C4 Questions in English

(C) Sugarcane is a $C_4$ plant. In $C_4$ plants,the primary fixation of $CO_2$ occurs in the mesophyll cells. The enzyme responsible for this initial carboxylation is $Phosphoenolpyruvate$ $(PEP)$ carboxylase. This enzyme catalyzes the reaction between $CO_2$ (or $HCO_3^-$) and $PEP$ to form oxaloacetic acid $(OAA)$,which is subsequently converted into malic acid or aspartic acid. Therefore,the correct enzyme is $PEP$ carboxylase.

(D) $C_4$ plants are highly efficient in photosynthesis because they possess a specialized mechanism to concentrate $CO_2$ around the enzyme Rubisco.
In $C_4$ plants,the primary fixation of $CO_2$ occurs in the mesophyll cells,where $PEP$ carboxylase $(PEPCase)$ acts as the primary carboxylating enzyme.
$PEPCase$ has a very high affinity for $CO_2$ and,unlike Rubisco,it does not show oxygenase activity.
This enzyme fixes $CO_2$ into a four-carbon compound (oxaloacetate),which is then transported to the bundle sheath cells.
In the bundle sheath cells,this compound is decarboxylated to release a high concentration of $CO_2$,which is then utilized by Rubisco for the Calvin cycle.
This mechanism minimizes photorespiration and ensures efficient carbon fixation even at low $CO_2$ concentrations.

(B) In $C_3$ plants,the synthesis of one molecule of glucose $(C_6H_{12}O_6)$ requires $18$ $ATP$ and $12$ $NADPH$.
In $C_4$ plants,an additional $ATP$ is required per $CO_2$ molecule fixed to regenerate phosphoenolpyruvate $(PEP)$ from pyruvate.
Since $6$ molecules of $CO_2$ are required to synthesize one molecule of hexose sugar,$6$ additional $ATP$ molecules are consumed in $C_4$ plants compared to $C_3$ plants.
Therefore,the total $ATP$ required for $C_4$ plants is $18 + 6 = 24$ $ATP$.

(D) In $C_4$ plants,the process of $CO_2$ fixation occurs in two stages in two different types of cells.
$1$. The primary $CO_2$ fixation occurs in the mesophyll cells,where $CO_2$ is accepted by Phosphoenolpyruvate $(PEP)$ to form a $4$-carbon compound called Oxaloacetic acid $(OAA)$.
$2$. This $OAA$ is then converted into malic acid (or aspartic acid) in the mesophyll cells themselves.
$3$. The malic acid is then transported to the bundle sheath cells,where it undergoes decarboxylation to release $CO_2$ for the Calvin cycle.
Therefore,the formation of malic acid takes place in the mesophyll cells.

(D) $C_4$ plants are more efficient than $C_3$ plants primarily because they lack photorespiration.
In $C_3$ plants,the enzyme $RuBisCO$ acts as an oxygenase under high $O_2$ concentrations,leading to photorespiration,which results in the loss of fixed carbon and energy.
$C_4$ plants have evolved a specialized anatomy called $Kranz$ anatomy,which separates the initial $CO_2$ fixation (in mesophyll cells) from the Calvin cycle (in bundle sheath cells).
This mechanism increases the concentration of $CO_2$ around the $RuBisCO$ enzyme in the bundle sheath cells,effectively suppressing the oxygenase activity of $RuBisCO$ and thereby minimizing photorespiration.

(A) The characteristics described,such as the absence of photorespiration,high water-use efficiency,high photosynthetic rate at high temperatures,and improved nitrogen-use efficiency,are the hallmark features of $C_4$ plants.
$C_4$ plants possess a specialized anatomy called Kranz anatomy,which allows them to concentrate $CO_2$ around the enzyme $RuBisCO$,thereby minimizing photorespiration.
These adaptations make $C_4$ plants highly efficient in tropical and arid environments compared to $C_3$ plants.

(B) In $C_4$ plants,the primary $CO_2$ acceptor is a $3$-carbon molecule called Phosphoenolpyruvate $(PEP)$.
This reaction occurs in the mesophyll cells and is catalyzed by the enzyme $PEP$ carboxylase $(PEPCase)$.
In contrast,$C_3$ plants use Ribulose $1,5$-bisphosphate $(RuBP)$ as the primary $CO_2$ acceptor.
Therefore,the correct option is $B$.

(D) $C_4$ cycle occurs in $1500$ species of $19$ angiospermic families. Most of these plants are monocots,which primarily belong to the families Gramineae (Poaceae) and Cyperaceae.

(C) The Assertion is correct because the $C_4$ pathway is an evolutionary adaptation found in many tropical plants to minimize photorespiration.
The Reason is incorrect because,in the $C_4$ pathway,the primary $CO_2$ acceptor is a $3$-carbon compound called Phosphoenolpyruvate $(PEP)$,but the $CO_2$ is fixed into a $4$-carbon compound called Oxaloacetic acid $(OAA)$. The statement 'fixed by $3C$ compound' is misleading as it implies the product is $3C$,whereas the fixation process involves the $3C$ acceptor to form a $4C$ stable product.

(A) $Amaranthus$ $sp.$ and sugarcane are $C_4$ plants,which are also known as Hatch and Slack plants because they follow the Hatch-Slack pathway (or $C_4$ cycle).
In the $C_4$ cycle,the fixation of $CO_2$ occurs twice. To produce one molecule of glucose $(C_6H_{12}O_6)$,$6$ molecules of $CO_2$ must be fixed through the Calvin cycle,which is integrated with the $C_4$ pathway. Thus,the statement that one glucose is formed by the fixation of $6 \,CO_2$ is correct.

(C) The Hatch and Slack pathway,also known as the $C_4$ cycle,occurs in $C_4$ plants.
In this pathway,the primary $CO_2$ acceptor is a $3$-carbon molecule called Phosphoenol pyruvate $(PEP)$.
This reaction is catalyzed by the enzyme $PEP$ carboxylase $(PEPCase)$ in the mesophyll cells,resulting in the formation of a $4$-carbon compound,Oxaloacetic acid $(OAA)$.

(N/A) No,one cannot distinguish whether a plant is $C_{3}$ or $C_{4}$ simply by observing its external morphological features.
$C_{4}$ plants possess a unique leaf anatomy known as $Kranz$ anatomy,which involves the presence of bundle sheath cells surrounding the vascular bundles.
This anatomical difference is internal and can only be observed at the cellular level using a microscope.
External appearance does not reveal these internal structural arrangements.
For instance,both wheat and maize are grasses,but wheat is a $C_{3}$ plant,whereas maize is a $C_{4}$ plant.

(N/A) The leaves of $C_{4}$ plants exhibit a specialized internal structure known as $Kranz$ anatomy,which distinguishes them from $C_{3}$ plants.
In $Kranz$ anatomy,the vascular bundles are surrounded by a ring of large,specialized cells called bundle-sheath cells.
These bundle-sheath cells contain a high density of chloroplasts,possess thick walls,and lack intercellular spaces.
Furthermore,these cells are impervious to gaseous exchange,which helps in maintaining a high concentration of $CO_{2}$ around the enzyme $RuBisCO$.
This unique anatomical arrangement minimizes photorespiration and enhances the photosynthetic efficiency of $C_{4}$ plants compared to $C_{3}$ plants.

(N/A) The productivity of a plant is measured by the rate at which it photosynthesizes. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis.
$C_{4}$ plants have a mechanism for increasing the concentration of carbon dioxide in the bundle-sheath cells.
In $C_{4}$ plants,the Calvin cycle occurs exclusively in the bundle-sheath cells.
The $C_{4}$ acid (malic acid or aspartic acid) formed in the mesophyll cells is transported to the bundle-sheath cells,where it is broken down to release $CO_{2}$.
This high concentration of $CO_{2}$ ensures that the enzyme $RuBisCO$ functions primarily as a carboxylase rather than an oxygenase.
This mechanism effectively prevents photorespiration,which would otherwise waste energy and carbon,thereby significantly increasing the overall rate of photosynthesis and productivity.

(N/A) The enzyme $RuBisCo$ is absent from the mesophyll cells of $C_{4}$ plants. It is present in the bundle-sheath cells surrounding the vascular bundles.
In $C_{4}$ plants,the Calvin cycle occurs in the bundle-sheath cells.
The primary $CO_{2}$ acceptor in the mesophyll cells is phosphoenol pyruvate $(PEP)$,a three-carbon compound.
It is converted into the four-carbon compound oxaloacetic acid $(OAA)$.
$OAA$ is further converted into malic acid or aspartic acid.
These acids are transported to the bundle-sheath cells,where they undergo decarboxylation,releasing $CO_{2}$.
This high concentration of $CO_{2}$ around $RuBisCo$ ensures that it functions primarily as a carboxylase rather than an oxygenase,thereby minimizing photorespiration.

(N/A) Plants adapted to dry tropical regions utilize the $C_4$ pathway.
These plants have $C_4$ oxaloacetic acid as the primary $CO_2$ fixation product. They utilize the $C_3$ pathway or the Calvin cycle as the main biosynthetic pathway. Key characteristics of $C_4$ plants include:
$(1)$ $C_4$ plants possess a specialized leaf anatomy and can tolerate higher temperatures.
$(2)$ They exhibit a positive response to high light intensities.
$(3)$ They lack the process of photorespiration.
$(4)$ They demonstrate greater biomass productivity.
$(5)$ In a vertical section of the leaves,mesophyll cells are arranged in a specific pattern.
$(6)$ Large cells surrounding the vascular bundles are called bundle sheath cells,and this arrangement is known as 'Kranz' anatomy.
$(7)$ 'Kranz' means 'wreath',reflecting the circular arrangement of cells.
$(8)$ The bundle sheath may consist of several layers around the vascular bundles.
$(9)$ These cells are characterized by numerous chloroplasts,thick walls impervious to gaseous exchange,and no intercellular spaces.
The pathway is also known as the Hatch and Slack pathway. It is a cyclic process consisting of the following steps:
Step $1$: Atmospheric $CO_2$ enters mesophyll cells where the primary $CO_2$ acceptor is a $3$-carbon molecule,phosphoenolpyruvate $(PEP)$. The enzyme responsible is $PEP$ carboxylase $(PEPcase)$. The $C_4$ acid,oxaloacetic acid $(OAA)$,is formed: $CO_2 + PEP \xrightarrow{PEPcase} OAA$.
Step $2$: $OAA$ is converted into other $4$-carbon compounds like malic acid or aspartic acid in the mesophyll,which are then transported to the bundle sheath cells. Here,these $C_4$ acids are broken down to release $CO_2$ and a $3$-carbon molecule (pyruvic acid).
Step $3$: The $3$-carbon molecule is transported back to the mesophyll,where it is converted back to $PEP$ in the presence of $ATP$,completing the cycle.
Step $4$: The $CO_2$ released in the bundle sheath cells enters the $C_3$ or Calvin pathway. The bundle sheath cells are rich in $RuBisCO$ but lack $PEPcase$. Thus,the Calvin pathway remains the basic mechanism for sugar formation in both $C_3$ and $C_4$ plants.

(N/A) The Hatch-Slack pathway,also known as the $C_{4}$ pathway,is observed in plants that are adapted to dry tropical regions.
These plants have $C_{4}$ oxaloacetic acid as the first $CO_{2}$ fixation product. They utilize the $C_{3}$ pathway or the Calvin cycle as the main biosynthetic pathway.
Internal characteristics of $C_{4}$ plant leaves:
$(1)$ $C_{4}$ plants possess a special type of leaf anatomy and can tolerate higher temperatures.
$(2)$ They show a positive response to high light intensities.
$(3)$ They lack a process called photorespiration.
$(4)$ They exhibit greater biomass productivity.
$(5)$ In a vertical section of the leaves,mesophyll cells are seen arranged in a specific pattern.
$(6)$ The particularly large cells around the vascular bundles of $C_{4}$ plants are called bundle sheath cells,and the leaves with such anatomy are said to have 'Kranz' anatomy.
$(7)$ Here,'Kranz' means 'wreath' and reflects the arrangement of cells.
$(8)$ The bundle sheath may consist of several layers around the vascular bundles.
$(9)$ These cells are characterized by having a large number of chloroplasts,thick walls impervious to gaseous exchange,and no intercellular spaces.
$(10)$ If we cut a section of the leaves of $C_{4}$ plants like maize or sorghum,we can observe the Kranz anatomy and the distribution of mesophyll cells.
Stages of the $C_{4}$ pathway:
Step $1$: Atmospheric $CO_{2}$ enters the mesophyll cells where the primary $CO_{2}$ acceptor is a $3-$carbon molecule,phosphoenolpyruvate $(PEP)$. The enzyme responsible for this $CO_{2}$ fixation is $PEP$ carboxylase $(PEPcase)$. The mesophyll cells lack $RuBisCO$. The $C_{4}$ acid $OAA$ is formed: $CO_{2} + PEP \xrightarrow{PEPcase} OAA$.
Step $2$: $OAA$ forms other $4-$carbon compounds like malic acid or aspartic acid in the mesophyll cells,which are transported to the bundle sheath cells. In the bundle sheath cells,these $C_{4}$ acids are broken down to release $CO_{2}$ and a $3-$carbon molecule (pyruvic acid).
Step $3$: The $3-$carbon molecule is transported back to the mesophyll where it is converted to $PEP$ again in the presence of $ATP$,thus completing the cycle.
Step $4$: The $CO_{2}$ released in the bundle sheath cells enters the $C_{3}$ or Calvin pathway. The bundle sheath cells are rich in the enzyme Ribulose bisphosphate carboxylase-oxygenase $(RuBisCO)$ but lack $PEPcase$.

(N/A) In $C_4$ plants,photorespiration does not occur.
This is because they possess a mechanism that increases the concentration of $CO_2$ at the enzyme site.
This occurs when the $C_4$ acid from the mesophyll cells is broken down in the bundle sheath cells to release $CO_2$.
This process results in increasing the intracellular concentration of $CO_2$.
In turn,this ensures that the $RuBisCO$ enzyme functions primarily as a carboxylase,thereby minimizing its oxygenase activity.
Consequently,the productivity of $C_4$ plants is significantly higher than that of $C_3$ plants.

(N/A) The internal anatomy of $C_{4}$ plant leaves is characterized by the following features:
$(1)$ $C_{4}$ plants possess a special type of leaf anatomy that allows them to tolerate higher temperatures.
$(2)$ They exhibit a positive response to high light intensities.
$(3)$ They lack the process known as photorespiration.
$(4)$ They demonstrate greater biomass productivity compared to $C_{3}$ plants.
$(5)$ In a vertical section of the leaf,two types of photosynthetic cells are observed: mesophyll cells and bundle sheath cells.
$(6)$ The particularly large cells surrounding the vascular bundles in $C_{4}$ plants are called bundle sheath cells,and this specific arrangement is known as 'Kranz' anatomy.
$(7)$ The term 'Kranz' means 'wreath' and reflects the circular arrangement of these cells around the vascular bundle.
$(8)$ The bundle sheath may consist of several layers of cells surrounding the vascular bundles.
$(9)$ These cells are characterized by a large number of chloroplasts,thick walls that are impervious to gaseous exchange,and the absence of intercellular spaces.
$(10)$ The absence of intercellular spaces in the bundle sheath prevents the leakage of $CO_{2}$ and maintains a high concentration of $CO_{2}$ for the Calvin cycle.

(N/A) $(1)$ Photorespiration: It is a light-dependent process occurring in chloroplasts where the enzyme $RuBisCO$ acts as an oxygenase instead of a carboxylase,leading to the consumption of $O_2$ and the release of $CO_2$ without the production of $ATP$ or $NADPH$.
$(2)$ Bundle sheath: These are specialized layers of cells (parenchymatous or sclerenchymatous) that surround the vascular bundles in the leaves of plants,particularly prominent in $C_4$ plants where they are the site of the Calvin cycle.

(N/A) $(1)$ In $C_4$ plants,the bundle sheath cells are arranged in a ring-like structure around the vascular bundles. This specific leaf anatomy is known as Kranz anatomy (where 'Kranz' means 'wreath').
$(2)$ Photo-oxidation refers to the process where,under conditions of insufficient $CO_2$ concentration,the light energy absorbed by chlorophyll cannot be fully utilized for photosynthesis. This excess energy leads to the oxidative destruction or decomposition of chlorophyll molecules.

(N/A) $(1)$ Location: Found in the periphery of mesophyll cells in a leaf.
Function: Site of photosynthesis.
$(2)$ Location: Surrounding the vascular bundle (specifically in $C_4$ plants/monocot leaves).
Function: Involved in the $C_4$ cycle and carbon fixation.

(N/A) $(1)$ $PEP$ stands for Phosphoenolpyruvate.
$(2)$ $PEPcase$ stands for Phosphoenolpyruvate carboxylase.

(A) Succulents,which are xerophytic plants,grow in environments where water is scarce. To prevent excessive water loss through transpiration,they keep their stomata closed during the day.
These plants utilize a specialized metabolic pathway known as Crassulacean Acid Metabolism $(CAM)$.
During the night,when the temperature is lower and humidity is higher,the stomata open,allowing $CO_2$ to enter. This $CO_2$ is fixed into organic acids,primarily malic acid,which is a $4$-carbon compound.
During the day,the stomata remain closed to conserve water. The stored malic acid is decarboxylated to release $CO_2$ internally,which then enters the Calvin cycle within the photosynthetic cells to facilitate photosynthesis.

(N/A) $\Rightarrow$ 'Kranz' anatomy is observed in $C_{4}$ plants. The leaves of these plants exhibit two types of chloroplasts.
$\Rightarrow$ Grana-containing chloroplasts: These are present in mesophyll cells. These chloroplasts contain well-developed grana. They are capable of fixing $CO_{2}$ even at low concentrations. They contain the enzyme $PEP$ carboxylase,which fixes $CO_{2}$ to form a $C_{4}$ compound called $OAA$ (Oxaloacetic Acid).
$\Rightarrow$ Agranul chloroplasts: These are found in bundle sheath cells,where the $C_{3}$ pathway and the enzyme $RuBisCO$ are present.
$\Rightarrow$ $C_{4}$ plants can efficiently absorb $CO_{2}$ even when its atmospheric concentration is low.
$\Rightarrow$ Consequently,the rate of photosynthesis is higher in $C_{4}$ plants. Even when stomata are closed or water is scarce,they can continue photosynthesis by utilizing stored water and efficient $CO_{2}$ fixation.
$\Rightarrow$ Furthermore,$PEP$ carboxylase is insensitive to $O_{2}$,meaning photorespiration does not occur in $C_{4}$ plants.
$\Rightarrow$ Thus,$C_{4}$ plants are better adapted to tropical and arid (desert) regions.

(N/A) Plants that exhibit these two types of cells (mesophyll and bundle sheath) are known as $C_4$ plants. Examples include plants from the Poaceae family,such as sugarcane and maize.
$(b)$ The first stable product of the $C_4$ cycle is a $4$-carbon compound called Oxaloacetic Acid $(OAA)$.
$(c)$ In mesophyll cells,the enzyme $PEP$ carboxylase $(PEPCase)$ is present,which fixes atmospheric $CO_2$ into $OAA$. In bundle sheath cells,the enzyme $RuBisCO$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase) is present,which performs the Calvin cycle to fix $CO_2$ into $3$-carbon $PGA$ (Phosphoglyceric acid).

(B) $Maize$ is a $C_{4}$ plant,which is more efficient in terms of photosynthetic activity. $C_{4}$ plants are better adapted to tropical environments because they can perform photosynthesis efficiently at higher temperatures,higher light intensities,and lower concentrations of $CO_{2}$,while minimizing photorespiration.

(B) $Maize$ is a $C_{4}$ plant and exhibits Kranz anatomy in its leaf. In $Maize$,the mesophyll cells contain chloroplasts with grana,while the bundle sheath cells contain large,granaless chloroplasts.
$Euphorbia$ (specifically most species) is a $C_{3}$ plant and does not possess the specialized Kranz anatomy characteristic of $C_{4}$ plants.

(B) $C_4$ plants are superior to $C_3$ plants under conditions of low $CO_2$ concentration and high temperature.
$(i)$ $C_4$ plants possess a specialized anatomy called Kranz anatomy,which allows them to concentrate $CO_2$ around the enzyme RuBisCO,minimizing photorespiration.
$(ii)$ They utilize the enzyme $PEP$ carboxylase,which has a high affinity for $CO_2$ and is insensitive to $O_2$,preventing the wasteful process of photorespiration that occurs in $C_3$ plants.
$(iii)$ Because they avoid photorespiration,$C_4$ plants maintain high photosynthetic efficiency even in hot and dry environments where $CO_2$ levels might be limiting.

(N/A) $C_4$ plants exhibit a unique leaf anatomy known as $Kranz$ anatomy.
In $Kranz$ anatomy,the bundle sheath cells form several layers around the vascular bundles,characterized by a large number of chloroplasts,thick walls impervious to gaseous exchange,and no intercellular spaces.
$C_4$ plants possess two types of chloroplasts: agranal chloroplasts in bundle sheath cells and granal chloroplasts in mesophyll cells.
This structure provides an advantage by minimizing photorespiration,which is a wasteful process common in $C_3$ plants.
The bundle sheath cells maintain a high concentration of $CO_2$ around the enzyme $RuBisCO$,ensuring it functions as a carboxylase rather than an oxygenase.
Consequently,$C_4$ plants are more efficient in photosynthesis under high temperatures and high light intensities compared to $C_3$ plants.

(B) In $C_4$ plants,the process of photosynthesis involves a specialized anatomy known as Kranz anatomy.
The mechanism in these plants increases the concentration of $CO_2$ at the enzyme site within the bundle sheath cells.
By concentrating $CO_2$ around the $RuBisCO$ enzyme,the oxygenase activity of $RuBisCO$ is minimized,and $O_2$ is prevented from binding to the enzyme.
Consequently,the competitive binding of $O_2$ to $RuBisCO$ is avoided,which effectively eliminates the possibility of photorespiration.

(C) In $C_{4}$ plants,the primary carboxylation occurs in mesophyll cells via $PEP$ carboxylase,but the Calvin cycle,where $RuBisCO$ functions,occurs exclusively in the bundle sheath cells. Therefore,the statement that $RuBisCO$ activity occurs in mesophyll cells of $C_{4}$ plants is incorrect.

(A) $C_{4}$ plants are more advantageous than $C_{3}$ plants because they can efficiently fix carbon even in low $CO_{2}$ concentrations.
In hot and dry conditions,plants close their stomata to prevent water loss,which leads to a decrease in internal $CO_{2}$ levels.
$C_{3}$ plants suffer from photorespiration under these conditions because $RuBisCO$ acts as an oxygenase,leading to the loss of $RuBP$ and inhibition of the Calvin cycle.
$C_{4}$ plants possess a specialized anatomy called $Kranz$ anatomy and a $PEP$ carboxylase enzyme that has a high affinity for $CO_{2}$,allowing them to maintain photosynthesis even when $CO_{2}$ levels are low.

(C) Phosphorylation is a process involving the addition of a phosphate group to an organic molecule,such as the conversion of $ADP$ to $ATP$.
In Glycolysis,substrate-level phosphorylation occurs.
In the Krebs cycle,substrate-level phosphorylation occurs (conversion of succinyl-$CoA$ to succinate).
In the $ETS$ (Electron Transport System),oxidative phosphorylation occurs.
However,the $C_{4}$ cycle (Hatch-Slack pathway) is a carbon fixation pathway that does not involve phosphorylation as a direct step in its cycle reactions; it consumes $ATP$ produced elsewhere but does not perform phosphorylation itself.

(D) In leaves,each vascular bundle is surrounded by a layer of cells known as the bundle sheath.
These cells are primarily parenchymatous in nature.
In many plants,especially those exhibiting $C_4$ photosynthesis,these cells contain numerous chloroplasts,making them chlorenchymatous as well.
Since they are parenchymatous and often contain chloroplasts (chlorenchymatous),the most accurate description covering their fundamental nature is parenchymatous,but they are also chlorenchymatous. Given the options,both $A$ and $C$ are correct,but since they are fundamentally parenchymatous cells that perform photosynthesis,they are often referred to as chlorenchymatous. However,in the context of standard biology questions,they are parenchymatous cells that may be chlorenchymatous. If forced to choose the most encompassing answer,they are parenchymatous cells.

(B) $CAM$ plants are adapted to survive in arid and hot environments. To minimize water loss through transpiration,their stomata remain closed during the day and open only at night. This type of stomatal movement is known as scotoactive. During the night,these plants take up $CO_{2}$ and store it as organic acids. This stored $CO_{2}$ is then released and used for photosynthesis during the day while the stomata remain closed.

(C) $C_{4}$-plants exhibit a specialized leaf anatomy known as Kranz anatomy.
In this anatomy,the vascular bundles are surrounded by a ring of bundle sheath cells.
These bundle sheath cells are large and contain numerous chloroplasts.
Unlike mesophyll cells,the chloroplasts in bundle sheath cells typically lack grana.
This structural arrangement is a key adaptation for efficient carbon fixation in plants like sugarcane,maize,and sorghum.

(B) Sunken stomata are a characteristic adaptation in $CAM$ (Crassulacean Acid Metabolism) plants,which are often xerophytes.
These stomata are situated in deep pits below the surface of the epidermis.
This structural arrangement helps in reducing the rate of transpiration by creating a humid microclimate around the stomatal pore,thereby conserving water in arid environments.

(C) In $CAM$-plants,carbon dioxide enters the leaf and is fixed to oxaloacetic acid,which is then converted to malic acid at night when stomata are open.
This malic acid is stored in the vacuoles of cells during the night.
Therefore,in $CAM$-plants,the concentration of organic acids increases during the night due to their accumulation.

(A) $C_4$-plants exhibit a specialized leaf anatomy known as Kranz anatomy,which consists of bundle sheath cells and mesophyll cells.
In $C_4$-plants,the primary $CO_2$ acceptor is phosphoenolpyruvate $(PEP)$,a $3$-carbon molecule,not RuBisCo.
RuBisCo is absent in the mesophyll cells of $C_4$-plants and is instead found in the bundle sheath cells.
Carboxylation occurs in the mesophyll cells where $PEP$ carboxylase fixes $CO_2$ into oxaloacetate.
Therefore,the statement that the $CO_2$ acceptor is RuBisCo in mesophyll cells is false.

(B) In $C_{4}$ plants,the primary $CO_{2}$ fixation occurs in the mesophyll cells,where the enzyme $PEP$ carboxylase is present,but RuBisCO is absent.
The $C_{4}$ acid formed is transported to the bundle sheath cells.
In the bundle sheath cells,the $C_{4}$ acid is broken down to release $CO_{2}$,which then enters the Calvin cycle.
Therefore,the RuBisCO enzyme is exclusively present in the bundle sheath cells of $C_{4}$ plants to facilitate the Calvin cycle.

(D) In $C_{4}$ plants,the bundle sheath cells are the site of the $C_{3}$ cycle (Calvin cycle).
These cells contain a high concentration of the enzyme $RuBisCo$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase),which is essential for the fixation of $CO_{2}$ during the Calvin cycle.
In contrast,$PEP$ carboxylase is primarily found in the mesophyll cells of $C_{4}$ plants.

(D) The $C_{4}$ pathway of $CO_{2}$ fixation,also known as the $C_{4}$ cycle or the Hatch-Slack pathway,was discovered and proposed by $M. D. Hatch$ and $C. R. Slack$ in $1966$. This pathway is an adaptation found in certain plants to minimize photorespiration and increase photosynthetic efficiency in high-temperature and high-light environments.

(A) In the $C_{4}$ pathway,the primary $CO_{2}$ acceptor is phosphoenolpyruvate $(PEP)$,which is a $3$-carbon molecule.
This reaction is catalyzed by the enzyme $PEP$ carboxylase.
The first stable product formed after $CO_{2}$ fixation is a $4$-carbon compound called oxaloacetic acid $(OAA)$.
Therefore,the correct option is $A$.

(C) $C_{4}$ plants exhibit 'Kranz' anatomy.
In this anatomy,vascular bundles are surrounded by bundle sheath cells.
The chloroplasts in these plants are dimorphic.
The bundle sheath cells contain large,agranal chloroplasts (lacking grana) which are often rich in starch grains.
In contrast,the mesophyll cells contain smaller,granal chloroplasts.
Therefore,the correct description is that mesophyll cells have small chloroplasts,while bundle sheath cells have larger agranal chloroplasts.

(B) In $C_{4}$ plants, the initial fixation of carbon dioxide occurs in the mesophyll cells.
The primary acceptor of $CO_{2}$ is phosphoenol pyruvate $(PEP)$.
It combines with carbon dioxide in the presence of the enzyme $PEP$ carboxylase to form oxaloacetic acid $(OAA)$.
The reaction is as follows:
$PEP + CO_{2} + H_{2}O \xrightarrow{PEP \text{ carboxylase}} \text{Oxaloacetic acid} + H_{3}PO_{4}$

(B) In plants,$CO_{2}$ assimilation during photosynthesis occurs primarily through two pathways:
$(i)$ $C_{3}$ pathway: In these plants,the first stable product of $CO_{2}$ fixation is a $3$-carbon acid,$3$-phosphoglyceric acid $(3-PGA)$.
$(ii)$ $C_{4}$ pathway: In these plants,the first stable product of $CO_{2}$ fixation is a $4$-carbon acid,oxaloacetic acid $(OAA)$.
Therefore,in the $C_{4}$ pathway,the first product identified is $OAA$.

(C) In $C_{4}$-plants,the bundle sheath cells are characterized by having a large number of chloroplasts that are typically agranal (lacking grana). These cells are the primary site for the Calvin cycle,and therefore,they contain a large amount of the enzyme $RuBisCO$ (Ribulose$-1,5-$bisphosphate carboxylase-oxygenase) to facilitate carbon fixation.

(D) In $C_{4}$ plants,the $C_{4}$ acids (malic acid or aspartic acid) formed in the mesophyll cells are transported to the bundle sheath cells.
In the bundle sheath cells,these $C_{4}$ acids undergo decarboxylation to release $CO_{2}$ and a $3$-carbon molecule.
Specifically,malic acid is decarboxylated to form pyruvic acid and $CO_{2}$,while aspartic acid is first converted to oxaloacetic acid and then to malic acid,or it undergoes deamination to form pyruvic acid.
Therefore,both processes can occur depending on the specific pathway taken by the $C_{4}$ acid in the bundle sheath cells.

(B) $CAM$ (Crassulacean Acid Metabolism) plants are adapted to arid environments.
In $CAM$-plants,the stomata open at night to minimize water loss through transpiration.
This allows the entry of atmospheric carbon dioxide into the leaves.
The carbon dioxide is converted into malic acid (a four-carbon compound) and stored in the vacuoles.
During the day,the stomata remain closed to prevent water loss.
The stored malic acid is then transported to the chloroplasts,where it undergoes decarboxylation to release carbon dioxide,which is then fixed by the Calvin cycle.