Kreb's cycle Questions in English

(D) In prokaryotes,mitochondria are absent.
However,the plasma membrane forms specialized infoldings known as mesosomes.
These mesosomes contain respiratory enzymes and are functionally analogous to mitochondria,serving as the site for the Krebs cycle and oxidative phosphorylation in bacteria.

(A) Mitochondria are known as the powerhouses of the cell because they are the sites of aerobic respiration.
They contain enzymes required for the Krebs cycle (in the matrix) and the electron transport chain (on the inner mitochondrial membrane) to produce $ATP$.

(B) Succinic dehydrogenase is an enzyme that catalyzes the oxidation of succinate to fumarate in the $Krebs$ cycle.
Unlike other enzymes of the $Krebs$ cycle which are found in the mitochondrial matrix,succinic dehydrogenase is embedded in the inner mitochondrial membrane.
It also functions as $Complex$ $II$ of the electron transport chain $(ETC)$.

(D) The enzyme $aconitase$ is a key enzyme in the $Krebs$ cycle (citric acid cycle) that catalyzes the isomerization of citrate to isocitrate.
This enzyme contains an iron-sulfur cluster $([4Fe-4S]^{2+})$ at its active site,which is essential for its catalytic activity.
Therefore,$Iron$ $(Fe)$ is the essential mineral element required for the activity of the $aconitase$ enzyme.

(B) The degradation of sugar (via glycolysis and pyruvate oxidation) and fat (via $\beta$-oxidation) leads to the formation of Acetyl $CoA$.
In eukaryotic cells,the conversion of pyruvate to Acetyl $CoA$ occurs within the mitochondrial matrix.
Furthermore,the subsequent processing of Acetyl $CoA$ through the Krebs cycle (Tricarboxylic Acid Cycle) also takes place exclusively in the mitochondrial matrix.
Therefore,if the mitochondrion is absent,the aerobic respiration pathway cannot proceed,and the degradation of these substrates to Acetyl $CoA$ and their further oxidation will not occur.

(D) The end product of glycolysis is pyruvic acid.
Pyruvic acid undergoes oxidative decarboxylation to form acetyl $CoA$.
Acetyl $CoA$ acts as the connecting link because it is the substrate that enters the Kreb's cycle ($TCA$ cycle) for further oxidation.

(B) The link reaction (also known as the transition reaction or oxidative decarboxylation of pyruvate) connects glycolysis to the Krebs cycle by converting pyruvic acid into acetyl $CoA$.
This reaction is catalyzed by the pyruvate dehydrogenase complex,which requires several cofactors to function,including thiamine pyrophosphate $(TPP)$,which is a derivative of Vitamin $B_1$ (thiamine),as well as $NAD^+$,$CoA$,lipoic acid,and $Mg^{2+}$ ions.
Therefore,Vitamin $B_1$ is essential for the catalysis of this reaction.

(D) The enzyme complex $Pyruvate$ dehydrogenase catalyzes the oxidative decarboxylation of $Pyruvate$ (a $3$-carbon molecule) into $Acetyl$ $CoA$ (a $2$-carbon molecule).
This reaction occurs in the mitochondrial matrix and serves as a crucial link between glycolysis and the $Krebs$ cycle.
During this process, one molecule of $CO_2$ is released and $NAD^+$ is reduced to $NADH + H^+$.

(C) Hans Krebs was awarded the Nobel Prize in Physiology or Medicine in $1953$ for his discovery of the Citric Acid Cycle,also known as the Krebs cycle or the Tricarboxylic Acid $(TCA)$ cycle.
This cycle is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-$CoA$ derived from carbohydrates,fats,and proteins into carbon dioxide and chemical energy in the form of $ATP$.

(C) Malate dehydrogenase is an enzyme that catalyzes the reversible oxidation of malate to oxaloacetate in the citric acid cycle (Krebs cycle).
During this reaction,$NAD^+$ is reduced to $NADH + H^+$.
Therefore,the product formed by the action of malate dehydrogenase on malate is oxaloacetic acid.

(B) The $TCA$ cycle (Tricarboxylic Acid cycle),also known as the $Krebs$ cycle,is an indirect oxidation pathway.
In this process,the acetyl group of acetyl-$CoA$ is oxidized to $CO_2$ and $H_2O$.
It is considered an indirect oxidation pathway because the oxidation of the substrate does not occur directly by oxygen; instead,electrons are transferred to electron carriers like $NAD^+$ and $FAD$ to form $NADH$ and $FADH_2$,which are later oxidized in the electron transport chain.

(C) During the process of dehydrogenation in the Krebs cycle,the enzyme malate dehydrogenase catalyzes the conversion of malic acid (malate) into oxaloacetic acid.
In this reaction,a pair of hydrogen atoms is removed from malate.
These hydrogen atoms are transferred to the coenzyme $NAD^+$,which acts as an electron acceptor.
Consequently,$NAD^+$ is reduced to $NADH + H^+$.

(C) The correct answer is $(c)$. $\alpha$-ketoglutaric acid is a key intermediate in the Krebs cycle ($TCA$ cycle). It serves as a precursor for the synthesis of amino acids,particularly glutamate,through the process of reductive amination. Similarly,oxaloacetic acid $(OAA)$ acts as a precursor for the synthesis of aspartate. These organic acids link the respiratory pathway to amino acid metabolism.

(C) The process by which the energy of glucose is released during aerobic respiration is known as the $Krebs$ cycle (or Citric Acid Cycle).
This metabolic pathway was discovered by Sir Hans $Krebs$ in $1937$.
Therefore,the correct option is $C$.

(C) The Krebs cycle (also known as the Citric Acid Cycle) occurs in the mitochondrial matrix. While the cycle itself does not directly use $O_2$ as a reactant,it is considered an aerobic process because it relies on the continuous operation of the Electron Transport System $(ETS)$. The $ETS$ requires $O_2$ as the final electron acceptor to regenerate $NAD^+$ and $FAD$ from $NADH$ and $FADH_2$. If $O_2$ is absent,the $ETS$ stops,leading to the accumulation of $NADH$ and $FADH_2$,which inhibits the enzymes of the Krebs cycle. Thus,aerobic conditions are essential for the continued operation of the cycle.

(B) In the $Krebs$ cycle,the conversion of succinyl-$CoA$ to succinate is catalyzed by the enzyme succinyl-$CoA$ synthetase. During this reaction,the energy released from the cleavage of the high-energy thioester bond of succinyl-$CoA$ is used to synthesize a high-energy phosphate molecule. In animal cells,this process results in the formation of $GTP$ (guanosine triphosphate) from $GDP$ and inorganic phosphate $(Pi)$ via substrate-level phosphorylation. $GTP$ can subsequently transfer its terminal phosphate to $ADP$ to form $ATP$.

(C) $Kreb's$ cycle involves various enzymatic reactions including dehydrogenation and decarboxylation.
$Acetylation$ of pyruvate to form $Acetyl-CoA$ is a preparatory step for the cycle.
$Oxidative$ phosphorylation is a distinct process that occurs in the electron transport system $(ETS)$ located in the inner mitochondrial membrane,where $ATP$ is synthesized using the energy from electron transfer.

(C) The Krebs cycle (also known as the Citric Acid Cycle) involves a series of enzymatic reactions to oxidize one molecule of acetyl-CoA (derived from one molecule of pyruvic acid) into $CO_2$ and $H_2O$.
$1$. Citrate synthase: Acetyl-CoA + Oxaloacetate $\rightarrow$ Citrate.
$2$. Aconitase: Citrate $\rightarrow$ Isocitrate.
$3$. Isocitrate dehydrogenase: Isocitrate $\rightarrow$ $\alpha$-Ketoglutarate.
$4$. $\alpha$-Ketoglutarate dehydrogenase: $\alpha$-Ketoglutarate $\rightarrow$ Succinyl-CoA.
$5$. Succinyl-CoA synthetase: Succinyl-CoA $\rightarrow$ Succinate.
$6$. Succinate dehydrogenase: Succinate $\rightarrow$ Fumarate.
$7$. Fumarase: Fumarate $\rightarrow$ Malate.
$8$. Malate dehydrogenase: Malate $\rightarrow$ Oxaloacetate.
Thus,there are $8$ distinct enzymatic steps in the cycle.

(C) The $Kreb's$ cycle is also known as the Citric acid cycle or Tricarboxylic acid $(TCA)$ cycle.
This is because the first stable product formed in this cycle is citric acid,which is a tricarboxylic acid containing three carboxyl groups.

(A) The complete oxidation of carbohydrates occurs during aerobic respiration,which involves the Krebs cycle and the Electron Transport System $(ETS)$.
In the Krebs cycle,the acetyl group of Acetyl $CoA$ is completely oxidized to $CO_2$ and $H_2O$,releasing a significant amount of energy in the form of $NADH$ and $FADH_2$.
These electron carriers then donate electrons to the $ETS$,where the majority of $ATP$ is synthesized through oxidative phosphorylation.
Therefore,the liberation of most of the energy occurs when pyruvic acid (after conversion to Acetyl $CoA$) is completely oxidized to $CO_2$ and $H_2O$.

(B) The link between glycolysis and the Kreb's cycle is Acetyl $CoA$.
Glycolysis occurs in the cytoplasm and produces pyruvate.
This pyruvate is then transported into the mitochondria,where it undergoes oxidative decarboxylation to form Acetyl $CoA$.
Acetyl $CoA$ acts as a connecting link because it is the substrate that enters the Kreb's cycle ($TCA$ cycle) for further oxidation.

(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,the enzymes required for the $Krebs$ cycle are located within the mitochondrial matrix.
Therefore,the entire process of the $Krebs$ cycle occurs in the matrix of the mitochondria.

(C) The largest amount of $ATP$ is produced during aerobic respiration.
In the complete oxidation of one molecule of glucose,$38$ $ATP$ molecules are produced in prokaryotes (or $36$ $ATP$ in eukaryotes).
Glycolysis produces a net gain of $2$ $ATP$ and $2$ $NADH_2$.
The Kreb's cycle ($TCA$ cycle) is the major stage where the majority of high-energy electron carriers ($NADH_2$ and $FADH_2$) are generated,which subsequently yield the largest amount of $ATP$ through the Electron Transport System $(ETS)$.
Therefore,the Kreb's cycle is the primary metabolic pathway associated with the production of the largest amount of phosphate bond energy.

(A) In the Krebs cycle,the enzyme succinate dehydrogenase catalyzes the oxidation of succinate to fumarate. During this dehydrogenation process,a pair of hydrogen atoms is removed from succinate. These hydrogen atoms are accepted by $FAD$ (Flavin Adenine Dinucleotide) to form $FADH_2$.

(D) The Krebs cycle (also known as the citric acid cycle) begins with the condensation reaction of an acetyl group from acetyl $CoA$ $(2C)$ with oxaloacetic acid $(4C)$ and water to form citric acid $(6C)$.
This reaction is catalyzed by the enzyme citrate synthase.
The reaction is: $\text{Acetyl } CoA + \text{Oxaloacetic acid} + H_2O \xrightarrow{\text{Citrate synthase}} \text{Citric acid} + CoA$.

(A) The respiratory enzymes involved in the $Krebs$ cycle are located in the mitochondrial matrix.
While the electron transport chain components are located on the cristae (inner mitochondrial membrane),the enzymes for the $Krebs$ cycle and the link reaction are found in the matrix.

(B) The correct answer is $B$.
In aerobic respiration,the pyruvic acid produced during glycolysis enters the mitochondrial matrix.
Inside the mitochondria,it undergoes oxidative decarboxylation to form Acetyl-$CoA$,which then enters the Kreb's cycle (also known as the Citric Acid Cycle or $TCA$ cycle).
In this cycle,pyruvic acid is completely oxidized in the presence of oxygen to release $CO_2$,$H_2O$,and energy in the form of $ATP$,$NADH$,and $FADH_2$.

(A) In the $Krebs$ cycle,the enzyme $isocitrate$ $dehydrogenase$ catalyzes the oxidation of $isocitric$ $acid$ to $oxalosuccinic$ $acid$ (an unstable intermediate) in the presence of $NAD^+$.
Subsequently,$oxalosuccinic$ $acid$ undergoes $decarboxylation$ to form $\alpha$-ketoglutaric $acid$.
Therefore,the correct sequence is: $Isocitric$ $acid$ $\to$ $Oxalosuccinic$ $acid$ $\to$ $\alpha$-ketoglutaric $acid$.

(B) In the Kreb's cycle,the enzyme succinate dehydrogenase catalyzes the oxidation of succinate to fumarate.
This reaction involves the removal of two hydrogen atoms (dehydrogenation) from succinate,which are transferred to $FAD$ to form $FADH_2$.
Since oxidation is defined as the removal of hydrogen or the loss of electrons,the removal of hydrogen from succinate results in its oxidation to fumarate.

(D) The correct answer is $D$.
Succinate dehydrogenase is the only enzyme of the Kreb's cycle that is embedded in the inner mitochondrial membrane.
It acts as a mitochondrial marker enzyme and functions as Complex $II$ of the electron transport chain.

(C) The Krebs cycle (also known as the Citric Acid Cycle) is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates,fats,and proteins into carbon dioxide and chemical energy in the form of $ATP$.
Biological oxidation in this context refers to the removal of electrons and hydrogen atoms from substrates (like isocitrate,alpha-ketoglutarate,etc.) by dehydrogenases,which are then transferred to coenzymes like $NAD^+$ and $FAD$.
Although the Krebs cycle is part of aerobic respiration,the cycle itself does not directly use $O_2$ as a reactant. However,the process is dependent on the presence of $O_2$ because the electron transport system $(ETS)$ requires $O_2$ as the final electron acceptor to regenerate $NAD^+$ and $FAD$ from $NADH$ and $FADH_2$. Among the given options,$O_2$ is the most relevant factor associated with the aerobic nature of biological oxidation in the context of cellular respiration.

(D) The $TCA$ cycle (also known as the Krebs cycle or Citric Acid cycle) involves several enzymes that require metal ion cofactors for their catalytic activity.
Specifically,the carboxylase enzymes involved in the $TCA$ cycle,such as isocitrate dehydrogenase and $\alpha$-ketoglutarate dehydrogenase,utilize $Mn^{++}$ (manganous ions) as a crucial mineral activator to facilitate the decarboxylation reactions.
Therefore,$Mn^{++}$ is the correct mineral activator.

(D) The Krebs cycle,also known as the citric acid cycle,is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates,fats,and proteins into carbon dioxide and chemical energy in the form of $ATP$ (or $GTP$). During this process,$ADP$ is phosphorylated to form $ATP$ via substrate-level phosphorylation. Therefore,the formation of $ATP$ from $ADP$ is a key outcome of the cycle.

(A) The $Krebs$ cycle,also known as the $Tricarboxylic$ $Acid$ $(TCA)$ cycle or $Citric$ $Acid$ cycle,begins with the condensation of an $Acetyl$ $CoA$ molecule $(2C)$ with $Oxaloacetic$ $acid$ ($OAA$,$4C$) in the presence of water to form $Citric$ $acid$ $(6C)$.
Since $Acetyl$ $CoA$ is derived from the oxidative decarboxylation of $Pyruvic$ $acid$,$Pyruvic$ $acid$ is the primary substrate that initiates the sequence leading into the cycle.

(D) The $Kreb's$ cycle is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-$CoA$ derived from carbohydrates,fats,and proteins into carbon dioxide and chemical energy in the form of $ATP$.
It is also known as the $TCA$ (Tricarboxylic Acid) cycle because the first stable product formed is citric acid,which contains three carboxylic acid groups.
Therefore,all the given options are correct names for the $Kreb's$ cycle.

(D) The Krebs 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 through the oxidation of acetyl-$CoA$ derived from carbohydrates,fats,and proteins into carbon dioxide and chemical energy in the form of $ATP$.
Since this process requires oxygen to function as the final electron acceptor in the electron transport chain,it is an integral part of aerobic respiration.
Therefore,the correct option is $D$.

(C) The Kreb's cycle,also known as the citric acid cycle,is a crucial stage of aerobic respiration.
It occurs in the mitochondrial matrix.
The primary significance of the Kreb's cycle is the generation of high-energy electron carriers ($NADH$ and $FADH_2$),which subsequently drive the production of a significant amount of $ATP$ through oxidative phosphorylation in the electron transport chain.
Therefore,the production of $ATP$ (via the electron transport chain linked to the cycle) is the main functional importance of this metabolic pathway.

(D) In the $Kreb's$ cycle (also known as the Citric Acid Cycle), the conversion of succinic acid to fumaric acid is catalyzed by the enzyme succinate dehydrogenase.
During this specific reaction, $FAD$ (Flavin Adenine Dinucleotide) acts as an electron acceptor and is reduced to $FADH_2$.
This is the only step in the $Kreb's$ cycle where $FAD$ is used as an electron carrier instead of $NAD^+$.
Therefore, the correct option is $D$.

(C) Glycolysis occurs in the cytoplasm,where glucose is broken down into two molecules of pyruvic acid.
In aerobic respiration,these pyruvic acid molecules enter the mitochondrial matrix.
Here,they undergo oxidative decarboxylation to form Acetyl-$CoA$,which then enters the Kreb's cycle (also known as the Citric Acid Cycle or $TCA$ cycle).
In the Kreb's cycle,the acetyl group is completely oxidized to $CO_2$ and $H_2O$,releasing energy in the form of $ATP$,$NADH$,and $FADH_2$.

(D) During the Krebs cycle,the conversion of succinyl $CoA$ to succinic acid is catalyzed by the enzyme succinyl $CoA$ synthetase.
This reaction is coupled with the phosphorylation of guanosine diphosphate $(GDP)$ to guanosine triphosphate $(GTP)$ in the presence of inorganic phosphate $(Pi)$.
Therefore,$GTP$ is the energy-storing compound formed during this step.

(C) The Krebs cycle,also known as the citric acid cycle or $TCA$ cycle,takes place in the mitochondrial matrix. It is a key metabolic pathway in aerobic respiration that generates energy by oxidizing acetyl-CoA derived from carbohydrates,fats,and proteins.

(A) In the $Kreb's$ cycle of aerobic respiration,the enzyme succinate dehydrogenase catalyzes the oxidation of succinic acid to fumaric acid. During this process,$FAD$ is reduced to $FADH_2$.

(C) The $Tricarboxylic$ $acid$ $cycle$ ($TCA$ $cycle$) is another name for the $Citric$ $acid$ $cycle$. Both terms refer to the same metabolic pathway that occurs in the mitochondrial matrix,where acetyl-$CoA$ is oxidized to produce $CO_2$,$ATP$,$NADH$,and $FADH_2$. This cycle is also commonly known as the $Krebs$ $cycle$ in honor of Sir Hans Krebs.

(A) The $Kreb's$ cycle is also known as the $Tricarboxylic$ $acid$ $cycle$ ($TCA$ cycle) because the first stable product formed in this cycle is citric acid,which is a tricarboxylic acid.
It is also referred to as the $Citric$ $acid$ $cycle$.

(A) The Kreb's cycle,also known as the Citric Acid Cycle or Tricarboxylic Acid $(TCA)$ cycle,begins with the condensation of Acetyl coenzyme-$A$ $(2C)$ with oxaloacetic acid $(4C)$ to form Citric acid $(6C)$.
Therefore,Citric acid is the first stable intermediate product formed during the Kreb's cycle.

(B) In the $TCA$ cycle,the enzyme succinate dehydrogenase catalyzes the oxidation of succinate to fumarate.
During this reaction,two hydrogen atoms are removed from succinate and transferred to the electron acceptor $FAD$ (flavin adenine dinucleotide).
This results in the reduction of $FAD$ to $FADH_2$.

(D) The $Kreb's$ cycle (also known as the Citric Acid Cycle or $TCA$ cycle) involves a series of enzymatic reactions that oxidize acetyl groups to $CO_2$.
$1$. Acetyl $CoA$ is the starting substrate that enters the cycle by condensing with oxaloacetate.
$2$. Citric acid is the first stable product formed in the cycle.
$3$. Succinic acid is an intermediate formed during the conversion of succinyl-$CoA$ to fumarate.
$4$. Lactic acid is the product of lactic acid fermentation,which occurs in the cytoplasm under anaerobic conditions,not in the $Kreb's$ cycle.

(B) The citric acid cycle,also known as the Krebs cycle,occurs in the mitochondrial matrix of eukaryotic cells.
During aerobic respiration,pyruvate produced in the cytosol enters the mitochondria,where it is converted into acetyl-CoA.
This acetyl-CoA then enters the citric acid cycle to produce $NADH$,$FADH_2$,and $ATP$ through a series of enzymatic reactions.
Therefore,the correct location for the citric acid cycle is the mitochondria.