C4 Questions in English

(D) The reaction $PEP + HCO_3^{-} \rightarrow OAA$ (Oxaloacetic acid) is the primary carboxylation step in the $C_4$ pathway of photosynthesis.
This reaction occurs in the mesophyll cells of $C_4$ plants.
The enzyme that catalyzes this reaction is $PEP$ carboxylase (Phosphoenolpyruvate carboxylase).
$PEP$ carboxylase has a high affinity for $CO_2$ (in the form of $HCO_3^{-}$) and does not show oxygenase activity,which helps in efficient carbon fixation even at low $CO_2$ concentrations.

(C) In the $C_3$ pathway (Calvin cycle),the fixation of one molecule of $CO_2$ requires $3$ $ATP$ and $2$ $NADPH$ molecules.
In the $C_4$ pathway,there is an additional step of $CO_2$ fixation by $PEP$ carboxylase and the regeneration of $PEP$ (Phosphoenolpyruvate) from pyruvate,which consumes $2$ additional $ATP$ molecules.
Therefore,the $C_4$ pathway requires a total of $5$ $ATP$ ($3$ for the Calvin cycle + $2$ for $PEP$ regeneration) per $CO_2$ molecule fixed.
The difference in $ATP$ requirement is $5 - 3 = 2$ $ATP$ molecules.

(B) Desert plants are adapted to survive in water-scarce environments.
$1$. They possess a thick cuticle on their leaf surfaces to minimize water loss through transpiration.
$2$. They follow the $CAM$ (Crassulacean Acid Metabolism) pathway for photosynthesis,which allows them to fix $CO_2$ at night.
$3$. Consequently,their stomata remain closed during the day to prevent water loss and open at night to take in $CO_2$.
$4$. Stomata in desert plants are typically 'sunken' (sunken stomata) to reduce water loss,not located on the surface of the epidermis. Therefore,the statement that stomata are located on the epidermis is incorrect.

(C) In $C_4$ plants,the photosynthetic process is divided between two types of cells: mesophyll cells and bundle sheath cells.
$1$. The $C_4$ cycle (initial $CO_2$ fixation) occurs in the mesophyll cells,where $CO_2$ is fixed into a $4$-carbon compound called oxaloacetate.
$2$. The $C_3$ cycle (Calvin cycle) occurs in the bundle sheath cells,where the $4$-carbon compound is transported and decarboxylated to release $CO_2$,which is then fixed by the enzyme $RuBisCO$ in the Calvin cycle.
Therefore,the site of the $C_3$ cycle in $C_4$ plants is the bundle sheath cell.

(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 (like maize),the $C_4$ cycle requires an additional $2$ $ATP$ molecules per $CO_2$ molecule fixed for the regeneration of 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,a $C_4$ plant requires $18 + 12 = 30$ $ATP$ molecules in total for the synthesis of one glucose molecule.

(D) In $C_4$ plants,the $C_4$ cycle involves two types of cells: mesophyll cells and bundle sheath cells.
$1$. In the mesophyll cells,$CO_2$ is fixed into a $4$-carbon compound (oxaloacetic acid),which is then converted into malic acid or aspartic acid.
$2$. These $4$-carbon acids are transported to the bundle sheath cells,where they undergo decarboxylation to release $CO_2$ for the Calvin cycle.
$3$. The remaining $3$-carbon compound,pyruvic acid,is then transported back from the bundle sheath cells to the mesophyll cells to regenerate the primary $CO_2$ acceptor,phosphoenolpyruvate $(PEP)$.

(B) In $C_4$ plants,the process of photosynthesis is divided between two types of cells: mesophyll cells and bundle sheath cells.
$1$. Mesophyll cells: These cells contain the enzyme $PEPCase$ (Phosphoenolpyruvate carboxylase) but lack the enzyme $\text{RuBisCo}$. Therefore,the initial fixation of $CO_2$ occurs here.
$2$. Bundle sheath cells: These cells contain the enzyme $\text{RuBisCo}$ and are the site of the Calvin cycle.
Since $\text{RuBisCo}$ is absent in the mesophyll cells of $C_4$ plants,option $(B)$ is the correct answer.

(C) The correct answer is $C$.
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.
Maize $(Zea \, mays)$ is a classic example of a $C_4$ plant, which exhibits this characteristic anatomy to minimize photorespiration.

(D) $PEP$ carboxylase $(PEPcase)$ is an enzyme that catalyzes the carboxylation of phosphoenolpyruvate $(PEP)$ in $C_4$ plants.
It is primarily involved in the initial carbon fixation step in $C_4$ plants and is not found in the mesophyll cells of $C_3$ plants.

(A) The correct answer is $A$.
In $C_3$ plants,the Calvin cycle ($C_3$ cycle) occurs exclusively in the mesophyll cells.
However,in $C_4$ plants,the process is compartmentalized: the initial $CO_2$ fixation occurs in the mesophyll cells to form a $4$-carbon compound,which is then transported to the bundle sheath cells.
Therefore,the $C_3$ cycle (Calvin cycle) takes place specifically in the bundle sheath cells of $C_4$ plants.

(D) The correct answer is $D$.
Maize is a $C_4$ plant. In $C_4$ plants,$CO_2$ fixation occurs in two stages in different cell types.
In the mesophyll cells,the primary $CO_2$ acceptor is phosphoenol pyruvate $(PEP)$,a $3$-carbon molecule. The enzyme responsible for this initial fixation is $PEP$ carboxylase $(PEPcase)$.
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 $RuBisCO$ via the Calvin cycle to produce sugars.
Therefore,the enzymes involved in mesophyll and bundle sheath cells are $PEP$ carboxylase and $RuBisCO$,respectively.