Light reaction Questions in English

(B) $DCMU$ ($3$-($3$,$4$-dichlorophenyl)$-1,1-$dimethylurea) is a potent herbicide that acts as an inhibitor of photosynthesis.
It specifically binds to the $Q_B$ binding site of the $D1$ protein in the $PS-II$ (Photosystem-$II$) complex.
By blocking the electron flow from $Q_A$ to $Q_B$,it prevents the reduction of plastoquinone,thereby halting the light-dependent reactions of photosynthesis.

(B) During the light-dependent reactions of photosynthesis,solar energy is converted into chemical energy.
This chemical energy is stored in the form of $ATP$ (Adenosine Triphosphate) and $NADPH$.
These molecules are then utilized in the light-independent reactions (also known as the Calvin cycle or dark reaction) to fix carbon dioxide into glucose.
Therefore,$ATP$ acts as the primary energy carrier between the light and dark reactions.

(B) Solar energy is converted into chemical energy ($ATP$ and $NADPH$) during the light-dependent reactions of photosynthesis. This process occurs in the thylakoid membranes of the chloroplast. Mitochondria are involved in cellular respiration where $ATP$ is produced from the breakdown of glucose,not solar energy. Ribosomes are sites of protein synthesis,and peroxisomes are involved in metabolic reactions like photorespiration and lipid metabolism. Therefore,the correct answer is chloroplast.

(C) Photosynthesis occurs most efficiently in the blue and red regions of the visible light spectrum.
Among the given options,red light is highly effective for photosynthesis as it is well-absorbed by chlorophyll $a$ and $b$.
While blue light is also very effective,red light is typically cited in the context of action spectra for photosynthesis in many textbooks.

(C) The Hill reaction refers to the light-driven splitting of water (photolysis) by isolated chloroplasts in the presence of an artificial electron acceptor.
Robert Hill demonstrated that when isolated chloroplasts are illuminated in the presence of an artificial electron acceptor like ferricyanide (potassium ferricyanide),they evolve $O_2$ even in the absence of $CO_2$.
Therefore,the presence of an artificial electron acceptor like ferricyanide is essential for the Hill reaction to occur in isolated chloroplasts.

(A) In the light-dependent reactions of photosynthesis,the two photosystems ($PS-II$ and $PS-I$) work in series to facilitate the non-cyclic electron transport chain ($Z$-scheme).
$PS-II$ (P680) absorbs light energy first,leading to the photolysis of water and the release of electrons.
These electrons are then transferred through an electron transport system to $PS-I$ (P700).
Therefore,the flow of energy and electrons occurs from $PS-II$ to $PS-I$.

(D) $NADPH_2$ (or $NADPH + H^+$) is produced during the light-dependent reactions of photosynthesis.
It acts as a primary electron donor in the Calvin cycle (dark reactions).
Because it provides the electrons and hydrogen ions necessary to reduce $CO_2$ into glucose,it is biologically referred to as a 'reducing power'.

(D) The Emerson enhancement effect is the increase in the rate of photosynthesis when chloroplasts are exposed to light of two different wavelengths simultaneously,compared to the sum of the rates when exposed to each wavelength separately.
Robert Emerson observed that the rate of photosynthesis drops significantly when plants are exposed to light with a wavelength greater than $680 \ nm$ (far-red light).
However,when light of a shorter wavelength (less than $680 \ nm$) is provided along with the far-red light,the rate of photosynthesis increases dramatically.
Therefore,the enhancement effect requires both wavelengths greater than $680 \ nm$ and wavelengths less than $680 \ nm$ to activate both Photosystem-$I$ and Photosystem-$II$ simultaneously.

(B) During the process of photosynthesis,the photolysis of water $(H_2O)$ occurs in the thylakoid lumen.
This process splits water molecules into protons $(H^+)$,electrons $(e^-)$,and oxygen $(O_2)$.
The oxygen released during photosynthesis is a byproduct of this water-splitting reaction,not from carbon dioxide $(CO_2)$.
This was experimentally proven by $C$.$B$. van Niel using purple and green sulfur bacteria.

(C) In Photosystem-$I$ $(PS-I)$,the reaction center is a special form of chlorophyll-$a$ that absorbs light at a wavelength of $700 \ nm$. Hence,it is known as $P-700$. In contrast,the reaction center of Photosystem-$II$ $(PS-II)$ absorbs light at $680 \ nm$ and is known as $P-680$.

(D) Manganese $(Mn^{2+})$ plays a crucial role in the process of photosynthesis,specifically in the photolysis of water. During the light-dependent reactions,the oxygen-evolving complex $(OEC)$ associated with Photosystem $II$ requires manganese ions to split water molecules $(2H_2O \rightarrow 4H^+ + 4e^- + O_2)$. This process releases electrons that are essential for the electron transport chain,thereby facilitating the synthesis of $ATP$ and $NADPH$.

(C) Photosynthesis is a redox reaction where water $(H_2O)$ is oxidized to oxygen $(O_2)$ and carbon dioxide $(CO_2)$ is reduced to glucose $(C_6H_{12}O_6)$.
During the light-dependent reactions,the photolysis of water occurs,where $H_2O$ molecules are split to release electrons,protons $(H^+)$,and oxygen $(O_2)$.
Since $H_2O$ loses electrons during this process,it undergoes oxidation.

(B) In the light-dependent reactions of photosynthesis, the $Z$-scheme involves both photosystems.
$PS-II$ absorbs light and releases electrons, which pass through an electron transport chain to $PS-I$.
$PS-I$ ($P700$) absorbs light and gets excited, releasing high-energy electrons.
These electrons are passed to ferredoxin and finally to the enzyme $NADP$ reductase, which reduces $NADP^+$ to $NADPH + H^+$.
Therefore, $PS-I$ is the pigment system that eventually donates electrons for the reduction of $NADP^+$.

(B) Photosystem-$I$ $(PS-I)$ is primarily associated with cyclic photophosphorylation.
In cyclic photophosphorylation,only $PS-I$ is functional.
The electrons are excited from the reaction center $P700$ and travel through an electron transport chain,returning back to $P700$.
This process results in the synthesis of $ATP$ but does not involve the photolysis of water or the production of $NADPH$.

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

(D) The $OEC$ (Oxygen Evolving Complex) is a protein complex associated with $Photosystem \ II$ $(PS \ II)$ in the thylakoid membrane of chloroplasts.
It is responsible for the photolysis of water,which releases oxygen as a byproduct.
The $OEC$ requires several inorganic cofactors to function,specifically $Manganese$ $(Mn^{2+})$,$Calcium$ $(Ca^{2+})$,and $Chloride$ $(Cl^-)$ ions.
Therefore,all the mentioned elements are essential components of the $OEC$.

(A) During the light-dependent reactions of photosynthesis,water molecules undergo photolysis (ionization) in the presence of light and the oxygen-evolving complex.
When water $(H_2O)$ splits,it releases electrons,protons $(H^+)$,and oxygen $(O_2)$.
The electrons released from the photolysis of water are immediately accepted by the oxidized form of the reaction center of Photosystem $II$,which is $P_{680}^+$.
Therefore,the primary electron acceptor that replaces the electrons lost by chlorophyll $a$ in Photosystem $II$ is the reaction center itself,but in the context of the electron transport chain,the electrons are transferred to the primary electron acceptor of Photosystem $II$,which is pheophytin.
However,among the given options,the question refers to the immediate electron donor/acceptor relationship in the context of the water-splitting complex. Since the question asks for the primary electron acceptor during the ionization of water,it refers to the reaction center $P_{680}$ which gets reduced after accepting electrons from water. Given the standard options provided,$P_{680}$ (Chlorophyll $a$) is the correct biological entity.

(B) The photolysis of water (splitting of water molecules) occurs during the light-dependent reactions of photosynthesis.
This process takes place in the thylakoid lumen and is catalyzed by the Oxygen Evolving Complex $(OEC)$.
$Mn^{2+}$ (Manganese) ions,along with $Cl^-$ and $Ca^{2+}$ ions,are essential cofactors for the functioning of the $OEC$,which splits water into $O_2$,protons $(H^+)$,and electrons $(e^-)$.
Therefore,$Mn$ is the essential cofactor for the photolysis of water.

(C) Chlorophyll is the primary photosynthetic pigment that absorbs light energy (photons).
This absorbed light energy is used to excite electrons,which drives the process of photosynthesis.
Specifically,in the light-dependent reactions,chlorophyll absorbs light energy to facilitate the photolysis (photochemical splitting) of water molecules into oxygen,protons,and electrons.
Therefore,the correct function includes both the absorption of light and the subsequent photochemical splitting of water.

(A) In the chloroplast,the light-dependent reactions of photosynthesis occur in the thylakoid membranes,which are stacked to form the $Grana$.
These membranes contain photosystems ($PS-I$ and $PS-II$) and the electron transport chain.
$PS-II$ is responsible for the photolysis of water,which results in the production of oxygen $(O_2)$.
Additionally,the electron transport chain facilitates the synthesis of $ATP$ through a process known as photophosphorylation.
Therefore,the $Grana$ is the site for both oxygen production and photosynthetic phosphorylation.

(B) In the chloroplast,the thylakoid membrane system is organized into grana and stroma lamellae (stroma thylakoids).
$PS$ $II$ is primarily located in the appressed regions of the grana thylakoids.
$PS$ $I$ is located in the non-appressed regions of the grana thylakoids and the entire stroma thylakoids (stroma lamellae).
Therefore,$PS$ $I$ is found in the non-appressed part of stroma thylakoids.

(D) In the light-dependent reactions of photosynthesis,$P_{680}$ (the reaction center of Photosystem $II$) absorbs light energy and becomes excited,releasing an electron. This high-energy electron is immediately captured by the primary electron acceptor,which is $Pheophytin$. From $Pheophytin$,the electron is then passed to the electron transport chain,starting with Plastoquinone.

(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 transferred from $PS-I$ to ferredoxin $(Fd)$ via the primary electron acceptor.
Therefore,ferredoxin is functionally associated with $PS-I$.

(B) Photosynthesis occurs in the chloroplasts of plant cells.
Light-dependent reactions involve two photosystems,$PS I$ and $PS II$.
These photosystems are embedded within the thylakoid membranes of the chloroplast.
The thylakoid membranes are organized into stacks called grana.
Therefore,both $PS I$ and $PS II$ are located in the grana of the chloroplast.

(C) The photolysis of water (splitting of water molecules) occurs during the light-dependent reactions of photosynthesis.
This process is catalyzed by the Oxygen Evolving Complex $(OEC)$,which is associated with Photosystem $II$.
Essential elements required for this reaction include Manganese $(Mn^{2+})$,Calcium $(Ca^{2+})$,and Chloride $(Cl^-)$ ions.
Among the given options,Manganese $(Mn)$ is the primary element essential for the water-splitting reaction.

(B) $DCMU$ ($3$-($3$,$4$-dichlorophenyl)$-1,1-$dimethylurea) is a well-known herbicide that specifically inhibits photosynthesis.
It acts by binding to the $Q_B$ binding site of the $D1$ protein in the $PS-II$ complex.
By blocking this site,it prevents the transfer of electrons from $Q_A$ to $Q_B$,thereby halting the electron transport chain at the level of $PS-II$.
Consequently,the light-dependent reactions of photosynthesis are inhibited.

(B) In the process of photosynthesis,light energy is used to split water molecules $(H_2O)$ into oxygen,protons,and electrons. This process is known as photolysis of water. Since electrons are removed from water,it undergoes oxidation. The electrons released are used to reduce $NADP^+$ to $NADPH$,and $CO_2$ is reduced to carbohydrates. Therefore,the correct statement is that $H_2O$ undergoes oxidation.

(B) In the process of photosynthesis, specifically during the light-dependent reactions, an electron transport chain $(ETC)$ is involved.
Cytochromes, such as $Cytochrome \, b_6f$ complex, are integral membrane proteins that act as electron carriers.
They facilitate the transfer of electrons from plastoquinone to plastocyanin, which is a crucial step in the generation of a proton gradient for $ATP$ synthesis.
Therefore, $Cytochrome$ is the correct component associated with electron transfer.

(B) 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 electrons pass through the electron transport system,a proton gradient is generated,which drives the synthesis of $ATP$ from $ADP$ and inorganic phosphate.
Unlike non-cyclic photophosphorylation,cyclic photophosphorylation does not involve the photolysis of water,so no $O_2$ is released.
Additionally,$NADP^+$ is not reduced to $NADPH_2$ because the electrons are returned to the photosystem rather than being used to reduce $NADP^+$.
Therefore,only $ATP$ is produced during this process.

(B) In the light-dependent reactions of photosynthesis,the reaction center of $PS-II$ (Photosystem $II$) is known as $P_{680}$ because it absorbs light at a wavelength of $680 \ nm$.
Conversely,the reaction center of $PS-I$ (Photosystem $I$) is known as $P_{700}$ because it absorbs light at a wavelength of $700 \ nm$.
Therefore,$P_{680}$ is associated with $PS-II$.

(A) 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.
It was discovered by Robert Hill in $1937$.
Since this process requires light energy to excite electrons in chlorophyll molecules,it occurs only in the presence of light.

(C) During the light-dependent reactions of photosynthesis,the photolysis of water (splitting of water molecules) occurs in the thylakoid lumen. This process is catalyzed by the Oxygen Evolving Complex $(OEC)$,which requires specific mineral elements as cofactors. Manganese $(Mn^{2+})$,along with Chloride $(Cl^-)$ and Calcium $(Ca^{2+})$,is essential for the functioning of this complex. Without manganese,the splitting of water into protons,electrons,and oxygen cannot occur,thereby halting the photosynthetic process.

(C) The process of synthesis of $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ in the presence of light in chloroplasts is known as photophosphorylation.
This process occurs during the light-dependent reactions of photosynthesis.
Oxidative phosphorylation occurs in mitochondria during cellular respiration,while photophosphorylation is specific to the light energy-driven process in plants.

(D) The light reaction of photosynthesis begins with the absorption of light energy by the pigment molecules (chlorophyll) in the photosystems.
When a photon of light strikes the chlorophyll molecule,it transfers its energy to the electrons of the chlorophyll.
This causes the electrons to become excited and move to a higher energy state,which is the primary photochemical event that initiates the entire process of photosynthesis.

(D) The photochemical phase (or light reaction) of photosynthesis occurs in the thylakoid membranes of the chloroplast.
During this phase,light energy is captured by chlorophyll pigments.
The primary events include the photolysis of water (splitting of water molecules into $H^+$,$e^-$,and $O_2$) and the synthesis of energy-rich compounds,namely $ATP$ and $NADPH$.
The process of converting $ADP$ to $ATP$ using light energy is known as photophosphorylation.
Therefore,the photochemical reaction is directly associated with the photolysis of water and the phosphorylation of $ADP$ to $ATP$.

(A) The process of photolysis of water occurs in the thylakoid lumen during the light-dependent reactions of photosynthesis.
This process is catalyzed by the Oxygen Evolving Complex $(OEC)$,which is associated with Photosystem-$II$ ($PS$-$II$).
Within the reaction center of $PS$-$II$,the primary pigment is a specialized form of Chlorophyll-$a$ known as $P_{680}$.
When $P_{680}$ absorbs light energy,it becomes excited and loses an electron,creating a strong oxidizing agent that extracts electrons from water molecules,resulting in the splitting of water $(2H_2O \rightarrow 4H^+ + 4e^- + O_2)$ and the release of molecular oxygen $(O_2)$.

(A) The chemiosmotic hypothesis,proposed by Peter Mitchell,explains how $ATP$ is synthesized in chloroplasts and mitochondria.
This process relies on the development of a proton gradient ($H^+$ gradient) across the membrane (thylakoid membrane in chloroplasts and inner mitochondrial membrane in mitochondria).
The movement of protons from a region of higher concentration to a region of lower concentration through the $ATP$ synthase enzyme provides the energy required to phosphorylate $ADP$ into $ATP$.

(A) Primitive photosynthetic organisms,such as early bacteria,performed photosynthesis using only cyclic photophosphorylation. In this process,electrons are cycled back to the reaction center,allowing for the synthesis of $ATP$ without the production of $NADPH$ or the evolution of $O_2$. The $Z$-scheme (non-cyclic photophosphorylation) evolved later with the development of $PS-II$,which allows for water splitting and oxygen production. Therefore,primitive plants relied on cyclic photophosphorylation.

(D) In the process of non-cyclic photophosphorylation ($Z$-scheme),electrons are transferred from $PS-II$ to $PS-I$ through an electron transport chain.
Finally,the electrons from $PS-I$ are passed to $NADP^+$ reductase enzyme,which reduces $NADP^+$ to $NADPH + H^+$.
This step is a crucial part of the light-dependent reactions where light energy is converted into chemical energy.

(A) The process described in the question is biologically incorrect as stated. In green plants exposed to light,$ATP$ is produced from $ADP$ and inorganic phosphate $(Pi)$ during the light-dependent reactions of photosynthesis,a process known as $Photophosphorylation$.
Glucose is the product of the Calvin cycle (dark reaction),and its breakdown to produce $ATP$ occurs during cellular respiration,specifically through $Oxidative$ $phosphorylation$ in the mitochondria.
Since the question asks for the process of $ATP$ production in light-exposed green plants (implying photosynthesis),the most relevant term is $Photophosphorylation$. However,if the question implies the breakdown of glucose to produce $ATP$ in light,that is $Respiration$ (not listed). Given the context of photosynthesis,$Photophosphorylation$ is the intended answer.

(A) According to the chemiosmotic hypothesis proposed by $P. Mitchell$ $(1978)$,the synthesis of $ATP$ is driven by the proton gradient across the membrane.
When protons $(H^+)$ accumulate in the intermembrane space or thylakoid lumen,a proton gradient is created.
The movement of these protons back across the membrane through the $ATP$ synthase enzyme complex releases energy,which is used to synthesize $ATP$ from $ADP$ and inorganic phosphate $(Pi)$.

(C) Herbicides,particularly those like $DCMU$ ($3$-($3$,$4$-dichlorophenyl)$-1,1-$dimethylurea),are known to act as inhibitors of photosynthesis.
Specifically,they interfere with the electron transport chain in the light-dependent reactions.
They block the electron flow from Photosystem $II$ to plastoquinone,which effectively inhibits the photolysis of water (the splitting of water molecules) and the subsequent production of $ATP$ and $NADPH$.

(B) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate $(Pi)$ using light energy.
This process occurs in the chloroplasts of plants.
Specifically,the light-dependent reactions,including the electron transport chain and the synthesis of $ATP$ by $ATP$ synthase,take place across the thylakoid membrane.
The thylakoid membrane provides the necessary environment for the establishment of a proton gradient,which drives $ATP$ synthesis.
Therefore,the thylakoid is the site required for photophosphorylation.

(C) 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.
The light-dependent reactions,including the electron transport system and the formation of a proton gradient,take place within the thylakoid membrane and lumen.
Therefore,the correct site for photophosphorylation is the thylakoid.

(A) Photo-phosphorylation 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.
The thylakoid membrane contains the photosystems ($PS-I$ and $PS-II$),electron transport chain components,and $ATP$ synthase enzyme,which are essential for the light-dependent reactions of photosynthesis.

(C) The given figure represents a chloroplast. The label '$A$' points to the thylakoids,which are stacked to form grana. Thylakoids contain chlorophyll pigments and are the primary sites for the light-dependent reactions of photosynthesis,where light energy is converted into chemical energy ($ATP$ and $NADPH$).

(A) $NADP$ stands for Nicotinamide Adenine Dinucleotide Phosphate.
It is a coenzyme that acts as an electron carrier in various metabolic reactions,particularly in photosynthesis (light-dependent reactions) and biosynthetic pathways.

(C) The correct answer is $C$. Oxygen is evolved during photosynthesis through the process of photolysis of water,which occurs in the membranes of grana thylakoids.
The phenomenon of breaking down water into hydrogen and oxygen in illuminated chloroplasts is called photolysis or photocatalytic splitting of water.
Light energy,an Oxygen Evolving Complex $(OEC)$,and electron carriers are required for this process. The Oxygen Evolving Complex is attached to the inner surface of the thylakoid membrane and contains four $Mn$ ions.
Light-energized changes in $Mn$ $(Mn^{2+}, Mn^{3+}, Mn^{4+})$ remove electrons from the $OH^-$ component of water,resulting in the formation of oxygen.
The liberation of $O_2$ also requires two other essential ions: $Ca^{2+}$ and $Cl^-$.
The reaction is as follows:
$4H_2O \rightleftharpoons 4H^+ + 4OH^-$
$4OH^- \xrightarrow[Mn^{2+}, Ca^{2+}, Cl^-]{\text{Oxygen evolving complex}} 2H_2O + O_2 \uparrow + 4e^-$

(D) During the light-dependent reactions of photosynthesis,protons $(H^+)$ are pumped from the stroma into the lumen of the thylakoids. This process is driven by the electron transport chain and the photolysis of water. As a result,a high concentration of protons accumulates within the thylakoid lumen,creating a proton gradient across the thylakoid membrane. This gradient is essential for the synthesis of $ATP$ via the enzyme $ATP$ synthase. Therefore,the highest concentration of protons is found in the lumen of the thylakoids.

(D) : Emerson et al. $(1957)$ found that if light of shorter wavelengths was provided at the same time as the longer red wavelengths,photosynthesis was even faster than the sum of the two rates with either colour alone. This synergism or enhancement became known as the Emerson enhancement effect. The two separate groups of pigments or photosystems cooperate in photosynthesis and that such long red wavelengths are absorbed only by one photosystem,called photosystem $I$ $(PS\ I)$. The second photosystem,photosystem $II$ $(PS\ II)$,absorbs wavelengths shorter than $690 \ nm$,and for maximum photosynthesis,wavelengths absorbed by both systems must function together. The two photosystems normally cooperate to cause photosynthesis at all wavelengths shorter than $690 \ nm$,because both photosystems absorb those wavelengths. The importance of Emerson's work is that it suggested the presence of two distinct photosystems.