C4 Questions in English

(C) Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants.
It is characterized by the presence of bundle sheath cells arranged in a ring-like structure around the vascular bundles.
This type of anatomy is observed in both monocots (e.g.,Sugarcane,Maize,and Sorghum) and some dicots (e.g.,Amaranthus edulis,Atriplex rosea).
Therefore,the correct option is $(c)$.

(B) In the $C_4$ photosynthetic pathway,the primary carboxylation occurs in the mesophyll cells.
$CO_2$ is accepted by a $3$-carbon molecule called Phosphoenolpyruvate $(PEP)$.
This reaction is catalyzed by the enzyme $PEP$ carboxylase $(PEPCase)$.
This results in the formation of a $4$-carbon compound,Oxaloacetic acid $(OAA)$.

(B) $C_4$ plants are more efficient than $C_3$ plants in terms of water use efficiency.
They can fix $CO_2$ even at low concentrations,which allows them to keep their stomata partially closed during the day to reduce water loss through transpiration while still maintaining high rates of photosynthesis.
Therefore,$C_4$ plants economize transpirational loss of water much better than $C_3$ plants.

(B) In $CAM$ (Crassulacean Acid Metabolism) plants,which are typically $Succulents$,the stomata remain closed during the day to prevent water loss through transpiration and open during the night to take in $CO_2$. Such stomata are known as $scotoactive$ stomata.

(C) Scotoactive stomata are those that remain closed during the day and open at night.
This mechanism is an adaptation to prevent excessive water loss in arid environments.
$CAM$ (Crassulacean Acid Metabolism) plants,which are typically succulent or fleshy,exhibit this characteristic to conserve water by performing gas exchange during the cooler night hours.

(D) $CAM$ (Crassulacean Acid Metabolism) plants exhibit a unique adaptation to arid environments.
They possess scotoactive stomata,which means their stomata open during the night to take in $CO_2$ and close during the day to minimize water loss through transpiration.
This mechanism allows them to survive in extremely dry conditions.

(B) In succulent plants ($CAM$ plants),the stomata open at night to take in $CO_2$.
During the night,carbohydrates are incompletely oxidized to form organic acids,primarily malic acid,through the process of dark fixation of $CO_2$.
This accumulation of organic acids increases the osmotic concentration of the guard cells,leading to the influx of water and the opening of stomata.
During the day,these organic acids are broken down to release $CO_2$ for photosynthesis,and the stomata close to prevent water loss.

(B) The $CAM$ ($Crassulacean$ $Acid$ $Metabolism$) pathway of $CO_2$ fixation was first described and proposed by $M.D.$ $Thomas$ and later extensively studied by $Rouhani$ and his associates in the $1950s$ and $1960s$.
This pathway is an adaptation found in succulent plants to minimize photorespiration and water loss by fixing $CO_2$ at night.

(C) $C_4$ pathway,also known as the Hatch-Slack pathway,was discovered by $M$.$D$. Hatch and $C$.$R$. Slack in $1966$. In this pathway,$CO_2$ is initially fixed in the mesophyll cells to form a $4$-carbon compound (oxaloacetate),which is then transported to the bundle sheath cells where it undergoes decarboxylation to release $CO_2$ for the Calvin cycle. Thus,the correct answer is $C$.

(A) Sugarcane is a ${C_4}$ plant.
In ${C_4}$ plants,the leaves exhibit Kranz anatomy,where chloroplasts are dimorphic (two morphologically distinct types).
The bundle sheath cells contain large chloroplasts that lack grana and are often associated with starch storage.
The mesophyll cells contain smaller,normal chloroplasts that possess well-developed grana for the light-dependent reactions.

(A) In $C_4$ plants,the primary $CO_2$ fixation occurs in the mesophyll cells.
$CO_2$ combines with a $3$-carbon compound called Phosphoenolpyruvate $(PEP)$ to form a $4$-carbon compound,Oxaloacetic acid $(OAA)$.
This reaction is catalyzed by the enzyme $PEP$ carboxylase (or $PEPCase$).
$RuBP$ carboxylase is primarily involved in the Calvin cycle ($C_3$ cycle) in the bundle sheath cells.

(C) In $C_4$ plants,the dark reaction (Calvin cycle) is spatially separated.
$1$. The initial $CO_2$ fixation occurs in the mesophyll cells via the Hatch-Slack cycle ($PEP$ carboxylase enzyme).
$2$. The Calvin cycle (dark reaction) enzymes,such as RuBisCO,are localized in the bundle sheath chloroplasts.
$3$. Since both cell types participate in the complete process of the dark reaction (fixation and reduction),the enzymes for the dark reaction are found in both the mesophyll and bundle sheath cells.

(C) In $C_4$ plants like sugarcane,the leaves exhibit Kranz anatomy.
These plants possess two types of chloroplasts:
$1$. Mesophyll chloroplasts,which contain well-developed grana and perform light reactions.
$2$. Bundle sheath chloroplasts,which are agranal (lack grana) and perform the Calvin cycle (dark reactions).
Therefore,the bundle sheath cells of sugarcane contain chloroplasts that do not have grana.

(B) The conversion of oxaloacetic acid $(OAA)$ into malic acid is a reduction process.
In the $C_4$ pathway,$OAA$ is reduced to malic acid in the presence of the enzyme malate dehydrogenase,utilizing $NADPH + H^+$ as a reducing agent.
This reaction is essential for the transport of $CO_2$ in $C_4$ plants,and the resulting malic acid is often stored in the vacuoles of mesophyll cells.

(D) In the $C_4$ photosynthetic pathway (Hatch-Slack cycle),oxaloacetic acid $(OAA)$ is the first stable product formed in the mesophyll cells.
This $OAA$ is subsequently reduced to malic acid (or aspartic acid) by the enzyme malate dehydrogenase.
This reaction requires $NADPH + H^+$ as a reducing agent,which is converted into $NADP^+$.
Therefore,the correct enzyme involved in this conversion is malate dehydrogenase.

(B) In the $C_4$ photosynthetic pathway (Hatch-Slack cycle),oxaloacetic acid $(OAA)$ is formed in the mesophyll cells.
This $OAA$ is then reduced to malic acid (or aspartic acid) by the enzyme malate dehydrogenase.
This reduction process requires $NADPH_2$ as a reducing agent,which provides the necessary hydrogen atoms for the conversion of the keto group to a hydroxyl group.

(C) Succulent xerophytes (like members of the family $Crassulaceae$) exhibit a specialized pathway of photosynthesis known as $Crassulacean$ $Acid$ $Metabolism$ $(CAM)$.
In these plants,stomata open during the night to take in $CO_2$,which is fixed into oxaloacetic acid and subsequently converted into malic acid.
This malic acid is stored in the vacuoles during the night and is decarboxylated during the day to release $CO_2$ for the $Calvin$ cycle,allowing the plant to conserve water in arid environments.

(C) In $C_4$ plants,the mesophyll cells and bundle sheath cells are structurally and functionally linked to facilitate the transport of metabolites like malate and pyruvate. This connection is established through $Plasmodesmata$,which are cytoplasmic channels that traverse the cell walls of plant cells,allowing for direct communication and transport of substances between adjacent cells.

(C) $CAM$ plants exhibit dark acidification. During the night,$CO_2$ is fixed into organic acids,primarily malic acid,which leads to an increase in acidity within the vacuoles. This process is known as dark acidification. During the daytime,this malic acid is decarboxylated to release $CO_2$,which is then utilized in the $C_3$ cycle.

(B) In the $C_4$ pathway,the primary $CO_2$ acceptor is phosphoenolpyruvate $(PEP)$,which is a $3-C$ compound.
$CO_2$ combines with $PEP$ in the mesophyll cells to form a $4-C$ compound called oxaloacetic acid $(OAA)$.
This reaction is catalyzed by the enzyme $PEP$ carboxylase.

(D) $C_4$ plants are adapted to high temperatures and high light intensities.
Maize ($Zea$ $mays$) is a classic example of a $C_4$ plant.
In $C_4$ plants,the initial $CO_2$ fixation occurs in mesophyll cells to form a $4$-carbon compound called oxaloacetate,which is why they are called $C_4$ plants.
Papaya,Pea,and Potato are examples of $C_3$ plants.

(A) $C_4$ plants are also known as Hatch and Slack type plants.
This is because the detailed study of the $C_4$ photosynthetic pathway was first described by $M$.$D$. Hatch and $C$.$R$. Slack in $1966$.
Due to their pioneering work in identifying this specific carbon fixation pathway,the $C_4$ cycle is commonly referred to as the Hatch and Slack cycle.

(A) In $C_4$ plants,the process of photosynthesis is divided between two types of cells: mesophyll cells and bundle sheath cells.
$1$. The initial fixation of $CO_2$ occurs in the mesophyll cells,where $PEP$ carboxylase fixes $CO_2$ into a $4$-carbon compound (oxaloacetic acid).
$2$. This $4$-carbon compound is transported to the bundle sheath cells.
$3$. In the bundle sheath cells,the $4$-carbon compound is broken down to release $CO_2$,which then enters the Calvin cycle.
$4$. Therefore,the Calvin cycle specifically takes place in the stroma of the bundle sheath chloroplasts.

(B) In $1965$,Kortschak,Hart,and Burr,while working with $^{14}CO_2$ on sugarcane leaves,found that $C_4$ dicarboxylic acids,specifically malate and aspartate,were the major labeled products during very short periods of photosynthesis.
This observation was confirmed by $M$.$D$. Hatch and $C$.$R$. Slack in $1967$. The Hatch-Slack pathway,also known as the $C_4$ pathway,is an alternative mechanism for $CO_2$ fixation found in tropical and subtropical grasses and certain dicotyledons,which allows them to fix $CO_2$ with high efficiency even at low concentrations.

(D) $C_4$ plants are adapted to high temperatures and high light intensities. Examples of $C_4$ plants include maize $(Zea \, mays)$, sugarcane $(Saccharum \, officinarum)$, and various grasses like $Saccharum \, munja$. $Euphorbia \, splendens$ is a $CAM$ (Crassulacean Acid Metabolism) plant, not a $C_4$ plant. Therefore, $C_4$ photosynthesis does not occur in $Euphorbia \, splendens$.

(A) In $C_4$ plants,the primary $CO_2$ acceptor is a $3$-carbon molecule called Phosphoenol pyruvate $(PEP)$.
This reaction occurs in the mesophyll cells and is catalyzed by the enzyme $PEP$ carboxylase.
The product formed after $CO_2$ fixation is Oxaloacetic acid $(OAA)$,which is a $4$-carbon compound.

(B) The correct answer is $B$. In the Hatch-Slack pathway ($C_4$ cycle),the first stable product of $CO_2$ fixation is a $4$-carbon compound known as oxaloacetic acid $(OAA)$.
Because this first stable product contains $4$ carbons,these plants are referred to as ${C_4}$ plants.
Subsequently,this acid is converted into another $4$-carbon acid,which is malic acid.

(C) The correct answer is $C_4$ plants.
$A$ fundamental characteristic of $C_4$ plants is the presence of "Kranz" (a German term meaning halo or wreath) anatomy in their leaves.
In $C_4$ leaves, the vascular bundles are surrounded by a layer of bundle sheath cells that contain a large number of chloroplasts.
The chloroplasts in $C_4$ leaves are dimorphic, meaning they exist in two morphologically distinct types: those in the mesophyll cells and those in the bundle sheath cells.

(C) $CAM$ (Crassulacean Acid Metabolism) is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions.
In $CAM$ plants,the stomata remain closed during the day to reduce water loss and open at night to collect $CO_2$.
Examples of $CAM$ plants include succulent plants such as $Agave$,$Opuntia$,$Pineapple$,and $Cactus$.
Maize and Sugarcane are $C_4$ plants,while Mango is a $C_3$ plant.

(A) $C_4$ plants,such as maize and sugarcane,have evolved a specialized leaf anatomy known as $Kranz$ anatomy.
This adaptation allows them to minimize photorespiration,which is a wasteful process that occurs in $C_3$ plants under high temperatures.
By concentrating $CO_2$ around the enzyme $RuBisCO$ in the bundle sheath cells,$C_4$ plants can maintain high rates of photosynthesis even in hot and dry environments where water conservation is necessary.

(B) $Atriplex$ $spongiosa$ is a well-known example of a $C_4$ plant.
$C_4$ plants are adapted to high temperatures and high light intensities.
They possess $Kranz$ anatomy,which allows them to minimize photorespiration by concentrating $CO_2$ around the enzyme $RuBisCO$ in the bundle sheath cells.

(A) $C_4$ plants exhibit Kranz anatomy,which involves two types of photosynthetic cells: mesophyll cells and bundle sheath cells.
These two cell types contain dimorphic chloroplasts,meaning they differ in structure and function.
Mesophyll chloroplasts are typically granal (containing well-developed thylakoids),while bundle sheath chloroplasts are agranal (lacking grana or having reduced thylakoids) and are specialized for the Calvin cycle.

(A) In $CAM$ (Crassulacean Acid Metabolism) plants,the stomata open during the night to allow $CO_2$ uptake.
During the night,$CO_2$ is fixed by the enzyme $PEP$ carboxylase to form oxaloacetic acid,which is then converted into malic acid.
This malic acid is stored in the vacuoles until daytime.
Therefore,during the formation of malic acid (which occurs at night),the stomata remain open.

(C) $C_4$ plants are those that utilize the $C_4$ carbon fixation pathway,which is an adaptation to minimize photorespiration in hot and dry environments.
$Spirogyra$ is an alga.
$Pinus$ is a gymnosperm ($C_3$ plant).
$Sorghum$ is a well-known $C_4$ grass.
$Funaria$ is a bryophyte.
Therefore,$Sorghum$ is the correct answer.

(B) The correct answer is $B$. In $C_4$ plants,the leaves exhibit Kranz anatomy,which involves two types of photosynthetic cells: mesophyll cells and bundle sheath cells.
Mesophyll cells contain normal chloroplasts with well-developed grana.
However,the bundle sheath cells contain agranal chloroplasts,which lack grana and are primarily involved in the Calvin cycle.

$(C)$ The plant $Panicum$ $milioides$ exhibits characteristics of both $C_3$ and $C_4$ photosynthetic pathways.
It shows reduced photorespiration compared to typical $C_3$ plants but does not possess the full Kranz anatomy seen in $C_4$ plants.
Therefore, it is considered an intermediate plant between $C_3$ and $C_4$ types.

(D) $C_4$ plants exhibit a specialized leaf anatomy known as Kranz anatomy.
$1$. The bundle sheath cells are large,thick-walled,and contain a large number of chloroplasts,which are essential for the Calvin cycle.
$2$. $C_4$ plants often have smaller stomatal pores to minimize water loss while maintaining efficient $CO_2$ fixation.
$3$. The mesophyll cells are arranged compactly around the bundle sheath,reducing intercellular spaces to facilitate the rapid diffusion of metabolites.
Therefore,all these features are characteristic of $C_4$ plants.

(C) The correlation between 'Kranz' anatomy and the $C_4$ pathway of $CO_2$ assimilation was first established by $W.J.S. \text{Downton}$ and $E.B. \text{Tregunna}$ in $1968$.
They observed that plants exhibiting the $C_4$ photosynthetic pathway possess a specialized leaf anatomy known as 'Kranz' anatomy, characterized by the presence of bundle sheath cells surrounding the vascular bundles.

(B) In the $C_4$ cycle (also known as the $Hatch-Slack$ pathway),the atmospheric $CO_2$ is first fixed in the mesophyll cells.
Phosphoenolpyruvate $(PEP)$,a $3$-carbon compound,acts as the primary $CO_2$ acceptor.
This reaction is catalyzed by the enzyme $PEP$ carboxylase $(PEPCase)$,resulting in the formation of a $4$-carbon compound called Oxaloacetic acid $(OAA)$.

(B) Members of the family $Crassulaceae$ are known for exhibiting $Crassulacean$ $Acid$ $Metabolism$ $(CAM)$.
In $CAM$ plants,the stomata remain closed during the day to minimize water loss and open at night to take in $CO_2$.
The $CO_2$ taken up at night is fixed into organic acids (mainly malic acid) and stored in vacuoles.
During the day,these organic acids are decarboxylated to release $CO_2$,which then enters the $Calvin$ cycle for sugar synthesis.
Therefore,the correct answer is $CAM$ photosynthesis.

(B) $C_4$ plants exhibit a specialized leaf anatomy known as Kranz anatomy.
In this anatomy,there are two distinct types of photosynthetic cells:
$1$. Mesophyll cells: These are loosely arranged and contain normal chloroplasts.
$2$. Bundle sheath cells: These are large,thick-walled cells surrounding the vascular bundles,containing specialized chloroplasts (often lacking grana).
Therefore,$C_4$ plants possess two types of photosynthetic cells.

(C) In $C_4$ plants,the bundle sheath cells are large,thick-walled,and contain a large number of chloroplasts.
These cells are specialized for the storage of starch,which is the end product of the Calvin cycle occurring within them.
Therefore,bundle sheath cells are prominently loaded with starch compared to mesophyll cells.

(B) Sugarcane is a $C_4$ plant. $C_4$ plants exhibit a specialized pathway for carbon fixation known as the $C_4$ pathway or the Hatch and Slack cycle. This pathway allows these plants to fix $CO_2$ more efficiently than $C_3$ plants,especially under conditions of high temperature and high light intensity,by minimizing photorespiration.

(B) In $C_4$ plants,the process of carbon dioxide fixation occurs in two stages in different cell types.
$1$. The primary $CO_2$ fixation occurs in the mesophyll cells,where $CO_2$ is accepted by Phosphoenolpyruvate $(PEP)$ to form a $4$-carbon compound,Oxaloacetic acid $(OAA)$.
$2$. This $4$-carbon compound is then transported to the bundle sheath cells,where it undergoes decarboxylation to release $CO_2$ for the Calvin cycle.
$3$. Therefore,the initial fixation of $CO_2$ happens in the mesophyll cells (which include both palisade and spongy types depending on the leaf structure,but specifically the mesophyll tissue).
$4$. Given the options,since the primary fixation is a characteristic of mesophyll cells,and the question asks for the site of fixation,the most accurate answer in the context of $C_4$ pathway initiation is the mesophyll cells.

(C) In ${C_4}$ plants,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.
The combination of $CO_2$ with $PEP$ results in the formation of a $4$-carbon compound,Oxaloacetic acid $(OAA)$,which is why these plants are called ${C_4}$ plants.

(C) ${C_4}$ plants are more efficient at photosynthesis under low $CO_2$ concentrations because they possess a specialized mechanism called the $C_4$ cycle or Hatch-Slack pathway.
In this pathway,$CO_2$ is initially fixed in mesophyll cells by the enzyme $PEP$ carboxylase to form a $4$-carbon compound (oxaloacetate).
This $4$-carbon compound is then transported to the bundle sheath cells,where it is decarboxylated to release a high concentration of $CO_2$ around the enzyme $Rubisco$.
This effective pumping mechanism ensures that $Rubisco$ is saturated with $CO_2$,thereby minimizing photorespiration and allowing photosynthesis to continue even when atmospheric $CO_2$ levels are low.

(C) $CAM$ (Crassulacean Acid Metabolism) is an adaptation found in plants living in arid or semi-arid environments.
These plants possess succulent or fleshy leaves (or stems) that help in water storage.
The fleshy nature of the leaves allows them to store organic acids produced during the night,which are then used for the Calvin cycle during the day when stomata are closed to prevent water loss.

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

(C) Sugarcane is a $C_4$ plant. In $C_4$ plants,the primary fixation of $CO_2$ occurs in the mesophyll cells.
The enzyme Phosphoenolpyruvate carboxylase (PEPCase) catalyzes the fixation of $CO_2$ by combining it with Phosphoenolpyruvate $(PEP)$ to form Oxaloacetic acid $(OAA)$.
This $OAA$ is then converted into malic acid or aspartic acid,which is transported to the bundle sheath cells.
Therefore,the correct enzyme involved in the initial fixation of $CO_2$ is Phosphoenolpyruvate carboxylase.

(D) $C_4$ plants are adapted to high temperatures and high light intensity. Examples include sugarcane,maize,and sorghum. Wheat is a $C_3$ plant,which follows the Calvin cycle for carbon fixation. Therefore,wheat is not a $C_4$ plant.