Kreb's cycle Questions in English

(C) Pyruvate is a $3$-carbon compound $(CH_3COCOOH)$.
During aerobic respiration,pyruvate undergoes oxidative decarboxylation to form Acetyl-CoA,releasing $1$ molecule of $CO_2$.
Subsequently,the Acetyl-CoA enters the Krebs cycle ($TCA$ cycle),where the remaining $2$ carbons are released as $CO_2$ molecules.
Therefore,the complete oxidation of $1$ molecule of pyruvate results in the production of $3$ molecules of $CO_2$.

(C) The given reaction represents the Krebs cycle (Tricarboxylic Acid cycle) occurring in the mitochondrial matrix.
In one turn of the Krebs cycle, one molecule of Acetyl CoA enters the cycle.
It reacts with $3 NAD^{+}$, $FAD^{+}$, $GDP$ (or $ADP$), and $P_i$ to produce $3 NADH + H^{+}$, $FADH_2$, $GTP$ (or $ATP$), and $2 CO_2$.
However, the question asks for the stoichiometry of the overall process starting from Acetyl CoA.
Looking at the options, option $C$ correctly identifies the input as Acetyl CoA and the products as $3 CO_2$ (if considering the complete oxidation of the acetyl group) and $3 NADH$.
Therefore, $A = \text{Acetyl CoA}$, $B = 3 NAD^{+}$, $C = 3 CO_2$, and $D = 3 NADH$.

(A) In the Krebs cycle (also known as the Citric Acid Cycle),the conversion of succinyl $\text{CoA}$ to succinic acid is catalyzed by the enzyme succinyl $\text{CoA}$ synthetase.
This reaction involves the coupling of the cleavage of the high-energy thioester bond of succinyl $\text{CoA}$ with the phosphorylation of $\text{GDP}$ (or $\text{ADP}$) to form $\text{GTP}$ (or $\text{ATP}$).
This process is known as substrate-level phosphorylation because the phosphate group is transferred directly from a substrate to $\text{GDP}$/$\text{ADP}$ without the involvement of the electron transport chain.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the Assertion.

(B) In aerobic respiration,one molecule of acetyl $\text{CoA}$ ($2$-carbon compound) enters the Krebs cycle ($TCA$ cycle).
During the Krebs cycle,decarboxylation occurs at two specific steps:
$1$. Conversion of isocitrate to $\alpha$-ketoglutarate (catalyzed by isocitrate dehydrogenase).
$2$. Conversion of $\alpha$-ketoglutarate to succinyl $\text{CoA}$ (catalyzed by $\alpha$-ketoglutarate dehydrogenase).
Each of these steps releases one molecule of $\text{CO}_2$.
Therefore,a total of $2$ decarboxylation events occur during the complete oxidation of one molecule of acetyl $\text{CoA}$.

(C) In the citric acid cycle (also known as the Krebs cycle),the conversion of succinyl $\text{CoA}$ to succinic acid is catalyzed by the enzyme succinyl $\text{CoA}$ synthetase.
This reaction involves the substrate-level phosphorylation where a molecule of $\text{GDP}$ (guanosine diphosphate) is phosphorylated to $\text{GTP}$ (guanosine triphosphate) using an inorganic phosphate group.
Therefore,the correct option is $C$.

(B) During aerobic respiration,the pyruvate produced in the cytoplasm via glycolysis enters the mitochondrial matrix.
Here,it undergoes a reaction catalyzed by the pyruvate dehydrogenase complex.
This reaction involves the removal of a carbon dioxide molecule (decarboxylation) and the oxidation of the remaining two-carbon fragment (oxidation) to form Acetyl CoA.
Therefore,the process is known as oxidative decarboxylation.

(C) The Krebs cycle (also known as the Citric Acid Cycle) begins with the condensation of an acetyl group from $Acetyl-CoA$ $(2C)$ with a $4C$ compound called $Oxaloacetate$ to form a $6C$ compound called $Citrate$. This reaction is catalyzed by the enzyme $Citrate$ $Synthase$.

(D) The Krebs cycle (also known as the Citric Acid Cycle) involves a series of enzymatic reactions in the mitochondrial matrix.
Starting from Succinate,the sequence of reactions is as follows:
$1$. Succinate is oxidized to Fumarate by the enzyme succinate dehydrogenase.
$2$. Fumarate is hydrated to form Malate by the enzyme fumarase.
$3$. Malate is oxidized to Oxaloacetate $(OAA)$ by the enzyme malate dehydrogenase.
Therefore,the correct sequence is $Succinate \rightarrow Fumarate \rightarrow Malate \rightarrow OAA$.

(D) In one turn of the Krebs cycle,$1$ $ATP$ (or $GTP$ in some organisms) is formed directly through substrate-level phosphorylation.
This occurs during the conversion of succinyl-$CoA$ to succinic acid,catalyzed by the enzyme succinyl-$CoA$ synthetase.

(A) In the citric acid cycle (Krebs cycle),the conversion of isocitrate to oxalosuccinate is catalyzed by the enzyme isocitrate dehydrogenase.
This reaction involves the removal of hydrogen atoms from the substrate,which are transferred to $NAD^+$ to form $NADH + H^+$.
Since the process involves the removal of hydrogen (a form of oxidation),this reaction is classified as an oxidation reaction.

(C) The citric acid cycle (also known as the Krebs cycle) begins with the condensation of an acetyl group from acetyl-CoA $(2C)$ with oxaloacetic acid ($OAA$,$4C$) and water $(H_2O)$ to form citric acid $(6C)$.
This reaction is catalyzed by the enzyme citrate synthase.

(C) In one turn of the Krebs cycle,there are four oxidation steps where electrons are removed and transferred to electron carriers:
$1$. Isocitrate to Oxalosuccinic acid (Reduction of $NAD^+$ to $NADH + H^+$).
$2$. $\alpha$-Ketoglutarate to Succinyl-CoA (Reduction of $NAD^+$ to $NADH + H^+$).
$3$. Succinate to Fumarate (Reduction of $FAD$ to $FADH_2$).
$4$. Malate to Oxaloacetate (Reduction of $NAD^+$ to $NADH + H^+$).
Therefore,there are $4$ oxidation steps in total.

(B) In aerobic respiration, the process begins with glycolysis, which occurs in the cytoplasm and does not release $CO_2$.
Following glycolysis, the pyruvic acid enters the mitochondrial matrix.
Here, it undergoes oxidative decarboxylation to form acetyl $CoA$, a reaction catalyzed by the pyruvate dehydrogenase complex.
During this step, one molecule of $CO_2$ is released for each molecule of pyruvic acid, making it the first instance of $CO_2$ release in aerobic respiration.
Therefore, the correct option is $B$.

(C) The correct matching is as follows:
$1$. $i.$ Isocitrate dehydrogenase catalyzes the conversion of Isocitrate to Oxalosuccinic acid $(i-d)$.
$2$. $ii.$ Fumarase catalyzes the hydration of Fumarate to Malate $(ii-c)$.
$3$. $iii.$ Aconitase catalyzes the conversion of Citrate to Cis-aconitate $(iii-a)$.
$4$. $iv.$ Succinate dehydrogenase catalyzes the oxidation of Succinate to Fumarate $(iv-b)$.
Therefore,the correct sequence is $i-d, ii-c, iii-a, iv-b$.

(B) The Krebs cycle (also known as the Citric Acid Cycle) processes one molecule of acetyl CoA at a time.
For one molecule of acetyl CoA,the products generated are $3 NADH$,$1 FADH_2$,and $1 GTP$ (or $ATP$).
Since the question asks for the products from two molecules of acetyl CoA,we multiply the products of one cycle by $2$.
Therefore,$2 \times (3 NADH + 1 FADH_2 + 1 GTP) = 6 NADH + 2 FADH_2 + 2 GTP$.

(C) Acetyl CoA is formed during the link reaction (pyruvate oxidation) where pyruvate $(3C)$ undergoes oxidative decarboxylation to form acetyl CoA $(2C)$.
The acetyl group $(CH_3CO-)$ attached to Coenzyme $A$ consists of $2$ carbon atoms.
Therefore,the correct answer is $2$.

(B) In the $TCA$ cycle (also known as the $Krebs$ cycle),decarboxylation occurs at two specific steps:
$1$. The conversion of isocitrate to $\alpha$-ketoglutarate,where one molecule of $CO_2$ is released.
$2$. The conversion of $\alpha$-ketoglutarate to succinyl-$CoA$,where another molecule of $CO_2$ is released.
Among the given options,$\alpha$-ketoglutarate is the compound formed after the first decarboxylation step of the cycle.

(B) In the Krebs cycle (also known as the Citric Acid Cycle),the oxidation of succinate to fumarate is catalyzed by the enzyme succinate dehydrogenase.
During this specific reaction,two hydrogen atoms are removed from succinate and transferred to the electron carrier $FAD$ (Flavin Adenine Dinucleotide).
This process results in the reduction of $FAD$ to $FADH_2$.
Therefore,$FADH_2$ is formed during the conversion of succinate to fumarate.

(D) In the Krebs cycle,isocitrate undergoes oxidation to form oxalosuccinate,which is a $6C$ compound.
Subsequently,oxalosuccinate undergoes decarboxylation (removal of $CO_2$) to form $\alpha$-ketoglutarate,which is a $5C$ compound.
This reaction is catalyzed by the enzyme isocitrate dehydrogenase.

(C) The Krebs cycle (also known as the Citric Acid Cycle) occurs in the mitochondrial matrix.
For every single molecule of Acetyl-CoA that enters the cycle,the following products are formed:
$1$. $3$ molecules of $NADH + H^+$
$2$. $1$ molecule of $FADH_2$
$3$. $1$ molecule of $GTP$ (or $ATP$)
Since one glucose molecule produces two Acetyl-CoA molecules,the cycle turns twice per glucose molecule. However,the question asks for a 'single turn'.
Therefore,a single turn yields $3 NADH_2$,$1 FADH_2$,and $1$ $GTP$. Note: Option $C$ is the most accurate representation of the products per turn.

(A) Citric acid is a tricarboxylic acid formed by the condensation of acetyl-CoA $(2C)$ with oxaloacetic acid $(4C)$,resulting in a $6C$ compound.
Oxaloacetic acid $(OAA)$ is a $4C$ dicarboxylic acid that acts as the initial acceptor in the Krebs cycle.
Pyruvic acid is the end product of glycolysis and is a $3C$ compound.
Therefore,the number of carbon atoms in citric acid,oxaloacetic acid,and pyruvic acid are $6, 4,$ and $3$ respectively.

(A) In the Krebs cycle (Tricarboxylic Acid Cycle),$1$ molecule of citrate $(6C)$ is converted back to oxaloacetate $(4C)$.
During this process,the following steps occur:
$1$. Decarboxylation: Two molecules of $CO_2$ are released. One during the conversion of isocitrate to $\alpha$-ketoglutarate,and another during the conversion of $\alpha$-ketoglutarate to succinyl-CoA.
$2$. Oxidation: Four oxidation steps occur where $NAD^+$ or $FAD$ are reduced to $NADH$ or $FADH_2$. These are: $(i)$ Isocitrate to $\alpha$-ketoglutarate,(ii) $\alpha$-ketoglutarate to succinyl-CoA,(iii) Succinate to fumarate,and (iv) Malate to oxaloacetate.
Therefore,there are $4$ oxidation steps and $2$ decarboxylation steps.

(D) In the Krebs cycle (also known as the Citric Acid Cycle),decarboxylation refers to the removal of a carbon atom in the form of $CO_2$.
$1$. The first decarboxylation occurs during the conversion of $Isocitrate$ $(6C)$ to $\alpha-Ketoglutarate$ $(5C)$.
$2$. The second decarboxylation occurs during the conversion of $\alpha-Ketoglutarate$ $(5C)$ to $Succinyl-CoA$ $(4C)$.
Therefore,decarboxylation takes place at $2$ steps in a single Krebs cycle.

(C) The correct answer is $C$.
The $TCA$ cycle (also known as the Krebs cycle or Citric acid cycle) begins with the condensation of an acetyl group $(2C)$ with Oxaloacetic acid $(4C)$ in the presence of water to form Citric acid $(6C)$.
This reaction is catalyzed by the enzyme Citrate synthase.

(A) The correct answer is $A$.
Glycolysis results in the production of $2$ molecules of $Pyruvic$ $Acid$ from one molecule of glucose.
In the presence of oxygen, $Pyruvic$ $Acid$ undergoes oxidative decarboxylation to form $Acetyl$ $CoA$.
$Acetyl$ $CoA$ acts as the connecting link between glycolysis and the $TCA$ cycle (Krebs cycle) by entering the mitochondrial matrix and initiating the cycle.

(C) The correct answer is $C$.
In the citric acid cycle (Krebs cycle),the organic acids have different numbers of carbon atoms.
$1$. Succinic acid is a $4C$ organic acid.
$2$. $\alpha$-Ketoglutaric acid is a $5C$ organic acid.
$3$. Citric acid is a $6C$ organic acid.
Therefore,the sequence $4C, 5C, 6C$ corresponds to Succinic acid,$\alpha$-Ketoglutaric acid,and Citric acid respectively.

(C) The correct answer is $C$.
In the given reaction,Pyruvic acid undergoes oxidative decarboxylation to form Acetyl CoA.
This reaction is catalyzed by the enzyme complex Pyruvate dehydrogenase in the presence of $Mg^{2+}$ ions.
Therefore,$X$ represents the enzyme Pyruvate dehydrogenase and $Y$ represents the product Acetyl CoA.
The overall reaction is:
$\text{Pyruvic acid} + \text{CoA} + \text{NAD}^+ \xrightarrow[\text{Pyruvate dehydrogenase}]{\text{Mg}^{2+}} \text{Acetyl CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+$

(C) The correct answer is $C$. Oxidative decarboxylation occurs during the formation of both acetyl CoA from pyruvate and succinyl CoA from $\alpha$-ketoglutarate.
In these reactions,a carboxyl group is removed as $CO_2$,and $NAD^+$ is reduced to $NADH$.
These steps are key parts of cellular respiration,particularly in the link reaction (pyruvate to acetyl CoA) and the Krebs cycle ($\alpha$-ketoglutarate to succinyl CoA).
$1. \text{Pyruvate} + CoA + NAD^+ \longrightarrow \text{Acetyl CoA} + CO_2 + NADH + H^+$
This reaction is catalyzed by the enzyme pyruvate dehydrogenase.
$2. \alpha\text{-Ketoglutarate} + CoA + NAD^+ \longrightarrow \text{Succinyl CoA} + CO_2 + NADH + H^+$
This reaction is catalyzed by the enzyme $\alpha$-ketoglutarate dehydrogenase.

(B) The correct answer is $B$ (mitochondrial matrix).
The Krebs cycle,also known as the citric acid cycle or $TCA$ cycle,takes place in the mitochondrial matrix.
During the cycle,acetyl $CoA$ is oxidized to produce $ATP$,$NADH$,$FADH_2$,and $CO_2$,which are essential for energy production in cells.
The mitochondrial matrix provides the necessary environment and enzymes for this process to occur efficiently.

(C) The correct answer is $C$. The conversion of pyruvate to acetyl CoA is a classic example of oxidative decarboxylation.
In this reaction,pyruvate $(3C)$ undergoes both oxidation and decarboxylation (loss of $CO_2$) to form acetyl CoA $(2C)$.
Simultaneously,$NAD^+$ is reduced to $NADH + H^+$.
This reaction is catalyzed by the pyruvate dehydrogenase complex and serves as a vital link between glycolysis and the Krebs cycle.

(A) matrix of mitochondria.
The pyruvate dehydrogenase complex,which is essential for the oxidative decarboxylation of pyruvic acid into acetyl $CoA$,is located in the mitochondrial matrix.
This reaction serves as a crucial link between glycolysis (which occurs in the cytoplasm) and the $Kreb's$ cycle (which occurs in the mitochondrial matrix).

(D) The correct match is $A-r, B-s, C-p, D-q$.
$1$. Oxaloacetate $(A)$ is a $4-C$ dicarboxylic acid involved in the Krebs cycle.
$2$. Phosphoglyceraldehyde $(B)$ is a $3-C$ triose phosphate intermediate in glycolysis.
$3$. Isocitrate $(C)$ is a $6-C$ tricarboxylic acid formed during the Krebs cycle.
$4$. $\alpha$-Ketoglutarate $(D)$ is a $5-C$ dicarboxylic acid formed after the decarboxylation of isocitrate in the Krebs cycle.