C3 and Dark reaction Questions in English

(A) RuBP carboxylase,also known as $RuBisCO$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase),is the most abundant protein in the chloroplast and on Earth.
It plays a critical role in the Calvin cycle by catalyzing the carboxylation of $RuBP$ during the light-independent reactions of photosynthesis.
While other enzymes like hexokinase,phosphatase,and nuclease are present in cells,$RuBisCO$ constitutes a significant portion of the total soluble protein content within the chloroplast stroma.

(A) Melvin Calvin used radioactive $ ^{14}C $ in algal photosynthesis studies.
He utilized the technique of radioactive labeling followed by paper chromatography and autoradiography to trace the path of carbon in the Calvin cycle.
Therefore,chromatography was the essential technique used to separate and identify the radioactive compounds formed during the carbon fixation process.

(A) Melvin Calvin $(1954)$ traced the pathway of carbon in photosynthesis and described the $C_3$ cycle,which is now known as the Calvin cycle. He was awarded the Nobel Prize for this significant work in $1960$.

(B) During the process of photosynthesis,$CO_2$ is reduced to form glucose $(C_6H_{12}O_6)$.
The carbon atoms present in the glucose molecule are derived from the carbon dioxide $(CO_2)$ molecules fixed during the Calvin cycle.
The oxygen released as a byproduct during photosynthesis comes from the photolysis of water $(H_2O)$,not from $CO_2$.
Therefore,carbon is obtained from $CO_2$ for the synthesis of glucose.

(B) Melvin Calvin successfully traced the path of carbon in the dark reaction of photosynthesis.
He used the radioactive isotope of carbon,$C^{14}$,in the form of $C^{14}O_2$.
By using this radioactive tracer,he was able to identify the intermediate compounds formed during the reduction of $CO_2$ in the Calvin cycle,which is a key part of the dark reaction.

(C) The synthesis of hexose sugars (glucose) occurs during the biosynthetic phase (Calvin cycle) of photosynthesis,which takes place in the stroma of the chloroplast.
Thylakoids are primarily responsible for the light-dependent reactions (photophosphorylation),where $ATP$ and $NADPH$ are produced.
The enzymes required for the Calvin cycle (such as $RuBisCO$) are located in the stroma,not within the thylakoids.
When thylakoids are removed and placed in a medium with $CO_2$ and $H_2O$,the light-dependent reactions may occur,but the enzymes necessary for carbon fixation and sugar synthesis are absent in the medium. Therefore,hexose sugars are not formed.

(B) The path of carbon in photosynthesis was elucidated by Melvin Calvin and his colleagues. They used the radioactive isotope $^{14}C$ in the green alga $Chlorella$ to trace the carbon fixation pathway,which is now known as the Calvin cycle.

(A) During the process of photosynthesis,the primary product formed in the light-independent reactions (Calvin cycle) is a triose phosphate sugar,which is a simple sugar (specifically $G3P$ or $PGAL$).
While plants eventually store energy as starch,transport it as sucrose,or use it to build cellulose,the immediate organic product synthesized from $CO_2$ and water is a simple sugar.

(B) The enzyme Ribulose$-1,5-$bisphosphate carboxylase-oxygenase,commonly known as $RuBisCO$,is primarily located in the stroma of the chloroplasts.
It plays a crucial role in the Calvin cycle during the light-independent reactions of photosynthesis.
$RuBisCO$ constitutes approximately $16\%$ of the total chloroplast protein.
It is widely recognized as the most abundant protein on this planet.

(B) The correct answer is $B$.
$CO_2$ is the most essential raw material for food formation in plants during photosynthesis.
During the Calvin cycle,$CO_2$ is fixed and reduced to form carbohydrates (glucose),which serve as the primary food material for the plant.

(B) The suspension of isolated thylakoids contains the machinery for the light-dependent reactions of photosynthesis,which produce $ATP$ and $NADPH$.
However,the synthesis of hexose (glucose) occurs during the light-independent reactions (Calvin cycle),which take place in the stroma of the chloroplast.
The stroma contains the necessary enzymes,such as $RuBisCO$,which are required to fix $CO_2$ into carbohydrates.
Since isolated thylakoids lack these stroma-localized enzymes,they cannot produce hexose even in the presence of $CO_2$ and $H_2O$.

(B) In the Calvin cycle ($C_3$ cycle),the enzyme phosphoketopentose epimerase (also known as ribulose-phosphate $3$-epimerase) catalyzes the reversible interconversion of xylulose-$5$-phosphate and ribulose-$5$-phosphate.
This step is crucial for the regeneration phase of the Calvin cycle,where ribulose-$5$-phosphate is subsequently phosphorylated by phosphoribulokinase to regenerate ribulose-$1,5$-bisphosphate $(RuBP)$,the primary $CO_2$ acceptor.

(D) The photosynthetic production of one mole of hexose $(C_6H_{12}O_6)$ via the Calvin cycle requires $6$ turns of the cycle.
Each turn of the Calvin cycle consumes $3$ $ATP$ and $2$ $NADPH$ molecules.
For $6$ turns,the total requirement is $18$ $ATP$ and $12$ $NADPH$.
Since $1$ $NADPH$ is equivalent to $3$ $ATP$ in terms of energy,$12$ $NADPH$ equals $36$ $ATP$.
Therefore,the total energy consumed is $18$ $ATP + 36$ $ATP = 54$ $ATP$ equivalents.

(C) The dark reaction of photosynthesis is known as $Blackman's$ reaction.
This is because the biochemical steps of the light-independent phase were first elucidated and established by the scientist $F.F. Blackman$ in $1905$.
Although it is called the 'dark reaction',it does not necessarily require darkness; rather,it is independent of light and relies on the products of the light reaction ($ATP$ and $NADPH$).

(A) The conversion of $1,3-$bisphosphoglyceric acid $(1,3-BPGA)$ to $3-$phosphoglyceraldehyde $(3-PGAL)$ is a reduction step in the Calvin cycle.
This reaction is catalyzed by the enzyme $Triose\ phosphate\ dehydrogenase$.
The reaction is represented as: $1,3-BPGA + 12 NADPH + 12 H^+ \xrightarrow{Triose\ phosphate\ dehydrogenase} 3-PGAL + 12 NADP^+ + 12 H_3PO_4$.

(B) The first step of the Calvin cycle involves the carboxylation of Ribulose-$1,5$-bisphosphate $(RuBP)$.
$RuBP$ acts as the $CO_2$ acceptor.
It reacts with $CO_2$ in the presence of the enzyme $RuBisCO$ to form $3$-phosphoglyceric acid ($3$-$PGA$).
$3$-$PGA$ is the first stable intermediate product of the $C_3$ cycle and it contains $3$ carbon atoms.

(B) Phosphatase enzymes catalyze the hydrolysis of phosphate esters. In the $C_3$ cycle (Calvin cycle), specifically during the regeneration phase, phosphatase enzymes are involved in the removal of a phosphate group from substrates like Fructose-$1,6$-bisphosphate to form Fructose-$6$-phosphate.
The reaction is: $\text{Fructose-}1,6\text{-bisphosphate} + H_2O \xrightarrow{\text{Phosphatase}} \text{Fructose-}6\text{-phosphate} + H_3PO_4$.

(A) The first step of the dark reaction (Calvin cycle) is carboxylation.
In this process,$CO_2$ is fixed by the enzyme $RuBisCO$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase) to a $5$-carbon sugar called Ribulose-$1,5$-bisphosphate $(RuBP)$.
This reaction results in the formation of two molecules of $3$-phosphoglyceric acid ($3$-$PGA$).
The reaction is represented as:
$6$ Ribulose-$1,5$-bisphosphate + $6$ $CO_2$ + $6$ $H_2O$ $\xrightarrow{RuBisCO}$ $12$ molecules of $3$-phosphoglyceric acid.

(D) The dark reaction of photosynthesis, also known as the Calvin cycle, involves the reduction of $CO_2$ to form carbohydrates.
Specifically, $1,3$-bisphosphoglyceric acid is reduced to form $3$-phosphoglyceraldehyde (also known as Glyceraldehyde $3$-phosphate or $G3P$) using $NADPH$ and $ATP$.
The reaction is catalyzed by the enzyme triose phosphate dehydrogenase:
$1,3\text{-bisphosphoglyceric acid} + 12 NADPH + 12 H^+ \xrightarrow{\text{Triose phosphate dehydrogenase}} 12 \text{ Phosphoglyceraldehyde} + 12 NADP^+ + 12 H_3PO_4$.
Thus, Phosphoglyceraldehyde is a primary product of the dark reaction.

(B) The photosynthetic pathway is divided into two main phases: the light-dependent reaction and the light-independent reaction,commonly known as the dark reaction or the Calvin cycle.
During the light reaction,solar energy is captured to produce $ATP$ and $NADPH$.
In the dark reaction,these products ($ATP$ and $NADPH$) are utilized to fix atmospheric $CO_2$ into carbohydrates.
Therefore,$CO_2$ fixation occurs specifically during the dark reaction phase of photosynthesis.

(B) The dark reaction of photosynthesis is termed so because it does not directly require light energy to proceed.
It is independent of light,unlike the light-dependent reactions.
However,it relies on the products generated during the light reaction,specifically $ATP$ and $NADPH$,to fix $CO_2$ into carbohydrates.

(A) The initial enzyme of the Calvin cycle is Ribulose $1, 5$-bisphosphate carboxylase-oxygenase,commonly known as $RuBisCO$.
This enzyme catalyzes the first step of the Calvin cycle,which is the carboxylation of Ribulose $1, 5$-bisphosphate $(RuBP)$ to form $3$-phosphoglyceric acid ($3$-$PGA$).
Therefore,the correct option is $A$.

(B) During the Calvin cycle of photosynthesis,$PGA$ (Phosphoglyceric acid) undergoes a reduction process to form phosphoglyceraldehyde (also known as $G3P$ or $PGAL$).
This reaction is catalyzed by the enzyme triose phosphate dehydrogenase and utilizes $ATP$ for phosphorylation and $NADPH$ as a reducing agent,which provides the necessary electrons and protons for the reduction of the carboxyl group to an aldehyde group.

(B) The enzyme $RuBisCO$ (Ribulose-$1, 5$-bisphosphate carboxylase-oxygenase) is the key enzyme in the Calvin cycle.
It catalyzes the primary carboxylation step in $C_3$ plants.
In this reaction,$CO_2$ is fixed by combining with a $5$-carbon sugar,ribulose $1, 5$-bisphosphate $(RuBP)$.
This reaction results in the formation of two molecules of $3$-phosphoglyceric acid ($3$-$PGA$).

(A) The Calvin cycle occurs in the stroma of chloroplasts.
In this process,the products of the light reaction,which are the assimilatory powers $NADPH + ATP$,are utilized to convert $CO_2$ into carbohydrates.
The stroma contains all the essential enzymes required for the fixation of $CO_2$ and the synthesis of sugars.

(B) The dark reaction of photosynthesis is also known as the biosynthetic phase or carbon assimilation. During this phase,the energy produced in the light reaction ($ATP$ and $NADPH$) is used to fix $CO_2$ into organic compounds like glucose. Therefore,$CO_2$ is reduced to form carbohydrates.

(B) Photosynthesis occurs in two main stages: the light-dependent reaction and the light-independent reaction (also known as the dark reaction or Calvin cycle).
$1$. The light-dependent reactions occur in the thylakoid membranes (grana) of the chloroplasts,where light energy is converted into chemical energy ($ATP$ and $NADPH$).
$2$. The dark reaction (Calvin cycle) takes place in the stroma of the chloroplasts.
$3$. In the stroma,the $ATP$ and $NADPH$ produced in the light reaction are used to fix $CO_2$ into carbohydrates (sugars).
Therefore,the correct location for the dark reaction is the stroma.

(A) In $C_3$ plants,the primary carboxylation step of the Calvin cycle involves the enzyme RuBisCO,which catalyzes the reaction of $CO_2$ with Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction results in the formation of two molecules of $3$-phosphoglyceric acid ($3$-$PGA$).
$3$-$PGA$ is a $3$-carbon compound and is the first stable product formed during the dark reaction of photosynthesis in $C_3$ plants.

(B) The photosynthetic process is divided into two main phases: the light-dependent reaction and the light-independent reaction (also known as the dark reaction or biosynthetic phase).
$1$. The light-dependent reaction (photochemical phase) requires light and involves the splitting of water and the production of $ATP$ and $NADPH$.
$2$. The light-independent reaction,also known as $Blackman's$ reaction or the Calvin cycle,does not require light directly. However,it is highly sensitive to temperature because it relies on enzymatic reactions (such as $RuBisCO$ activity) to fix $CO_2$ into carbohydrates.
Therefore,$Blackman's$ reaction is the correct answer.

(B) The Calvin cycle is also known as the light-independent reaction or the dark reaction of photosynthesis.
It does not directly require light energy to drive its chemical reactions.
Instead,it utilizes the products of the light-dependent reactions,specifically $ATP$ and $NADPH$,to fix atmospheric $CO_2$ into glucose.
Therefore,it is classified as not dependent on light.

(B) The Calvin cycle fixes one molecule of $CO_2$ per turn.
Since a hexose molecule (glucose,$C_6H_{12}O_6$) contains $6$ carbon atoms,the cycle must complete $6$ turns to incorporate $6$ molecules of $CO_2$ to produce one molecule of glucose.

(C) In $C_3$ plants,the primary $CO_2$ acceptor is a $5$-carbon ketose sugar called Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction is catalyzed by the enzyme RuBisCO,which leads to the formation of two molecules of $3$-phosphoglyceric acid ($3$-$PGA$),which is the first stable product of the Calvin cycle.

(C) The Calvin cycle involves the process of reductive carboxylation.
In the carboxylation phase,$CO_2$ is fixed to a $5$-carbon sugar,Ribulose $1,5$-bisphosphate $(RuBP)$,to form $3$-phosphoglycerate ($3$-$PGA$).
This reaction is catalyzed by the enzyme $RuBisCO$.
Following this,the reduction phase uses $ATP$ and $NADPH$ (produced in the light reaction) to convert $3$-$PGA$ into triose phosphate,which is a reductive process.
Therefore,the overall pathway is characterized as reductive carboxylation.

(B) Carbon dioxide fixation occurs during the $Calvin$ cycle,which is also known as the $Dark$ reaction or light-independent reaction of photosynthesis.
In this process,$CO_2$ is combined with $Ribulose$ $1,5-bisphosphate$ $(RuBP)$ to produce glucose,utilizing the $ATP$ and $NADPH$ generated during the light-dependent reactions.

(A) In the Calvin cycle (dark reaction),the primary carboxylation step involves the fixation of $CO_2$ into a $5$-carbon compound called Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction is catalyzed by the enzyme $RuBisCO$.
The resulting product is an unstable $6$-carbon intermediate,which immediately breaks down into two molecules of $3$-phosphoglyceric acid ($3$-$PGA$).
Since $3$-$PGA$ is the first stable compound formed in this process,it is considered the first stable product of the dark reaction.

(C) The dark reaction (Calvin cycle) begins with the carboxylation phase.
In this step,$CO_2$ is fixed by attaching it to a $5$-carbon sugar called Ribulose $1,5$-bisphosphate $(RuBP)$.
This reaction is catalyzed by the enzyme $RuBisCO$,resulting in the formation of $3$-phosphoglycerate $(3-PGA)$.

(A) In $C_3$ plants,the entire process of the Calvin cycle,including the synthesis of starch or sucrose,occurs within the chloroplasts of the mesophyll cells.
In $C_4$ plants,the mesophyll cells lack the enzyme RuBisCO and are primarily involved in the initial fixation of $CO_2$ into a $4$-carbon compound (oxaloacetate),which is then transported to the bundle sheath cells.
The synthesis of starch or sucrose in $C_4$ plants occurs in the chloroplasts of the bundle sheath cells,not the mesophyll cells.
Therefore,only the mesophyll chloroplasts of $C_3$ plants are capable of synthesizing starch or sucrose.

(B) The Calvin cycle consists of three distinct phases: carboxylation,reduction,and regeneration.
Carboxylation is the first step of the Calvin cycle,where $CO_2$ is fixed by the enzyme RuBisCO to form $3$-phosphoglycerate ($3$-$PGA$).
This process occurs only once per cycle for each molecule of $CO_2$ fixed.
Therefore,the correct answer is $1$.

(B) The conversion of $3$-phosphoglyceraldehyde $(3PGAL)$ and dihydroxyacetone phosphate $(DHAP)$ to fructose $1,6$-diphosphate is a key step in the Calvin cycle (dark reaction) of photosynthesis.
This reaction is catalyzed by the enzyme $Aldolase$.
The reaction can be represented as:
$3PGAL + DHAP \xrightarrow{Aldolase} Fructose-1,6-diphosphate$.
Therefore,the correct option is $B$.

(B) In the dark phase of photosynthesis (Calvin cycle),the process of carbon fixation begins with the carboxylation of Ribulose$-1,5-$bisphosphate $(RuBP)$.
$RuBP$ is a $5$-carbon sugar that acts as the primary carbon dioxide acceptor.
This reaction is catalyzed by the enzyme $RuBisCO$,resulting in the formation of two molecules of $3$-phosphoglycerate $(3-PGA)$.

(D) In $C_3$ plants,the first step of the Calvin cycle is the carboxylation of $RuBP$ (Ribulose$-1,5-$bisphosphate).
During this process,$CO_2$ is fixed by the enzyme $RuBisCO$ to form a $6$-carbon unstable intermediate.
This unstable intermediate immediately breaks down into two molecules of $3$-phosphoglyceric acid $(PGA)$.
Since the radioactive $C^{14}$ is part of the $CO_2$ molecule,it will first appear in the stable $3$-carbon compound,which is $PGA$.

(A) The $CAM$ (Crassulacean Acid Metabolism) cycle is a specialized photosynthetic adaptation found in plants living in arid conditions.
In these plants,stomata open at night to take in $CO_2$,which is fixed into organic acids (primarily malic acid) by the enzyme $PEP$ carboxylase,leading to 'night acidification'.
During the day,the stomata remain closed to prevent water loss,and the stored organic acids are decarboxylated to release $CO_2$ for the Calvin cycle,a process known as 'light deacidification'.

(C) $RuBP$ carboxylase (also known as $Rubisco$) is the most abundant protein in the chloroplast and on Earth. It constitutes approximately $16\%$ of the total chloroplast protein. It plays a crucial role in the Calvin cycle by catalyzing the carboxylation of $RuBP$.

(A) Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
$1$. The first system is the light reaction,which traps light energy to produce $ATP$ and $NADPH$.
$2$. The second system is the light-independent reaction (Calvin cycle),which utilizes the $ATP$ and $NADPH$ produced in the first stage to fix $CO_2$ into organic compounds (sugars).
Therefore,the second system is responsible for the fixation of $CO_2$.

(C) In the carboxylation phase of the Calvin cycle,Ribulose-$1,5$-bisphosphate $(RuBP)$,which is a $5$-carbon compound,acts as the primary $CO_2$ acceptor.
Each molecule of $RuBP$ reacts with one molecule of $CO_2$ in the presence of the enzyme $RuBisCO$ to form an unstable $6$-carbon intermediate.
This intermediate immediately splits into two molecules of $3$-phosphoglyceric acid $(PGA)$,which is a $3$-carbon compound.
Therefore,the carbon atoms in $PGA$ are derived from both the $5$-carbon $RuBP$ and the $1$-carbon $CO_2$ molecule.

(D) The Calvin cycle,also known as the $C_3$ cycle or the reductive pentose phosphate pathway,is the primary pathway for carbon fixation in plants.
It occurs in the stroma of the chloroplasts.
This cycle is present in all photosynthetic plants,regardless of whether they follow the $C_3$,$C_4$,or $CAM$ pathway,because it is the fundamental mechanism used to synthesize sugars from $CO_2$ and $H_2O$ using the energy generated during the light-dependent reactions.