduct of photosynthesis.

(A) The term 'Assimilatory power' was coined by Arnon $(1956)$ to refer to the products of the light-dependent reactions of photosynthesis,which are $ATP$ and $NADPH_2$.
These molecules are essential for the subsequent light-independent reactions (Calvin cycle) where they provide the energy and reducing power required to fix $CO_2$ into carbohydrates.
Note that $NADPH_2$ alone is often referred to as 'reducing power'.

(B) The process of synthesis of $ATP$ from $ADP$ and inorganic phosphate in the presence of light is known as photophosphorylation.
Since this process is driven by light energy,it is a photo process.
Therefore,photosynthetic phosphorylation is a photo process.

(C) During the light-dependent reactions of photosynthesis,the energy from sunlight is used to split water molecules $(H_2O)$ into oxygen $(O_2)$,protons $(H^+)$,and electrons $(e^-)$. This process is known as the photolysis of water. The oxygen released during this process is a byproduct that is liberated into the atmosphere.

(B) The correct answer is $B$. In angiosperms,the synthesis of chlorophyll is a light-dependent process. The conversion of protochlorophyllide to chlorophyllide requires light,which is then reduced to chlorophyll. In contrast,some gymnosperms can synthesize chlorophyll in the dark due to the presence of specific enzymes.

(B) The correct answer is $B$.
During the light-dependent reactions of photosynthesis,water molecules $(H_2O)$ undergo photolysis (splitting of water) in the presence of light and the oxygen-evolving complex.
This process releases electrons,protons $(H^+)$,and oxygen gas $(O_2)$.
The oxygen released is a byproduct of this water-splitting process,which is pure $O_2$.

(A) The process of adding a phosphate group to $ADP$ or $AMP$ is called phosphorylation.
In photosynthesis,specifically during the light-dependent reactions,photophosphorylation occurs where $ADP + iP \rightarrow ATP$.
This process generates $ATP$,which is essential for the dark reaction (Calvin cycle) to synthesize sugars.

(C) $NADP$ stands for Nicotinamide adenine dinucleotide phosphate.
It is a coenzyme that acts as an electron carrier in various metabolic processes,including the light-dependent reactions of photosynthesis.

(B) During the light reaction of photosynthesis,photophosphorylation occurs,which results in the production of $ATP$ and $NADPH_2$.
These molecules act as energy carriers.
$ATP$ and $NADPH_2$ are transported from the thylakoid membranes (where light reactions occur) to the stroma (where dark reactions occur).
Together,$ATP$ and $NADPH_2$ are referred to as assimilatory power,which provides the necessary energy and reducing power to fix $CO_2$ into carbohydrates during the dark reaction (Calvin cycle).

(C) The Emerson effect,discovered by $R$. Emerson and $C$.$M$. Lewis $(1943)$,demonstrates that the rate of photosynthesis increases significantly when plants are exposed to both far-red light and red light simultaneously,compared to the sum of the rates when exposed to each wavelength separately. This phenomenon provided evidence for the existence of two distinct photosystems ($PS-I$ and $PS-II$) working in tandem during the light-dependent reactions of photosynthesis.

(B) . $Grana$ are the sites for the light reaction of photosynthesis and consist of photosynthetic units called 'quantasomes'. These are found on the surface of the thylakoids.

(B) The first step in the light-dependent reactions of photosynthesis is the absorption of light energy by chlorophyll molecules.
When a photon of light energy strikes a chlorophyll molecule,it causes the excitation of an electron from its ground state to a higher energy level (excited singlet state).
This photoexcitation leads to the ejection of an electron from the chlorophyll molecule,which then initiates the electron transport chain.
Therefore,the correct sequence begins with the photoexcitation of chlorophyll.

(A) During the light-dependent reactions of photosynthesis,$ATP$ is synthesized through the process of photophosphorylation.
In cyclic photophosphorylation,electrons cycle through the photosystem $I$ $(PSI)$,leading to the production of $ATP$.
In non-cyclic photophosphorylation,both $PSI$ and $PSII$ are involved,resulting in the production of both $ATP$ and $NADPH$.
Additionally,the oxygen evolved during photosynthesis is derived from the photolysis of water,not from carbon dioxide.

(D) The process of photosynthesis begins with the absorption of light energy by chlorophyll pigments.
When chlorophyll molecules absorb photons from sunlight,they become excited and reach a higher energy state.
This activation of chlorophyll is the primary and initiating step that triggers the subsequent light-dependent reactions,including the photolysis of water and the synthesis of $ATP$ and $NADPH$.

(C) The process of photosynthesis begins with the absorption of light energy by chlorophyll pigments.
When a photon of light strikes a chlorophyll molecule,it transfers its energy to an electron,causing the electron to move to a higher energy state (excited state).
This excitation of the electron is the primary photochemical event that initiates the entire process of photosynthesis,leading to subsequent electron transport and energy conversion.

(A) The light-dependent reaction of photosynthesis,which occurs in the thylakoid membranes of the chloroplast,is known as the $Hill$ reaction. This process was discovered by $Robert$ $Hill$ in $1939$,who demonstrated that isolated chloroplasts could evolve $O_2$ in the presence of light and an artificial electron acceptor,even in the absence of $CO_2$.

(C) Ferredoxin is an iron-sulfur protein ($Fe-S$ protein) that acts as an electron carrier in the photosynthetic electron transport chain.
It contains iron $(Fe)$ atoms coordinated with sulfur atoms.
Therefore,the correct answer is Iron.

(B) In non-cyclic photophosphorylation (also known as $Z$-scheme),the electrons released from $PS-II$ do not return to the same photosystem. Instead,they flow through an electron transport chain to $PS-I$ and are finally accepted by $NADP^+$ to form $NADPH$. Since the electrons move in a single direction from water to $NADP^+$,this flow is described as unidirectional.

(C) During the light reaction of photosynthesis,the chlorophyll molecule absorbs light energy and becomes excited.
This excited chlorophyll molecule loses an electron,which is a process of oxidation.
Subsequently,the oxidized chlorophyll molecule regains an electron from the photolysis of water (or other electron donors),which is a process of reduction.
Therefore,chlorophyll undergoes both oxidation and reduction cycles during the light-dependent reactions.

(B) Ferredoxin is an iron-sulfur protein that acts as an electron carrier in the photosynthetic electron transport chain.
It is specifically associated with Photosystem-$I$ $(PS-I)$.
During the light reaction,electrons are transferred from the primary electron acceptor of $PS-I$ to ferredoxin,which then transfers them to $NADP^+$ reductase to reduce $NADP^+$ to $NADPH$.

(B) During the light-dependent reactions of photosynthesis,water molecules $(H_2O)$ undergo photolysis (splitting of water) in the thylakoid lumen.
This process releases oxygen $(O_2)$ as a byproduct,along with protons $(H^+)$ and electrons $(e^-)$.
Since the light reaction occurs before the dark reaction (Calvin cycle),where glucose is synthesized,oxygen is the first product released during the process of photosynthesis.

(A) In the light-dependent reactions of photosynthesis,when the reaction center of $PS-II$ $(P680)$ absorbs light energy,it becomes excited and releases an electron.
This high-energy electron is immediately captured by the primary electron acceptor,which is pheophytin.
From pheophytin,the electron is then transferred to the first mobile electron carrier in the electron transport chain,which is plastoquinone $(PQ)$.

(A) The photolysis of water,also known as the water-splitting reaction,occurs in the thylakoid lumen of the chloroplast.
This process is catalyzed by the Oxygen Evolving Complex $(OEC)$,which is associated with Photosystem $II$ $(PSII)$.
The essential elements required for this reaction are Manganese $(Mn)$,Calcium $(Ca)$,and Chloride $(Cl^-)$ ions.
Among the given options,Manganese $(Mn)$ is the primary element involved in the water-splitting complex.

(D) In the light-dependent reaction of photosynthesis,the photolysis of water occurs to provide electrons for the electron transport chain. The reaction is represented as: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$.
To produce one molecule of $NADPH_2$ (or $NADPH$),two electrons are required $(NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+)$.
Since the photolysis of one $H_2O$ molecule releases two electrons,the photolysis of one $H_2O$ molecule provides the two electrons necessary to reduce one $NADP^+$ to $NADPH$. Therefore,$1$ molecule of water is photolysed to form one $NADPH_2$ molecule.

(C) The Emerson effect is defined as the increase in the rate of photosynthesis when light of two different wavelengths (short and long) is provided simultaneously,compared to the sum of the rates when each wavelength is provided separately.
Mathematically,the Emerson effect $(E)$ is calculated as the ratio of the rate of oxygen production in a mixed beam of light $(x)$ to the sum of the rates of oxygen production in low wavelength light $(y)$ and high wavelength red light $(z)$.
Therefore,the formula is $E = \frac{x}{y + z}$.

(A) Manganese $(Mn^{2+})$ and chloride $(Cl^-)$ ions are essential cofactors for the oxygen-evolving complex $(OEC)$ of Photosystem $II$.
These ions play a crucial role in the photolysis of water (splitting of water molecules) into oxygen,protons $(H^+)$,and electrons $(e^-)$.
This process is vital for providing electrons to the electron transport chain during photosynthesis.

(D) Clayton $(1966)$ discovered another form of chlorophyll $a$ which has an absorption maximum at $700 \, nm$. This specific form of chlorophyll is designated as $P-700$ because it functions as the reaction center of Photosystem $I$ and absorbs light most efficiently at a wavelength of $700 \, nm$.

(B) In the light-dependent reactions of photosynthesis,the $Z$-scheme involves both Photosystem-$II$ $(PS-II)$ and Photosystem-$I$ $(PS-I)$.
During the electron transport chain,electrons are excited in $PS-I$ and passed through a series of carriers to the enzyme $NADP$ reductase.
This enzyme facilitates the reduction of $NADP^+$ to $NADPH + H^+$ (often referred to as $NADPH_2$) using electrons derived from the electron transport chain and protons from the stroma.
Therefore,the generation of $NADPH_2$ is specifically associated with the activity of $PS-I$ in the non-cyclic photophosphorylation pathway.

(B) During the process of photophosphorylation,light energy is absorbed by the antenna complex and transferred to the reaction center.
In both Photosystem $I$ $(PS I)$ and Photosystem $II$ $(PS II)$,the reaction center is composed of a specialized molecule of Chlorophyll $a$.
When this reaction center absorbs the excitation energy,it becomes excited and ejects an electron,which then enters the electron transport system.
Therefore,Chlorophyll $a$ is the specific pigment responsible for the ejection of electrons.

(B) Photosystem-$II$ is a photosynthetic pigment system,along with some electron carriers,that is located in the appressed part of the grana thylakoids. Therefore,it is found in the grana of the chloroplast.

(C) The oxygen evolved during photosynthesis in green plants originates from the photolysis of water $(H_2O)$.
This was experimentally proven by $C$.$B$. van Niel,who demonstrated that purple and green sulfur bacteria use $H_2S$ instead of $H_2O$ as a hydrogen donor,releasing sulfur instead of oxygen.
Later,this was confirmed using the oxygen isotope $^{18}O$ by Ruben and Kamen,which proved that the $O_2$ released comes from water and not from carbon dioxide $(CO_2)$.

(B) In chloroplasts,the light-dependent reactions of photosynthesis occur within the thylakoid membranes.
These membranes are organized into stacked structures known as $Grana$.
Oxygen evolution (photolysis of water) occurs in the lumen of the thylakoids,while photosynthetic phosphorylation ($ATP$ synthesis) occurs across the thylakoid membrane.
Therefore,the $Grana$ stacks are the primary site for these processes.

(B) In the process of photosynthesis,light energy is primarily absorbed by chlorophyll molecules. This energy is then used to perform photolysis of water,which is the splitting of water molecules $(H_2O)$ into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$. This process is essential to provide the electrons required for the electron transport chain and to generate the proton gradient necessary for $ATP$ synthesis.

(A) Cyclic photophosphorylation involves only Photosystem $I$ $(PSI)$.
In this process,electrons are cycled back to the reaction center $(P700)$ through an electron transport chain.
As electrons move through the transport chain,protons are pumped 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 Photosystem $II$ $(PSII)$,and therefore,it does not result in the photolysis of water or the production of $NADPH_2$ and $O_2$.
Thus,only $ATP$ is produced during this process.

(B) The correct answer is $B$.
In non-cyclic photophosphorylation,both $PS-I$ and $PS-II$ work in series.
The flow of electrons is unidirectional,starting from water and ending at $NADP^+$.
During this process,the electrons released from the electron transport chain are used to reduce $NADP^+$ to $NADPH_2$ (or $NADPH + H^+$),which is essential for the biosynthetic phase of photosynthesis.

(A) During the light reaction of photosynthesis,light energy is captured by chlorophyll and converted into chemical energy in the form of $ATP$ and $NADPH$.
This process is known as photophosphorylation.
In this reaction,an inorganic phosphate $(Pi)$ is added to adenosine diphosphate $(ADP)$ to form adenosine triphosphate $(ATP)$.
The reaction is represented as: $ADP + Pi \rightarrow ATP$.

(C) The light-dependent reactions (photochemical phase) of photosynthesis occur in the thylakoid membranes of the chloroplast. These membranes contain photosystems ($PS I$ and $PS II$) and the electron transport chain,which are essential for capturing light energy and converting it into chemical energy ($ATP$ and $NADPH$). The functional units of these membranes are known as quantasomes.

(C) In photosystem-$I$ $(PS-I)$,the reaction centre is a special form of chlorophyll-$a$ molecule that absorbs light at a wavelength of $700 \ nm$.
This specific chlorophyll-$a$ molecule is known as $P-700$.
In contrast,the reaction centre of photosystem-$II$ $(PS-II)$ is $P-680$,which absorbs light at $680 \ nm$.

(C) Pigment system-$I$ $(PS-I)$ is involved in both cyclic and non-cyclic photophosphorylation.
In cyclic photophosphorylation,the electrons released from the reaction center $(P700)$ of $PS-I$ are cycled back to the reaction center through an electron transport chain,resulting in the synthesis of $ATP$.
In non-cyclic photophosphorylation,both $PS-II$ and $PS-I$ work in tandem. $PS-I$ receives electrons from $PS-II$ via the electron transport chain and subsequently reduces $NADP^+$ to $NADPH$.