C4 Questions in English

(D) The $C_4$ pathway is an adaptation found in plants to minimize photorespiration and maximize photosynthetic efficiency in hot,dry environments.
$1$. It requires more energy $(ATP)$ than the $C_3$ pathway because it involves an additional cycle to concentrate $CO_2$ in bundle sheath cells.
$2$. It effectively eliminates photorespiration by maintaining a high concentration of $CO_2$ around the enzyme RuBisCO.
$3$. The primary $CO_2$ acceptor in the mesophyll cells is Phosphoenolpyruvate $(PEP)$,which is a $C_3$ compound.
$4$. The $C_4$ pathway is not inhibited by high $CO_2$ concentration; in fact,it is highly efficient at low $CO_2$ levels and functions well under high light and temperature conditions. Therefore,the statement that it is inhibited by high $CO_2$ concentration is false.

(B) In the $C_4$ cycle,carboxylation occurs twice.
First,carboxylation takes place in the mesophyll cells where Phosphoenolpyruvate $(PEP)$ is carboxylated by the enzyme $PEP$ carboxylase to form Oxaloacetic acid $(OAA)$.
Second,carboxylation occurs in the bundle sheath cells where Ribulose $1, 5$-bisphosphate $(RuBP)$ is carboxylated by the enzyme $RuBisCO$ during the Calvin cycle.

(B) The Hatch and Slack pathway ($C_4$ cycle) occurs in plants exhibiting Kranz anatomy.
In this anatomy,the bundle sheath cells contain larger chloroplasts that lack grana ($Agranal$ chloroplasts) and often contain starch grains.
In contrast,the mesophyll cells contain smaller chloroplasts that possess well-developed grana and lack starch grains.

(B) Kranz anatomy is a characteristic feature of $C_4$ plants.
In $C_4$ plants,the bundle sheath cells contain large,agranal (or less developed grana) chloroplasts,while the mesophyll cells contain smaller,well-developed,granal chloroplasts.
Option $B$ states that mesophyll cells have large chloroplasts and more developed grana; while the second part is true,the first part is incorrect as mesophyll chloroplasts are typically smaller than those in the bundle sheath.
Therefore,the statement in option $B$ is false.

(A) $C_4$ plants are highly efficient because they possess a specialized mechanism called the $C_4$ cycle (Hatch-Slack pathway) to concentrate $CO_2$ around the enzyme $RuBisCO$.
This internal $CO_2$ concentrating mechanism allows them to maintain high rates of photosynthesis even when external $CO_2$ concentrations are very low.
Therefore,$C_4$ plants show efficiency even in low $CO_2$ concentration.

(B) In $CAM$ plants,the stomata open at night to take in $CO_2$,which is fixed into malic acid and stored in vacuoles.
During the day,the stomata remain closed to prevent water loss.
The stored malic acid is transported out of the vacuole and decarboxylated to release $CO_2$ and pyruvic acid.
This released $CO_2$ is then utilized in the $C_3$ cycle (Calvin cycle) for photosynthesis.

(A) In most plants,stomata open during the day to facilitate gas exchange for photosynthesis and close at night to prevent water loss.
However,$CAM$ (Crassulacean Acid Metabolism) plants exhibit a unique adaptation to arid environments.
In these plants,stomata open at night to take in $CO_2$,which is then stored as organic acids (malic acid).
During the day,the stomata remain closed to conserve water,and the stored $CO_2$ is released internally for the Calvin cycle.
Therefore,$CAM$ plants are the exception.

(C) The ultrastructure of bundle sheath cells in $C_4$ plants was studied by $Fujita$ and $Miyachi$ in $1970$.
These cells are characterized by having a large number of chloroplasts,thick walls impervious to gaseous exchange,and no intercellular spaces.

(D) In ${C_3}$ plants,the synthesis of one molecule of glucose $(C_6H_{12}O_6)$ requires $18$ $ATP$ and $12$ $NADPH$ molecules.
In ${C_4}$ plants,an additional $C_4$ cycle (Hatch-Slack pathway) operates to concentrate $CO_2$ around the enzyme $RuBisCO$.
This process 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 a total of $30$ $ATP$ $(18 + 12)$ to produce one molecule of glucose.

(D) $C_4$ plants are found in both monocotyledonous and dicotyledonous plants.
Examples of $C_4$ monocots include sugarcane,maize,and species of Cyperus.
Examples of $C_4$ dicots include Amaranthus and Atriplex.
Therefore,$C_4$ pathway is not restricted to a single group of angiosperms.

(D) In the $C_4$ cycle (Hatch and Slack pathway),the primary $CO_2$ acceptor is phosphoenolpyruvate $(PEP)$.
This reaction occurs in the mesophyll cells of the leaves.
The enzyme $PEP$ carboxylase catalyzes the carboxylation of $PEP$ to form a $4-carbon$ compound known as oxaloacetic acid $(OAA)$.
Therefore,the first stable product of $CO_2$ fixation in the $C_4$ cycle is oxaloacetate,formed in the mesophyll cells.

(A) $CAM$ (Crassulacean Acid Metabolism) plants are typically found in arid environments. The ability to keep their stomata closed during the hottest and driest parts of the day significantly reduces water loss through transpiration,thereby aiding in water conservation.

(D) The $C_3$ cycle (Calvin cycle) involves the fixation of $CO_2$ into a three-carbon compound called $3$-phosphoglyceric acid ($3$-$PGA$).
However,in the context of carbon metabolism,$OAA$ (Oxaloacetic acid) is a four-carbon compound.
While $OAA$ is primarily associated with the $C_4$ cycle (Hatch-Slack pathway) as the first stable product,among the given options,$OAA$ is the only four-carbon compound.
$Erythrose-P$ is a four-carbon sugar phosphate,but it is an intermediate in the regeneration phase of the Calvin cycle,not typically classified as the primary four-carbon product in the context of cycle comparisons.
Given the standard options provided,$OAA$ is the correct choice as it is a well-known four-carbon organic acid.

(B) Sugarcane is a $C_4$ plant. In $C_4$ plants,the primary fixation of $CO_2$ occurs in the mesophyll cells.
$CO_2$ reacts with phosphoenolpyruvate $(PEP)$ to form oxaloacetic acid $(OAA)$,which is a $4$-carbon compound.
This reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase,commonly known as $PEP-case$.
$OAA$ is then converted into malic acid or aspartic acid,which are transported to the bundle sheath cells.
Therefore,the enzyme responsible for the initial fixation of $CO_2$ in sugarcane is $PEP-case$.

(B) Sugarcane is a $C_4$ plant. $C_4$ plants are adapted to tropical environments and possess a specialized pathway for carbon fixation known as the $C_4$ pathway or the Hatch-Slack cycle. In this pathway,$CO_2$ is initially fixed by the enzyme $PEP$ carboxylase in mesophyll cells,which has a much higher affinity for $CO_2$ than $RuBisCO$. This mechanism minimizes photorespiration and allows for highly efficient $CO_2$ fixation even under high temperature and low $CO_2$ concentrations.

(B) In $C_4$ plants,the photosynthetic pathway involves two types of cells: mesophyll cells and bundle sheath cells.
$1$. The initial fixation of $CO_2$ occurs in the mesophyll cells,where $CO_2$ is converted into a $4$-carbon compound (oxaloacetate).
$2$. This $4$-carbon compound is transported to the bundle sheath cells.
$3$. In the bundle sheath cells,the $4$-carbon compound undergoes decarboxylation to release $CO_2$,which then enters the Calvin cycle ($C_3$ cycle).
$4$. The Calvin cycle,which results in the synthesis of sugars like fructose,takes place specifically within the chloroplasts of the bundle sheath cells.

(C) In $C_4$ plants,photosynthesis is less limited by atmospheric $CO_2$ levels because they possess an efficient $CO_2$ concentrating mechanism.
In these plants,the primary fixation of $CO_2$ occurs in the mesophyll cells via the enzyme $PEP$ carboxylase,which has a very high affinity for $CO_2$ and does not bind with $O_2$.
This fixed $CO_2$ is then transported to the bundle sheath cells in the form of a $4$-carbon organic acid.
In the bundle sheath cells,this acid is decarboxylated to release $CO_2$,which significantly increases the local concentration of $CO_2$ around the $RuBisCO$ enzyme.
This high concentration ensures that $RuBisCO$ functions efficiently in the Calvin cycle and minimizes the rate of photorespiration.

(D) In $C_4$ plants,the process of photosynthesis involves two types of cells: mesophyll cells and bundle sheath cells.
$1$. The primary fixation of $CO_2$ occurs in the mesophyll cells.
$2$. $CO_2$ is accepted by Phosphoenolpyruvate $(PEP)$ with the help of the enzyme $PEP$ carboxylase to form a $4$-carbon compound,Oxaloacetic acid $(OAA)$.
$3$. This $OAA$ is then converted into other $4$-carbon compounds like malic acid or aspartic acid in the mesophyll cells.
$4$. Therefore,the synthesis of malic acid occurs in the mesophyll cells.

(B) In the $C_4$ photosynthetic pathway,the primary carbon dioxide fixation occurs in the mesophyll cells.
$PEP$ (Phosphoenolpyruvate) carboxylase is the key enzyme responsible for the initial fixation of $CO_2$ into a $4$-carbon compound called oxaloacetate.
Unlike $C_3$ plants,where $RuBP$ carboxylase $(RuBisCO)$ is the primary enzyme,$C_4$ plants utilize $PEP$ carboxylase because it has a high affinity for $CO_2$ and does not show oxygenase activity.

(D) $C_4$ plants are those that utilize the $C_4$ dicarboxylic acid pathway for carbon fixation.
Common examples of $C_4$ plants include maize ($Zea$ $mays$), sugarcane ($Saccharum$ $officinarum$), and various species of the genus $Atriplex$.
Since all the options provided ($Maize$, $Atriplex$, and $Sugarcane$) are $C_4$ plants, the correct answer is $D$.

(B) $C_4$ plants are more efficient in photosynthesis than $C_3$ plants primarily because they possess a specialized leaf anatomy known as $Kranz$ anatomy.
In $C_4$ plants,the bundle sheath cells contain a large number of chloroplasts,which allows for a more efficient $CO_2$ fixation process.
This structural adaptation minimizes photorespiration,which is a wasteful process that occurs in $C_3$ plants,thereby increasing the overall photosynthetic efficiency of $C_4$ plants.

(A) $CAM$ stands for Crassulacean Acid Metabolism.
$CAM$ plants are primarily xerophytes,which are plants adapted to survive in environments with little liquid water,such as deserts or ice-covered regions.
These plants have evolved the $CAM$ pathway to minimize photorespiration and water loss by opening their stomata only at night to take in $CO_2$ and fixing it into organic acids,which are then used for photosynthesis during the day.

(B) In $C_4$ plants,the leaves exhibit a special anatomy called Kranz anatomy.
In this anatomy,the bundle sheath cells are large,thick-walled,and contain a large number of chloroplasts.
These chloroplasts are responsible for the Calvin cycle,which occurs in the bundle sheath cells of $C_4$ plants,unlike $C_3$ plants where it occurs in the mesophyll cells.

(B) $C_4$ plants exhibit a specialized leaf anatomy known as $Kranz$ anatomy.
In this anatomy,the leaves possess two distinct types of photosynthetic cells:
$1$. Mesophyll cells: These cells perform the initial $CO_2$ fixation using the enzyme $PEP$ carboxylase.
$2$. Bundle sheath cells: These cells perform the Calvin cycle ($C_3$ cycle) by utilizing the $CO_2$ released from the $C_4$ acids.
Therefore,there are two types of photosynthetic cells present in $C_4$ plants.

(B) In $C_4$ plants,the process of photosynthesis involves two types of cells: mesophyll cells and bundle sheath cells.
$1$. In the mesophyll cells,$CO_2$ is initially fixed by the enzyme $PEP$ carboxylase to form a $4$-carbon compound,oxaloacetate.
$2$. This $4$-carbon compound is then transported to the bundle sheath cells.
$3$. In the bundle sheath cells,the $4$-carbon compound is decarboxylated to release $CO_2$.
$4$. This released $CO_2$ is then fixed by the enzyme $RuBisCO$ using $RuBP$ (Ribulose $1,5$-bisphosphate) in the Calvin cycle,which occurs exclusively in the bundle sheath cells of $C_4$ plants.

(C) In the $C_4$ pathway (Hatch-Slack cycle),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.
The $CO_2$ combines with $PEP$ to form a $4$-carbon compound called Oxaloacetic acid $(OAA)$.
Therefore,the first step involves the fixation of $CO_2$ by $PEP$.

(C) $C_4$ plants exhibit a specialized leaf anatomy known as Kranz anatomy.
In this anatomy,the bundle sheath cells are large and contain numerous chloroplasts,which are essential for the $C_4$ photosynthetic pathway.
This pathway is highly efficient as it minimizes photorespiration by concentrating $CO_2$ around the enzyme $RuBisCO$ in the bundle sheath cells,allowing these plants to thrive in high-temperature and high-light intensity environments.

(B) In $C_4$ plants,the process of carbon fixation occurs in two stages in different types of cells.
$1$. The primary $CO_2$ fixation occurs in the mesophyll cells,where $CO_2$ is converted into a $4$-carbon compound called oxaloacetate by the enzyme $PEP$ carboxylase.
$2$. The $4$-carbon compound is then transported to the bundle sheath cells.
$3$. In the bundle sheath cells,the $4$-carbon compound is broken down to release $CO_2$,which is then fixed again by the $RuBisCO$ enzyme in the Calvin cycle.
Therefore,the final carbon fixation (re-fixation) occurs in the chloroplasts of the bundle sheath cells.

(B) In $C_4$ plants,the primary $CO_2$ fixation occurs in the mesophyll cells.
$1$. The enzyme $PEP$ carboxylase $(PEPCase)$ captures $CO_2$ in the mesophyll cells to form a $4$-carbon compound called oxaloacetic acid $(OAA)$.
$2$. This $OAA$ is then converted into malic acid (or aspartic acid) within the same mesophyll cells.
$3$. The malic acid is subsequently transported to the bundle sheath cells for the Calvin cycle.

(D) $C_4$ plants are more efficient because they have evolved a mechanism to minimize photorespiration.
In $C_4$ plants,the $CO_2$ released during photorespiration (if any occurs) or the $CO_2$ concentrated in the bundle sheath cells is efficiently refixed by the enzyme $PEP$ carboxylase.
$PEP$ carboxylase has a very high affinity for $CO_2$ and does not bind with $O_2$,unlike $RuBisCO$.
This mechanism ensures that $CO_2$ loss is minimized and the photosynthetic rate remains high even under conditions of low $CO_2$ concentration.

(A) $C_4$ plants,such as maize and sugarcane,are specifically adapted to environments with high temperatures and intense light,which are often associated with dry conditions.
These plants possess a specialized anatomy known as $Kranz$ anatomy,which allows them to minimize photorespiration.
By concentrating $CO_2$ around the enzyme $RuBisCO$,$C_4$ plants maintain high photosynthetic efficiency even under conditions where $C_3$ plants would lose significant energy due to photorespiration.
Therefore,$C_4$ plants are highly adapted to hot and dry climates.

(C) The '$Kranz$' anatomy is a specialized structure found in the leaves of $C_4$ plants (e.g.,maize,sugarcane).
In this anatomy,the mesophyll cells are arranged in a ring-like manner around the bundle sheath cells.
This arrangement facilitates the efficient transport of $CO_2$ and minimizes photorespiration,allowing these plants to thrive in high-temperature and high-light intensity environments.

(B) The $CO_2$ compensation point is the concentration of $CO_2$ at which the rate of photosynthesis exactly equals the rate of respiration.
$C_4$ plants are more efficient at photosynthesis than $C_3$ plants because they possess a $CO_2$ concentrating mechanism (Kranz anatomy) that minimizes photorespiration.
Due to this mechanism,$C_4$ plants can maintain a positive net photosynthetic rate even at very low concentrations of $CO_2$.
Therefore,$C_4$ plants have a lower $CO_2$ compensation point $(0-10 \ ppm)$ compared to $C_3$ plants $(20-100 \ ppm)$.

(A) $CAM$ stands for Crassulacean Acid Metabolism.
This is a specialized photosynthetic pathway adapted by plants growing in arid or semi-arid environments.
In $CAM$ plants,the stomata remain closed during the day to prevent excessive transpiration and open only at night to take in $CO_2$.
This mechanism significantly reduces water loss,thereby helping the plants survive in water-scarce conditions.
Therefore,$CAM$ primarily helps in water conservation.

(A) In $C_4$ plants,the primary $CO_2$ fixation occurs in the mesophyll cells.
$PEP$ carboxylase (Phosphoenolpyruvate carboxylase) acts on $PEP$ (a $3$-carbon compound) and $CO_2$ to form oxaloacetate,which is a $4$-carbon compound.
This process is the initial step of the $C_4$ pathway (Hatch-Slack pathway) and takes place specifically in the mesophyll cells before the $CO_2$ is transported to the bundle sheath cells for the Calvin cycle.

(A) 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.
$RuBP$ is the primary $CO_2$ acceptor in $C_3$ plants,not $C_4$ plants.

(D) In $C_3$ plants,the synthesis of one molecule of glucose $(C_6H_{12}O_6)$ requires $18$ $ATP$ and $12$ $NADPH$ molecules.
In $C_4$ plants,an additional $6$ $ATP$ molecules are required for the regeneration of phosphoenolpyruvate $(PEP)$ in the mesophyll cells,totaling $30$ $ATP$ $(18 + 12 = 30)$ and $12$ $NADPH$ molecules.
Therefore,the additional $ATP$ requirement for $C_4$ plants compared to $C_3$ plants is $30 - 18 = 12$ $ATP$ molecules per hexose molecule.

(C) $C_4$ plants are characterized by the presence of Kranz anatomy,which allows them to perform efficient photosynthesis even in high-temperature and low-$CO_2$ conditions.
These plants are not restricted to a single group of angiosperms.
$C_4$ pathway is found in many species of both monocots (e.g.,maize,sugarcane,sorghum) and dicots (e.g.,Amaranthus,Atriplex).
Therefore,$C_4$ plants are found in both monocots and dicots.

(A) Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants (such as maize,sugarcane,and sorghum).
In this anatomy,the mesophyll cells are arranged in a ring-like manner around the bundle sheath cells.
The bundle sheath cells contain large numbers of chloroplasts,which are essential for the $C_4$ photosynthetic pathway to minimize photorespiration.
Therefore,the correct option is $A$.

(A) In $C_4$ plants,the primary fixation of atmospheric $CO_2$ occurs in the mesophyll cells.
This process is catalyzed by the enzyme $PEP$ carboxylase ($Phosphoenolpyruvate$ carboxylase).
$PEP$ carboxylase combines $CO_2$ with $Phosphoenolpyruvate$ $(PEP)$ to form a $4$-carbon compound called Oxaloacetic acid $(OAA)$.

(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 facilitates the efficient fixation of $CO_2$ and minimizes photorespiration,which is a key adaptation for plants in high-temperature and high-light environments.

(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 $(PEPCase)$.
$PEP + CO_2 + H_2O \xrightarrow{PEPCase} \text{Oxaloacetic acid } (OAA)$.
$RuBP$ is the primary $CO_2$ acceptor in $C_3$ plants, while $OAA$ is the first stable product formed in $C_4$ plants.

(C) 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 called oxaloacetate.
$2$. This $4$-carbon compound is then 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 in $C_4$ plants takes place exclusively in the chloroplasts of the bundle sheath cells.

(B) $CAM$ (Crassulacean Acid Metabolism) plants exhibit a unique adaptation to conserve water in arid environments.
During the night,these plants open their stomata and fix $CO_2$ into organic acids,primarily malic acid,using the enzyme $PEP$ carboxylase.
This malic acid is stored in the vacuoles of the cells.
During the day,the stomata remain closed to prevent water loss.
The stored malic acid is then transported out of the vacuole and decarboxylated to release $CO_2$ internally.
This released $CO_2$ is then utilized by the Calvin cycle ($C_3$ cycle) for photosynthesis.
Therefore,malic acid serves as the internal source of $CO_2$ during the day.

(D) $C_4$ plants differ from $C_3$ plants primarily in the mechanism of carbon fixation.
In $C_3$ plants,the primary acceptor of $CO_2$ is $RuBP$ (a $5$-carbon compound),and the first stable product is $3-PGA$ (a $3$-carbon compound).
In $C_4$ plants,the primary acceptor of $CO_2$ is $PEP$ (Phosphoenolpyruvate,a $3$-carbon compound),and the first stable product is $OAA$ (Oxaloacetic acid,a $4$-carbon compound).
Therefore,they differ in the substrate used for carbon assimilation and the nature of the first stable product formed.

(B) In $C_4$ plants,the primary $CO_2$ fixation occurs in the mesophyll cells.
$CO_2$ is initially accepted by phosphoenolpyruvate $(PEP)$ to form oxaloacetic acid $(OAA)$ in the presence of the enzyme $PEP$ carboxylase.
This process takes place in the chloroplasts of the mesophyll cells,which lack the enzyme $RuBisCO$ but are rich in $PEP$ carboxylase.

(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.
The $CO_2$ fixation results in the formation of a $4$-carbon compound,Oxaloacetic acid $(OAA)$,which is the first stable product of the $C_4$ cycle.

(B) Kranz anatomy is a specialized structure found in $C_4$ plants like maize,sugarcane,and Atriplex.
In this anatomy,the bundle sheath cells are large and contain numerous chloroplasts with poorly developed or absent grana.
The mesophyll cells,on the other hand,contain smaller chloroplasts with well-developed grana.
Therefore,the statement that mesophyll cells contain large chloroplasts is incorrect.
$C_4$ plants are indeed more efficient in photosynthesis compared to $C_3$ plants due to the absence of photorespiration.

(C) Kranz anatomy is a specialized structure found in the leaves of $C_4$ plants (like maize and sugarcane).
In this anatomy,the bundle sheath cells are arranged in a ring around the vascular bundles.
These bundle sheath cells are characterized by having thick walls that are impervious to gaseous exchange,the absence of intercellular spaces,and the presence of a large number of chloroplasts to facilitate the Calvin cycle.

(D) The $C_4$ pathway of photosynthesis is primarily found in plants adapted to high temperatures and high light intensities.
Many monocotyledonous plants,particularly those belonging to the family $Gramineae$ (also known as $Poaceae$),exhibit the $C_4$ photosynthetic pathway.
Examples include sugarcane,maize,and various grasses.
Therefore,the correct option is $D$.