Kreb's cycle Questions in English

(B) The Kreb's cycle,also known as the Citric Acid Cycle or Tricarboxylic Acid $(TCA)$ cycle,is a series of chemical reactions used by all aerobic organisms to release stored energy.
It begins with the condensation of acetyl-CoA with oxaloacetic acid $(OAA)$ to form citric acid.
Throughout the cycle,a series of enzymatic reactions occur,involving various intermediates such as isocitrate,alpha-ketoglutarate,succinyl-CoA,succinate,fumarate,and malate.
The cycle is completed when malate is oxidized to regenerate oxaloacetic acid $(OAA)$,which then acts as the acceptor for another molecule of acetyl-CoA to restart the cycle.
Therefore,the regeneration of $OAA$ marks the completion of the cycle.

(D) The Krebs' cycle begins with the condensation reaction of acetyl Co-$A$ $(2C)$ with oxaloacetic acid $(4C)$ to form citric acid $(6C)$.
This reaction is catalyzed by the enzyme citrate synthase in the presence of water.

(D) In the Krebs' cycle,the conversion of Succinic acid to Fumaric acid is catalyzed by the enzyme Succinic acid dehydrogenase. In this specific reaction,$FAD$ is reduced to $FADH_2$ instead of $NAD$ being reduced to $NADH + H^+$.
In contrast,the conversion of Isocitric acid to $\alpha$-ketoglutaric acid,Malic acid to Oxaloacetic acid,and Pyruvic acid to Acetyl Co-$A$ all involve the reduction of $NAD^+$ to $NADH + H^+$.

(B) Aerobic respiration occurs within the mitochondria. The final product of glycolysis,pyruvate,is transported from the cytoplasm into the mitochondria.
The major events in aerobic respiration are:
$1$. The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms,which releases $3$ molecules of $CO_{2}$.
$2$. The passing on of the electrons removed as part of the hydrogen atoms to molecular $O_{2}$ with the simultaneous synthesis of $ATP$.

(C) Pyruvic acid,generated in the cytosol,is transported into the mitochondria to initiate the second phase of respiration.
Before pyruvic acid enters the Krebs cycle,which occurs in the mitochondrial matrix,it undergoes a reaction known as oxidative decarboxylation.
In this process,one of the three carbon atoms of pyruvic acid is oxidized to $CO_2$,and the remaining two-carbon acetyl group is attached to Coenzyme $A$ to form Acetyl-$CoA$.

(B) Pyruvic acid is a $3$-carbon compound.
In the process of oxidative decarboxylation,one carbon atom of pyruvic acid is oxidized to carbon dioxide $(B = \text{CO}_2)$.
Pyruvate is first decarboxylated and then oxidized by the enzyme complex pyruvate dehydrogenase.
The remaining $2$-carbon acetate unit combines with coenzyme $A$ (CoA) to form acetyl CoA $(A = \text{Acetyl CoA})$.
The reaction is: $\text{Pyruvic acid} + \text{CoA} + \text{NAD}^+ \xrightarrow{\text{Mg}^{2+}} \text{Acetyl CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+$.

(B) In the Krebs' cycle,the sequence of carbon compounds is as follows:
$1$. Citric acid $(6C)$ is converted to Isocitric acid $(6C)$.
$2$. Isocitric acid is oxidized to Oxalosuccinic acid,which is a $6$-carbon compound.
$3$. Oxalosuccinic acid then undergoes decarboxylation to form $\alpha$-ketoglutarate,which is a $5$-carbon compound.
$4$. Malate is a $4$-carbon compound.
$5$. Pyruvic acid is a $3$-carbon compound.
Therefore,Oxalosuccinic acid is a $6$-carbon compound.

(A) In the first reaction of the citric acid cycle,one molecule of acetyl Co-$A$ ($2$ carbons) condenses with a $4$-carbon compound,oxaloacetic acid $(OAA)$,in the presence of water to form a $6$-carbon compound,citric acid,and Co-$A$ is released. This reaction is catalyzed by the enzyme citrate synthase.

(B) In the Krebs' cycle,when Succinic acid undergoes oxidation (dehydrogenation) to form Fumaric acid,two hydrogen atoms are transferred to $FAD$.
$FAD$ is reduced to $FADH_2$,and the enzyme involved in this specific step is Succinic acid dehydrogenase.

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

(D) The $TCA$ cycle (also known as the Krebs cycle) occurs primarily in the mitochondrial matrix.
However,the enzyme $Succinate$ $dehydrogenase$ is an exception.
It is embedded in the inner mitochondrial membrane in eukaryotes,where it also functions as $Complex$ $II$ of the electron transport chain.
In prokaryotes,this enzyme is located in the cytosol.

(D) Mitochondria generate the majority of biological energy through the oxidation of $TCA$ (tricarboxylic acid) cycle substrates.
In the $TCA$ cycle,substrates are oxidized,releasing $NADH + H^{+}$,which subsequently enters the electron transport chain to produce $ATP$ molecules.
Specifically,for one glucose molecule,the $TCA$ cycle produces $6$ $NADH + H^{+}$,which yields $18$ $ATP$ via the electron transport chain,and $2$ $FADH_{2}$,which yields $4$ $ATP$.
Thus,the oxidation of $TCA$ substrates is the primary mechanism for energy production in mitochondria.

(B) The synthesis of $ATP$ or $GTP$ from $ADP$ or $GDP$ during metabolic pathways without the involvement of an electron transport chain is known as substrate-level phosphorylation.
In the Krebs' cycle,the conversion of succinyl-$CoA$ to succinate is catalyzed by the enzyme succinyl-$CoA$ synthetase.
This reaction is coupled with the phosphorylation of $GDP$ to $GTP$,which is a classic example of substrate-level phosphorylation.

(A) In aerobic respiration,one molecule of glucose produces two molecules of acetyl Co-$A$ through glycolysis and the transition reaction.
Each acetyl Co-$A$ molecule enters the Krebs' cycle and produces $1$ $ATP$ (or $GTP$) molecule via substrate-level phosphorylation.
Since there are two acetyl Co-$A$ molecules per glucose,the total yield of $ATP$ directly from the Krebs' cycle is $2$ $ATP$ molecules.

(A) The conversion of Pyruvic acid to Acetyl $CoA$ is known as the link reaction or oxidative decarboxylation of pyruvate.
This reaction is catalyzed by the multi-enzyme complex known as Pyruvate dehydrogenase.
This complex requires several cofactors including $NAD^+$,$CoA$,$TPP$,$Mg^{2+}$,and Lipoic acid to function.

(B) During glycolysis,one molecule of glucose ($6$ carbons) is broken down into two molecules of pyruvic acid ($3$ carbons each).
Each molecule of pyruvic acid undergoes oxidative decarboxylation to form one molecule of acetyl $CoA$ ($2$ carbons).
Since there are two molecules of pyruvic acid produced from one glucose molecule,the total number of acetyl $CoA$ molecules formed is $2 \times 1 = 2$.

(A) Acetyl $CoA$ is a two-carbon compound.
It is formed by the oxidative decarboxylation of pyruvate ($3$ carbons) during the link reaction (pyruvate dehydrogenase complex).
The acetyl group $(CH_3CO-)$ contains two carbon atoms,which are then attached to Coenzyme $A$ $(CoA)$.
Therefore,the correct answer is $2$.

(C) The Krebs cycle (also known as the Citric Acid Cycle) occurs in the mitochondrial matrix.
During this process,acetyl $CoA$ is oxidized to produce $CO_2$,$ATP$ (or $GTP$),$NADH$,and $FADH_2$.
Among the given options,$ATP$ is a direct product of the Krebs cycle.

(B) In the Krebs cycle, the first step is the condensation reaction where the $2$-carbon compound Acetyl $CoA$ combines with the $4$-carbon compound Oxaloacetate $(OAA)$ in the presence of water and the enzyme Citrate synthase to form a $6$-carbon compound called Citric acid (Citrate).
Reaction: $OAA + \text{Acetyl } CoA + H_2O \xrightarrow{\text{Citrate synthase}} \text{Citric acid} + CoA + H^+$

(C) In the Krebs cycle ($TCA$ cycle),$NADH + H^+$ is produced during three oxidative decarboxylation or oxidation steps:
$1$. Conversion of Isocitrate to $\alpha$-ketoglutaric acid by the enzyme Isocitrate dehydrogenase.
$2$. Conversion of $\alpha$-ketoglutaric acid to Succinyl-CoA by the enzyme $\alpha$-ketoglutaric dehydrogenase.
$3$. Conversion of Malic acid to Oxaloacetic acid $(OAA)$ by the enzyme Malic dehydrogenase.
Therefore,the correct enzymes are Isocitrate dehydrogenase,$\alpha$-ketoglutaric dehydrogenase,and Malic dehydrogenase.

(C) In one turn of the Krebs cycle,two molecules of $CO_2$ are released (one during the conversion of isocitrate to $\alpha$-ketoglutarate and one during the conversion of $\alpha$-ketoglutarate to succinyl-CoA).
Since one glucose molecule produces two molecules of acetyl-CoA,the Krebs cycle runs twice for every glucose molecule.
Therefore,in two turns of the Krebs cycle,$2 \times 2 = 4$ molecules of $CO_2$ are released.

(C) The Krebs cycle (also known as the Citric Acid Cycle) occurs in the mitochondrial matrix.
In one complete turn of the Krebs cycle,one molecule of Acetyl-CoA is oxidized.
The net gain from one turn of the cycle is $3 NADH_2$,$1 FADH_2$,and $1 ATP$ (or $GTP$ which is equivalent to $ATP$ in energy terms).
Therefore,the correct option is $C$.

(B) Succinate dehydrogenase is an enzyme found in the inner mitochondrial membrane that plays a crucial role in the Krebs cycle (Citric Acid Cycle).
It catalyzes the oxidation of succinate to fumarate.
During this reaction,the electrons removed from succinate are transferred to the coenzyme $FAD$ (Flavin Adenine Dinucleotide),which gets reduced to $FADH_{2}$.
The reaction is: $Succinate + FAD \xrightarrow{\text{Succinate dehydrogenase}} Fumarate + FADH_{2}$.

(D) During a single Krebs cycle,$3$ molecules of $NADH$ are produced.
Each $NADH$ molecule yields $3$ $ATP$ molecules through oxidative phosphorylation in the electron transport system.
Therefore,the total number of $ATP$ molecules produced from $NADH$ in one Krebs cycle is $3 \times 3 = 9$ $ATP$.

(A) The $TCA$ cycle (also known as the Krebs cycle) begins with the condensation of an acetyl group from acetyl $CoA$ with a four-carbon compound, oxaloacetic acid $(OAA)$, to form a six-carbon compound, citric acid.
Therefore, $OAA$ acts as the primary acceptor of the acetyl group in the $TCA$ cycle.
Reaction: $OAA + \text{Acetyl } CoA \rightarrow \text{Citric acid}$.

(A) The $TCA$ cycle (Tricarboxylic Acid cycle) or Krebs cycle involves a series of enzymatic reactions where one molecule of Acetyl $CoA$ is oxidized.
During one turn of the $TCA$ cycle,the following steps produce reduced coenzymes:
$1$. Isocitrate to $\alpha$-Ketoglutarate: $1 NADH + H^+$ is produced.
$2$. $\alpha$-Ketoglutarate to Succinyl-$CoA$: $1 NADH + H^+$ is produced.
$3$. Succinate to Fumarate: $1 FADH_2$ is produced.
$4$. Malate to Oxaloacetate: $1 NADH + H^+$ is produced.
Thus,for one molecule of Acetyl $CoA$,the total yield is $3 NADH + H^+$ and $1 FADH_2$.

(C) In the $TCA$ cycle (Tricarboxylic Acid Cycle),also known as the $Krebs$ cycle,the $5C$ (five-carbon) intermediate molecule is $\alpha$-ketoglutaric acid.
It is formed by the oxidative decarboxylation of $isocitrate$.

(B) In the $TCA$ cycle (also known as the Krebs cycle),one molecule of Acetyl $CoA$ undergoes a series of reactions to produce the following energy-rich molecules:
$1$. $3$ molecules of $NADH$ are produced,which yield $3 \times 3 = 9$ $ATP$ molecules via oxidative phosphorylation.
$2$. $1$ molecule of $FADH_2$ is produced,which yields $2$ $ATP$ molecules via oxidative phosphorylation.
$3$. $1$ molecule of $GTP$ (or $ATP$) is produced directly via substrate-level phosphorylation,which is equivalent to $1$ $ATP$.
Adding these together,the total $ATP$ yield per Acetyl $CoA$ molecule is $9 + 2 + 1 = 12$ $ATP$.

(C) Substrate-level phosphorylation is a process where a phosphate group is transferred from a high-energy substrate molecule directly to $ADP$ or $GDP$ to form $ATP$ or $GTP$,respectively.
In the $TCA$ cycle (also known as the Krebs cycle),this occurs during the conversion of succinyl-$CoA$ to succinic acid.
This reaction is catalyzed by the enzyme succinyl-$CoA$ synthetase (or thiokinase).
During this step,the energy released from the cleavage of the high-energy thioester bond in succinyl-$CoA$ is used to synthesize $GTP$ (which is then converted to $ATP$) from $GDP$ and inorganic phosphate $(Pi)$.

(C) The enzyme isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate in the $TCA$ cycle.
This enzyme requires divalent metal ions as cofactors for its catalytic activity.
Specifically,$Mn^{2+}$ (manganese) or $Mg^{2+}$ (magnesium) ions act as essential mineral activators for this enzyme.
Among the given options,$Mn$ is the recognized mineral activator for isocitrate dehydrogenase.

(C) In the $Krebs$ cycle (also known as the $TCA$ cycle or Citric Acid Cycle),the enzyme $Fumarase$ (or $Fumarate$ $hydratase$) catalyzes the reversible hydration of $Fumaric$ $acid$ to $Malic$ $acid$.
This reaction involves the addition of a water molecule $(H_2O)$ across the double bond of $Fumaric$ $acid$ to produce $L-Malate$ (Malic acid).
Therefore,the correct conversion is $Fumaric$ $acid$ to $Malic$ $acid$.

(D) The Krebs cycle intermediate that serves as a raw material for chlorophyll synthesis is $Succinyl \ Co-A$.
This intermediate is produced during the conversion of $\alpha-Ketoglutaric \ acid$ to $Succinyl \ Co-A$.
This specific reaction involves the removal of a carbon dioxide molecule $(decarboxylation)$ and the oxidation of the substrate $(oxidation)$,coupled with the addition of Coenzyme $A$.
Therefore,this step is known as $Oxidative \ decarboxylation$.

(C) The Krebs cycle involves various intermediate organic acids that contain $4, 5,$ and $6$ carbon atoms.
For example:
- Citric acid,Isocitric acid,and Oxalosuccinic acid are $6C$ compounds.
- $\alpha$-Ketoglutaric acid is a $5C$ compound.
- Succinic acid,Fumaric acid,Malic acid,and Oxaloacetic acid are $4C$ compounds.
Therefore,option $C$ is the correct statement.

(B) The $TCA$ cycle (Krebs cycle) provides several intermediates that serve as precursors for the biosynthesis of various amino acids. Specifically,$\alpha$-ketoglutaric acid is a key intermediate in the $TCA$ cycle that undergoes transamination to form glutamic acid,which is a fundamental amino acid used in protein synthesis. Thus,it acts as a connecting link between respiratory metabolism and protein synthesis.

(D) The Krebs cycle (also known as the Citric Acid Cycle) involves a series of enzymatic reactions.
Starting from Isocitric acid,it undergoes oxidative decarboxylation to form $\alpha$-ketoglutaric acid.
Subsequently,$\alpha$-ketoglutaric acid undergoes further reactions to eventually regenerate Oxaloacetic acid,which is the starting substrate for the cycle.
Therefore,the correct sequence among the given options is: Isocitric acid $\rightarrow \alpha$-ketoglutaric acid $\rightarrow$ Oxaloacetic acid.

(B) The $SDH$ (Succinate dehydrogenase) enzyme is a unique component of the Krebs cycle because it is embedded in the inner mitochondrial membrane. It acts as a link between the Krebs cycle and the electron transport chain,functioning as Complex $II$.

(B) Succinyl $CoA$ is a key intermediate in the $TCA$ cycle (Krebs cycle).
It also serves as a precursor for the synthesis of porphyrins,which are pyrrole group-containing compounds such as chlorophyll,cytochromes,and hemoglobin.
Since both statements are scientifically accurate and the role of Succinyl $CoA$ in the $TCA$ cycle is a distinct metabolic fact from its role as a biosynthetic precursor,the Reason does not explain the Assertion.

(B) The given organelle is a mitochondrion.
$P$ represents the cristae (inner membrane folds).
$Q$ represents the matrix.
$R$ represents the intermembrane space.
$S$ represents the outer membrane.
The Krebs cycle (also known as the citric acid cycle) occurs in the mitochondrial matrix $(Q)$.
Therefore,the correct option is $B$.

(C) The $Krebs$ cycle,also known as the citric acid cycle or $TCA$ cycle,is a series of chemical reactions used by all aerobic organisms to generate energy. In eukaryotic cells,this process occurs in the mitochondrial matrix. The enzymes required for the $Krebs$ cycle are located within the matrix,except for succinate dehydrogenase,which is bound to the inner mitochondrial membrane.

(A) The reaction shown is the oxidative decarboxylation of pyruvic acid,which acts as a link reaction between glycolysis and the Krebs cycle.
This reaction is catalyzed by the Pyruvate Dehydrogenase Complex.
The Pyruvate Dehydrogenase Complex requires several cofactors,including $Mg^{2+}$ ions,$NAD^+$,$CoA$,$TPP$ (Thiamine pyrophosphate),and Lipoic acid.
Among the given options,Pyruvate dehydrogenase with $Mg^{2+}$ is the correct combination.

(C) When pyruvic acid enters the Krebs cycle (after conversion to Acetyl-CoA),it undergoes oxidation at the following steps:
$1$. Isocitrate to $\alpha$-ketoglutarate (Isocitrate dehydrogenase).
$2$. $\alpha$-ketoglutarate to Succinyl-CoA ($\alpha$-ketoglutarate dehydrogenase).
$3$. Succinate to Fumarate (Succinate dehydrogenase).
$4$. Malate to Oxaloacetate (Malate dehydrogenase).
Thus,there are $4$ steps of oxidation in the Krebs cycle.

(B) The chemical cycle shown in the image is the Krebs cycle (also known as the Citric Acid Cycle or $TCA$ cycle).
In eukaryotic cells,the Krebs cycle takes place in the mitochondrial matrix.
Pyruvate,produced during glycolysis in the cytoplasm,enters the mitochondria and is converted into Acetyl-CoA,which then enters the Krebs cycle within the matrix.

(B) The reaction involving the conversion of succinic acid to fumaric acid is catalyzed by the enzyme succinate dehydrogenase.
This enzyme is unique in the Krebs cycle because it is embedded in the inner mitochondrial membrane.
It also functions as Complex-$II$ of the electron transport chain $(ETC)$.
Therefore,the correct location for this specific enzyme is the inner mitochondrial membrane.

(D) The given reaction represents the $Krebs$ cycle (also known as the $Tricarboxylic$ $Acid$ $(TCA)$ cycle or $Citric$ $Acid$ cycle).
$1$. The $Krebs$ cycle occurs in the mitochondrial matrix of eukaryotic cells.
$2$. During this process, $Pyruvic$ $acid$ (derived from glycolysis) is completely oxidized to release $CO_2$, and high-energy electrons are transferred to $NAD^+$ and $FAD$ to form $NADH$ and $FADH_2$, respectively.
$3$. $ATP$ is produced via substrate-level phosphorylation.
Therefore, the correct location for this process is the mitochondrial matrix.

(A) The complete oxidation of one molecule of pyruvic acid involves the link reaction,the Krebs cycle,and the electron transport system.
$1$. In the link reaction (pyruvate to acetyl-CoA),no $ATP$ is produced by substrate-level phosphorylation.
$2$. In the Krebs cycle,one molecule of $GTP$ (which is equivalent to $ATP$) is produced per turn via substrate-level phosphorylation (conversion of succinyl-CoA to succinic acid).
$3$. Therefore,for one molecule of pyruvic acid,$1$ $ATP$ is produced by substrate-level phosphorylation.

(B) In the $TCA$ cycle (Krebs cycle),decarboxylation refers to the removal of a carbon atom in the form of $CO_2$.
$1$. The first decarboxylation occurs when isocitrate $(6C)$ is converted to $\alpha$-ketoglutarate $(5C)$ by the enzyme isocitrate dehydrogenase.
$2$. The second decarboxylation occurs when $\alpha$-ketoglutarate $(5C)$ is converted to succinyl-$CoA$ $(4C)$ by the $\alpha$-ketoglutarate dehydrogenase complex.
Therefore,decarboxylation occurs twice during a single $TCA$ cycle.

(A) The $TCA$ cycle (Tricarboxylic Acid cycle),also known as the Krebs cycle,involves a series of enzymatic reactions in the mitochondrial matrix.
In this cycle,isocitrate $(6-C)$ undergoes oxidative decarboxylation to form $\alpha$-ketoglutaric acid $(5-C)$.
This is the only step in the cycle where a $5-C$ compound is produced.
Therefore,the correct answer is $\alpha$-ketoglutaric acid.

(B) Oxidation involves the loss of electrons or hydrogen atoms from a substrate,which is typically coupled with the reduction of an electron carrier like $NAD^+$ or $FAD$.
In the tricarboxylic acid cycle ($TCA$ cycle):
$1$. Isocitrate $\rightarrow \alpha$-ketoglutaric acid involves the reduction of $NAD^+$ to $NADH + H^+$.
$2$. Succinic acid $\rightarrow$ Malic acid involves the reduction of $FAD$ to $FADH_2$.
$3$. Malic acid $\rightarrow$ Oxaloacetic acid involves the reduction of $NAD^+$ to $NADH + H^+$.
$4$. The conversion of Succinyl-$CoA$ to Succinic acid is a substrate-level phosphorylation step where $GDP$ (or $ADP$) is phosphorylated to $GTP$ (or $ATP$). This step does not involve the removal of electrons or hydrogen atoms from the substrate,hence it is not an oxidation reaction.

(D) In the citric acid cycle (Krebs cycle),substrate-level phosphorylation occurs during the conversion of succinyl-CoA to succinate.
This step is catalyzed by the enzyme succinyl-CoA synthetase,which produces one molecule of $GTP$ (or $ATP$) per turn of the cycle.
Since one molecule of glucose produces two molecules of acetyl-CoA through glycolysis and the link reaction,the citric acid cycle runs twice for each glucose molecule.
Therefore,the total number of substrate-level phosphorylations for one molecule of glucose in the citric acid cycle is $1 \times 2 = 2$.

(B) In the Krebs cycle ($TCA$ cycle):
$1$. Acetyl CoA combines with Oxaloacetic acid to form $A = \text{Citric acid}$.
$2$. $\alpha$-Ketoglutaric acid is converted to $C = \text{Succinyl CoA}$,releasing $B = CO_2$ and producing $NADH+H^+$.
$3$. Succinyl CoA is converted to Succinic acid,producing $GTP$ (or $ATP$).
$4$. Succinic acid is converted to Fumaric acid,producing $D = FADH_2$.
$5$. Fumaric acid is converted to Malic acid.
$6$. Between Citric acid $(A)$ and Malic acid,two molecules of $CO_2$ are released (one during the conversion of Isocitric acid to $\alpha$-Ketoglutaric acid and one during the conversion of $\alpha$-Ketoglutaric acid to Succinyl CoA).
Therefore,the correct labels are $A = \text{Citric acid}, B = CO_2, C = \text{Succinyl CoA}, D = FADH_2$ and the number of $CO_2$ molecules evolved is $2$.