C4 Questions in English

(D) Maize is a $C_4$ plant.
In $C_4$ plants,the synthesis of one molecule of hexose (glucose) requires $30 \text{ ATP}$ and $12 \text{ NADPH}$.
To calculate the requirement for $6$ hexose molecules,we multiply these values by $6$:
$\text{ATP requirement} = 6 \times 30 = 180 \text{ ATP}$.
$\text{NADPH requirement} = 6 \times 12 = 72 \text{ NADPH}$.
Therefore,the total requirement is $180 \text{ ATP}$ and $72 \text{ NADPH}$.

(C) $CAM-$ pathway (Crassulacean Acid Metabolism) is a mechanism of photosynthesis involving double fixation of carbon dioxide.
This process occurs in succulent plants belonging to families like Crassulaceae,Cactaceae,and Euphorbiaceae,which are typically found in dry habitats.
In these plants,the stomata remain closed during the daytime to prevent water loss and open only at night to take in $CO_{2}$.

(A) $PEP$ carboxylase ($PEP$ case) has a significant advantage over $RuBisCo$ because $PEP$ carboxylase does not bind with oxygen $(O_{2})$.
$RuBisCo$ has an affinity for both $CO_{2}$ and $O_{2}$. When it binds with $O_{2}$,it initiates photorespiration.
Photorespiration is a wasteful process that consumes energy and releases $CO_{2}$ without producing sugars,thereby decreasing the overall photosynthetic yield.
Since $PEP$ carboxylase is insensitive to $O_{2}$,it ensures efficient carbon fixation even in the presence of oxygen.

(A) In $C_{4}$ plants,the primary $CO_{2}$ acceptor is a $3$-carbon compound,phosphoenolpyruvate $(PEP)$,which is present in the mesophyll cells.
Since the primary acceptor is a $3$-carbon molecule,option $A$ is false.
The initial carboxylation reaction occurs in the mesophyll cells using the enzyme $PEP$ carboxylase.
$C_{4}$ plants exhibit Kranz anatomy,meaning their leaves contain two types of photosynthetic cells: mesophyll cells and bundle sheath cells.
Mesophyll cells in $C_{4}$ plants lack the enzyme Rubisco,which is instead found in the bundle sheath cells.

(A) $C_{4}$ plants exhibit specific anatomical and biochemical features.
$I$. Kranz anatomy: They possess a specialized leaf anatomy where mesophyll cells surround the bundle sheath cells.
$II$. Oxaloacetic acid: The primary $CO_{2}$ acceptor is phosphoenolpyruvate $(PEP)$,which forms the $4$-carbon compound oxaloacetic acid $(OAA)$ in mesophyll cells.
$III$. Large bundle sheath cells: These cells are large and contain numerous chloroplasts,which is a hallmark of Kranz anatomy.
$IV$. Found only in desert area: This is incorrect; while $C_{4}$ plants are adapted to high temperatures and light intensities,they are found in various tropical and subtropical regions,not just deserts.
Therefore,characteristics $I, II,$ and $III$ are correct.

(B) In the $C_{4}$-cycle,the primary $CO_{2}$ fixation product is $OAA$ (Oxaloacetic acid),which is a $4$-carbon acid formed in the mesophyll cells.
This $OAA$ is subsequently converted into other $4$-carbon acids such as malic acid or aspartic acid.
Picric acid is not involved in the $C_{4}$-cycle.
Therefore,the acids formed during the $C_{4}$-cycle are $OAA$ $(II)$,malic acid $(III)$,and aspartic acid $(IV)$.

(C) $C_{4}$-plants exhibit several specialized adaptations to optimize photosynthesis in tropical environments.
$1$. Special leaf anatomy: They possess $Kranz$ anatomy,where mesophyll cells surround bundle sheath cells.
$2$. High temperature tolerance: They are adapted to thrive in warmer climates compared to $C_{3}$-plants.
$3$. Lack of photorespiration: By concentrating $CO_{2}$ in the bundle sheath cells,they effectively eliminate the wasteful process of photorespiration.
$4$. Greater productivity: Due to the absence of photorespiration and efficient carbon fixation,they show higher biomass production.

(C) $C_{4}$-plants exhibit Kranz anatomy,which involves dimorphic chloroplasts: granal and agranal.
In $C_{4}$-plants,the mesophyll cells contain granal chloroplasts,which possess well-developed thylakoids stacked into grana.
Conversely,the bundle-sheath cells of $C_{4}$-plants contain agranal chloroplasts,meaning they lack grana,and thylakoids exist only as stroma lamellae.
This structural adaptation allows for the efficient separation of the light-dependent reactions and the Calvin cycle.

(A) In $C_4$ plants,the primary $CO_2$ fixation occurs in the mesophyll cells.
$CO_2$ combines with phosphoenolpyruvate $(PEP)$ to form a $4$-carbon compound called oxaloacetic acid $(OAA)$ with the help of the enzyme $PEP$ carboxylase.
This oxaloacetic acid is then converted into other $4$-carbon compounds like malic acid or aspartic acid within the same mesophyll cells.
These $4$-carbon acids are then transported to the bundle sheath cells for further processing in the Calvin cycle.
Therefore,both oxaloacetic acid and malic acid/aspartic acid are found in the mesophyll cells.

(B) $C_{4}$-plants are more efficient because they possess a mechanism to minimize photorespiration. In $C_{4}$-plants,the enzyme $PEP$ carboxylase fixes $CO_{2}$ in mesophyll cells,which has a high affinity for $CO_{2}$ and does not bind with $O_{2}$. This process pumps $CO_{2}$ into the bundle sheath cells,creating a high concentration of $CO_{2}$ around the enzyme $RuBisCO$. This high concentration effectively suppresses the oxygenase activity of $RuBisCO$,thereby preventing photorespiration and ensuring higher photosynthetic efficiency compared to $C_{3}$-plants.

(A) In the $C_{4}$ pathway (Hatch and Slack pathway),the process occurs in two types of cells: mesophyll cells and bundle sheath cells.
$1$. In the mesophyll cell $(A)$,atmospheric $CO_{2}$ is fixed into a $C_{4}$ acid by the enzyme $PEP$ carboxylase. This step is called Fixation $(B)$.
$2$. The $C_{4}$ acid is then transported to the bundle sheath cell $(C)$.
$3$. In the bundle sheath cell,the $C_{4}$ acid undergoes Decarboxylation $(D)$ to release $CO_{2}$ for the Calvin cycle.

(D) In $C_{3}$ plants,the synthesis of one molecule of glucose $(C_{6}H_{12}O_{6})$ requires $18$ $ATP$ and $12$ $NADPH$ molecules.
In $C_{4}$ plants,an additional $CO_{2}$ fixation cycle (the $C_{4}$ cycle) occurs to concentrate $CO_{2}$ in the bundle sheath cells.
This $C_{4}$ cycle requires $2$ additional $ATP$ molecules per $CO_{2}$ molecule fixed to regenerate phosphoenolpyruvate $(PEP)$.
Since $6$ molecules of $CO_{2}$ are required to produce one molecule of hexose sugar,the total additional $ATP$ required is $6 \times 2 = 12$ $ATP$ molecules.
Therefore,$C_{4}$ plants require $30$ $ATP$ $(18 + 12)$ and $12$ $NADPH$ for the synthesis of one glucose molecule.

(C) $C_4$ plants exhibit a specialized leaf anatomy known as Kranz anatomy.
In these plants,the bundle sheath cells contain large,agranal chloroplasts (lacking grana),which are adapted for the Calvin cycle.
Therefore,the statement that granal chloroplasts are present in bundle sheath cells is incorrect.

(A) In $C_4$ plants,the primary $CO_2$ acceptor is phosphoenolpyruvate $(PEP)$,which combines with $CO_2$ in the presence of $PEP$ carboxylase to form a $4$-carbon compound called Oxaloacetic acid $(OAA)$.
This $OAA$ is the first stable product formed during the $C_4$ photosynthetic pathway.

(A) In $C_4$ plants,the initial carbon fixation occurs in the mesophyll cells to form oxaloacetate,which is then converted to malate or aspartate.
These compounds are transported to the bundle sheath cells.
In the bundle sheath cells,decarboxylation occurs,releasing $CO_2$ for the Calvin cycle.
Therefore,the actual synthesis of sugar (glucose) via the Calvin cycle takes place exclusively in the bundle sheath cells.

(C) $C_4$ plants are adapted to high temperatures and high light intensities. Maize ($Zea$ $mays$),sugarcane,and sorghum are classic examples of $C_4$ plants. In these plants,the primary $CO_2$ acceptor is phosphoenolpyruvate $(PEP)$,and the first stable product is a four-carbon compound,oxaloacetic acid $(OAA)$.

(B) The $C_4$ pathway,also known as the Hatch and Slack pathway,involves two types of photosynthetic cells: mesophyll cells and bundle sheath cells.
In the mesophyll cells,the primary $CO_2$ fixation occurs. The enzyme $PEP$ carboxylase $(PEPCase)$ captures $CO_2$ and converts it into a $4$-carbon compound called oxaloacetate $(OAA)$.
Therefore,the initial fixation of $CO_2$ takes place in the mesophyll cells.

(A) Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants.
In this anatomy,the mesophyll cells are arranged in a ring-like manner around the bundle sheath cells.
This arrangement helps in concentrating $CO_2$ around the enzyme $RuBisCO$,thereby minimizing photorespiration and increasing photosynthetic efficiency.

(A) In the $C_4$ cycle,the primary $CO_2$ fixation occurs in the mesophyll cells.
This process is catalyzed by the enzyme Phosphoenolpyruvate $(PEP)$ carboxylase,also known as $PEP$ carboxylase.
This enzyme has a high affinity for $CO_2$ and does not show oxygenase activity,making it highly efficient for $CO_2$ fixation.

(C) In $C_4$ plants,the $C_4$ acid (malic acid or aspartic acid) is transported to the bundle sheath cells.
Inside the bundle sheath cells,this $C_4$ acid undergoes decarboxylation to release $CO_2$.
This process increases the intracellular concentration of $CO_2$ around the $RuBisCO$ enzyme.
High $CO_2$ concentration ensures that $RuBisCO$ functions primarily as a carboxylase rather than an oxygenase,thereby minimizing photorespiration.

(B) In the mesophyll cells of $C_4$ plants,the primary acceptor of atmospheric $CO_2$ is Phosphoenol pyruvic acid $(PEP)$,which is a $3$-carbon compound.
This reaction is catalyzed by the enzyme $PEP$ carboxylase $(PEPCase)$.

(D) In $C_4$ plants,the initial carbon fixation occurs in the mesophyll cells where the enzyme $PEP$case (Phosphoenolpyruvate carboxylase) is located in the cytoplasm.
After the formation of $C_4$ acids,they are transported to the bundle sheath cells.
In the bundle sheath cells,the $C_4$ acids are broken down to release $CO_2$,which is then fixed by the enzyme $RuBisCO$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase) located in the stroma of the chloroplasts.

(A) Sugarcane is a $C_4$ plant.
$C_4$ plants are highly efficient in $CO_2$ fixation because they lack photorespiration,a wasteful process that occurs in $C_3$ plants.
In $C_4$ plants,the enzyme $PEP$ carboxylase fixes $CO_2$ into a $4$-carbon compound,which prevents the oxygenation of $RuBP$ and thus avoids the loss of fixed carbon.

(A) $C_4$ plants show saturation at $CO_2$ concentrations of about $360$ $ppm$.
This is because $C_4$ plants have a more efficient mechanism for $CO_2$ fixation (via $PEP$ carboxylase) compared to $C_3$ plants.
$C_3$ plants show saturation at $450$ $ppm$ to $500$ $ppm$ of $CO_2$.

(D) The $C_4$ cycle involves the following steps:
$1$. $CO_2$ fixation: $CO_2$ is fixed in the mesophyll cells by the enzyme $PEP$ carboxylase to form a $4$-carbon compound (oxaloacetate).
$2$. Transport: This $4$-carbon compound is transported from the mesophyll cells to the bundle sheath cells.
$3$. Decarboxylation: In the bundle sheath cells,the $4$-carbon compound undergoes decarboxylation to release $CO_2$ for the Calvin cycle.
Therefore,the correct sequence is $CO_2$ fixation $\rightarrow$ Transport $\rightarrow$ Decarboxylation.

(A) Both sugarcane and Sorghum are $C_4$ plants.
$C_4$ plants do not show saturation at low light intensities.
They respond to higher light intensities and show saturation at about $50\%$ of full sunlight.
In contrast,$C_3$ plants show saturation at about $10\%$ of full sunlight.

(B) $CO_2$ concentrating mechanisms,also known as the $C_4$ pathway or Hatch-Slack pathway,are adaptations to minimize photorespiration.
In these plants,$CO_2$ is initially fixed in mesophyll cells into a $4$-carbon compound (oxaloacetate),which is then converted to malate or aspartate.
This malate is transported to bundle sheath cells,where it undergoes decarboxylation to release high concentrations of $CO_2$ around the enzyme $RuBisCO$.
Among the given options,Sugarcane is a $C_4$ plant,while Rice,Wheat,and Tomato are $C_3$ plants.

(B) Kranz anatomy is a characteristic feature of $C_4$ plants. In this anatomy,the bundle sheath cells are arranged in a ring around the vascular bundles,and these are further surrounded by a concentric ring of mesophyll cells. This arrangement allows for the efficient separation of the light-dependent reactions and the Calvin cycle.

(A) In $C_4$ plants,the process of photosynthesis is divided between two different cell types to minimize photorespiration.
$1$. Mesophyll cells: These cells contain chloroplasts that perform the light-dependent reactions (light reaction) to produce $ATP$ and $NADPH$.
$2$. Bundle sheath cells: These cells contain chloroplasts that perform the Calvin cycle (dark reaction) where $CO_2$ is fixed into sugars.
Therefore,mesophyll cells perform the light reaction and bundle sheath cells perform the dark reaction.

(D) Agranal chloroplasts are characterized by the absence of grana (thylakoid stacks).
These are typically found in the bundle sheath cells of $C_4$ plants.
Sugarcane is a classic example of a $C_4$ plant,where the bundle sheath cells contain chloroplasts that lack grana,allowing for the efficient operation of the Calvin cycle without the light-dependent reactions occurring in the same compartment.

(C) Double carboxylation is a characteristic feature of $C_4$ plants, where $CO_2$ is fixed twice: first in the mesophyll cells by $PEP$ carboxylase and then in the bundle sheath cells by $RuBisCO$.
$Zea \text{ } mays$, $Sugarcane$, and $Sorghum$ are examples of $C_4$ plants.
$Pisum \text{ } sativum$ (pea) is a $C_3$ plant, which performs single carboxylation.
Therefore, $Pisum \text{ } sativum$ is the odd one.

(D) In $C_4$ plants,the leaves exhibit a specialized anatomy known as $Kranz$ anatomy. In this arrangement,the bundle sheath cells are large,thick-walled,and contain a high density of chloroplasts. These chloroplasts are typically agranal (lacking grana) and are specialized for the $Calvin$ cycle,which occurs in the bundle sheath cells to prevent photorespiration.

(B) $C_4$ plants are adapted to high-temperature environments and have an optimum temperature range of $30-40^{\circ}C$.
At low temperatures,the enzyme $PEP$ synthetase (pyruvate phosphate dikinase),which is responsible for regenerating $PEP$ (phosphoenolpyruvate),becomes cold-sensitive and its activity decreases significantly.
This inhibition of the $PEP$ regeneration cycle limits the overall rate of $CO_2$ fixation in $C_4$ plants,making them less efficient compared to $C_3$ plants in cooler climates.

(A) $C_4$ plants are known for their ability to tolerate saline conditions and water stress.
This tolerance is primarily attributed to the accumulation of organic acids within their cells.
The accumulation of these organic acids helps in maintaining osmotic balance,which allows the plants to survive in high-salinity environments.

(B) $C_4$ plants are not significantly benefitted by the $CO_2$ fertilisation effect because they have a mechanism to concentrate $CO_2$ around the enzyme $RuBisCO$. These plants possess $Kranz$ anatomy,which allows them to maintain high levels of $CO_2$ in bundle sheath cells,effectively suppressing photorespiration. Since $C_4$ plants are already saturated with $CO_2$ at current atmospheric levels,an increase in $CO_2$ concentration does not enhance their photosynthetic rate as much as it does for $C_3$ plants.

(B) In $C_4$ plants,the $CO_2$ fixation process involves the regeneration of Phosphoenolpyruvate $(PEP)$ from Pyruvate. This regeneration process is energy-intensive and requires the conversion of $ATP$ to $AMP$,which effectively consumes two $ATP$ equivalents. This extra energy expenditure is necessary to maintain the $CO_2$ concentrating mechanism in the bundle sheath cells,which is absent in $C_3$ plants.

(C) In $C_4$ plants,the bundle sheath cells contain chloroplasts that are agranal (lacking grana) because they lack $PS$ $II$. Therefore,the Assertion is incorrect.
$PS$ $II$ is indeed primarily located in the appressed regions of the thylakoid membranes within the grana. Therefore,the Reason is correct.
Thus,the correct choice is $C$.

(C) In $CAM$ (Crassulacean Acid Metabolism) plants,stomata open at night to take in $CO_2$,which is fixed into organic acids like malic acid. This process is known as dark acidification.
During the day,these organic acids are decarboxylated to release $CO_2$ for the Calvin cycle.
Therefore,the Assertion is correct,but the Reason is incorrect because decarboxylation occurs during the day,not at night.

(C) : Tropical plants (mostly $C_4$ plants) are more efficient in $CO_2$ utilization because they have a mechanism to concentrate $CO_2$ around the $RuBisCO$ enzyme,which minimizes photorespiration.
$R$: $C_3$ plants do not have a specialized mechanism to minimize oxygenase activity of $RuBisCO$. In $C_3$ plants,$RuBisCO$ often functions as an oxygenase when $CO_2$ levels are low or temperature is high,leading to photorespiration. Therefore,the Reason is incorrect because $C_3$ plants are specifically known for their inability to fully prevent oxygenase activity compared to $C_4$ plants.

(B) Sorghum is a $C_{4}$ plant.
In $C_{4}$ plants,the primary $CO_{2}$ acceptor is Phosphoenolpyruvate $(PEP)$,which is a $3$-carbon compound.
This reaction is catalyzed by the enzyme $PEP$ carboxylase.
The first stable product formed during the $C_{4}$ cycle is Oxaloacetic acid $(OAA)$,which is a $4$-carbon compound.
Therefore,the correct option is $B$.

(B) $C_{4}$ plants are twice as efficient as $C_{3}$ plants in terms of fixing $CO_{2}$.
This means that for the same amount of $CO_{2}$ fixed,$C_{4}$ plants lose only half as much water as $C_{3}$ plants.
This efficiency is due to the $C_{4}$ pathway,which minimizes photorespiration and allows for better water-use efficiency.

(A-D) Hatch-Slack pathway: The $C_{4}$ pathway is an alternative carbon fixation process found in certain plants. It was discovered by $M. D. Hatch$ and $C. R. Slack$,hence it is named the Hatch-Slack pathway.
$(b)$ Calvin cycle: In $C_{4}$ plants,the Calvin cycle occurs specifically within the bundle sheath cells. This is where $CO_{2}$ is finally fixed into glucose via the enzyme $RuBisCO$.
$(c)$ $PEP$ carboxylase: This is the primary $CO_{2}$ fixing enzyme found in the mesophyll cells of $C_{4}$ plants. It catalyzes the reaction between $CO_{2}$ and phosphoenolpyruvate $(PEP)$ to form the $4-C$ compound oxaloacetic acid $(OAA)$.
$(d)$ Bundle sheath cells: These are large,specialized parenchymatous cells arranged in layers around the vascular bundles of $C_{4}$ plant leaves. They contain large numbers of chloroplasts (often agranal) and are the site where the Calvin cycle takes place.

(D) Statement $I$ is correct: In $C_{4}$ plants,the primary $CO_{2}$ acceptor is a $3$-carbon molecule called phosphoenolpyruvate $(PEP)$,which is present in the mesophyll cells.
Statement $II$ is correct: The mesophyll cells of $C_{4}$ plants lack the enzyme $RuBisCo$. The $RuBisCo$ enzyme is exclusively present in the bundle sheath cells of $C_{4}$ plants,where the Calvin cycle occurs.
Therefore,both statements are correct.

(A) In $C_{4}$ plants,the leaves exhibit a special anatomy called Kranz anatomy.
Large bundle sheath cells are present around the vascular bundles.
These cells contain a large number of chloroplasts,which are essential for the operation of the Calvin cycle.
By concentrating $CO_{2}$ in these cells,the plant minimizes photorespiration and ensures efficient carbon fixation,allowing the plant to thrive in high-temperature and high-light environments.

(D) $C_4$ plants are evolutionarily adapted to high-temperature and high-light environments.
$1$. Productivity: Due to the absence of photorespiration,$C_4$ plants show higher biomass production and productivity compared to $C_3$ plants.
$2$. Water Use Efficiency: $C_4$ plants are more efficient in water usage. They fix the same amount of $CO_2$ while losing only about half the water that $C_3$ plants lose through transpiration.
$3$. Rubisco Efficiency: In $C_4$ plants,the $CO_2$ concentration mechanism (Kranz anatomy) ensures that the concentration of $CO_2$ around the enzyme Rubisco is high,which minimizes the oxygenase activity of Rubisco and maximizes its carboxylase activity,making it more efficient than in $C_3$ plants.
Therefore,all the given statements are correct.

(C) In $C_4$ plants,the process of photosynthesis is divided between two different cell types: mesophyll cells and bundle sheath cells.
$1$. In mesophyll cells,the primary $CO_2$ fixation occurs via the enzyme $PEP$ carboxylase,forming a $4$-carbon compound $(OAA)$.
$2$. This $4$-carbon compound is transported to the bundle sheath cells.
$3$. In the bundle sheath cells,the $4$-carbon compound is decarboxylated to release $CO_2$,which then enters the Calvin cycle.
$4$. The enzyme $RuBisCO$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase) is exclusively present in the chloroplasts of the bundle sheath cells to facilitate the Calvin cycle,thereby avoiding photorespiration.

(D) In the $C_4$ pathway,the primary $CO_2$ acceptor is Phosphoenolpyruvate $(PEP)$,a $3$-carbon compound.
$CO_2$ fixation occurs in the mesophyll cells,where $PEP$ combines with $CO_2$ in the presence of the enzyme $PEP$ carboxylase $(PEPCase)$.
This reaction results in the formation of a $4$-carbon compound called Oxaloacetic acid $(OAA)$.
$OAA$ is the first stable product formed in the $C_4$ cycle.

(B) Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants (such as maize or sugarcane).
In this anatomy,the mesophyll cells are arranged in a ring-like manner around the bundle sheath cells.
This arrangement helps in concentrating $CO_2$ around the enzyme $RuBisCO$,thereby minimizing photorespiration and increasing the efficiency of photosynthesis.

(C) In $C_4$ plants,the process of photosynthesis is spatially separated into two different cell types to minimize photorespiration.
$1$. The light-dependent reaction occurs in the chloroplasts of the mesophyll cells,specifically within the grana,where $ATP$ and $NADPH$ are produced.
$2$. The Calvin cycle (dark reaction) occurs in the chloroplasts of the bundle sheath cells,specifically within the stroma,where $CO_2$ is fixed into sugars using the $ATP$ and $NADPH$ generated in the mesophyll cells.

(C) In the Hatch and Slack pathway ($C_4$ cycle):
$1$. In the mesophyll cells, $CO_2$ is fixed by $PEP$ to form a $C_4$ acid (e.g., oxaloacetate or malate/aspartate), which is represented by $P$.
$2$. This $C_4$ acid is transported to the bundle sheath cells, represented by $Q$.
$3$. In the bundle sheath cells, decarboxylation occurs, releasing $CO_2$ for the Calvin cycle and leaving behind a $C_3$ acid (e.g., pyruvate), represented by $R$.
$4$. This $C_3$ acid is transported back to the mesophyll cells, where it is converted back into $PEP$ (regeneration), represented by $S$.
Therefore, $P$ is a $C_4$ acid, $Q$ is a $C_4$ acid, $R$ is a $C_3$ acid, and $S$ is a $C_3$ acid.