Question

Which of the following statement is incorrect? [NEET 2021]

(a) During aerobic respiration, role of oxygen is limited to the terminal stage

(b) In ETC (Electron Transport Chain), one molecule of $\mathrm{NADH}+\mathrm{H}^{+}$gives rise to 2 ATP molecules and one $\mathrm{FADH}_2$ gives rise to 3 ATP molecules

(c) ATP is synthesised through complex V

(d) Oxidation-reduction reactions produce proton gradient in respiration

Answer/Explanation

Ans. (b)

Oxidation of one molecule of NADH gives rise to 3 molecules of ATP and one molecule of $\mathrm{FADH}_2$ produces 2 molecules of ATP. NADH and $\mathrm{FADH}_2$ are two different types of electron donors. They differ in the ways they feed electron during electron transport chain. NADH feeds its electrons into the electron transport chain at the beginning (ComplexI). $\mathrm{FADH}_2$ feeds into the electron transport chain at Complex II (at a lower energy level down the chain). The high energy electrons from NADH have sufficient energy to result in 3 ATP whereas the lower energy electrons in $\mathrm{FADH}_2$ have energy for 2ATP production.

Question

 Pyruvate dehydrogenase activity during aerobic respiration requires [NEET (Oct.) 2020]

(a) calcium

(b) iron

(c) cobalt

(d) magnesium

Answer/Explanation

Ans. (d)

Pyruvate dehydrogenase enzyme is involved in the conversion of pyruvate to acetyl Co-A, after the completion of glycolysis and before the start of Krebs’ cycle. This enzyme is made up of decarboxylase, lipoic acid, transacetylase and $\mathrm{Mg}^{2+}$ ion. The reaction occur in following way
Acetyl $\mathrm{Co}-\mathrm{A}+\mathrm{NADH}+\mathrm{H}^{+}+\mathrm{CO}_2$ In this reaction $\mathrm{Mg}^{2 *}$ acts as $\mathrm{a}$ cofactor.

Question

 The number of substrate level phosphorylation in one turn of citric acid cycle is [NEET (Sep.) 2020]

(a)one

(b) two

(c) three

(d)zero

Answer/Explanation

Ans. (a)

The number of substrate level phosphorylation in one turn of citric acid cycle is 1. During Krebs’ or citric acid cycle, succinyl-Co-A is acted upon by enzyme succinyl-Co-A synthetase to form succinate (a $4 \mathrm{C}$ compound). The reaction releases sufficient energy to form ATP (in plants) or GTP (in animals) by substrate-level phosphorylation. GTP can form ATP through a coupled reaction. Succinyl Co-A+GDP/ADP $+\mathrm{H}_3$ $\mathrm{PO}_4$ synthetase. Succinyl Co-A Succinate $+$ Co-A+GTP/ATP.

Question

 Where is respiratory Electron Transport System (ETS) located in plants? [NEET (Odisha) 2019]

(a) Mitochondrial matrix

(b) Outer mitochondrial membrane

(c) Inner mitochondrial membrane

(d) Intermembrane space

Answer/Explanation

Ans. (c)

Respiratory Electron Transport System (ETS) in plants is located in inner mitochondrial membrane. It serves as the site of oxidative phosphorylation through the action of ATP synthase.

Question

 Which one of these statements is incorrect? [NEET 2018]

(a) Glycolysis operates as long as it is supplied with NAD that can pick up hydrogen atoms.

(b) Glycolysis occurs in cytosol

(c) Enzymes of TCA cycle are present in mitochondrial matrix

(d) Oxidative phosphorylation takes place in outer mitochondrial membrane

Answer/Explanation

Ans. (d)

Oxidative phosphorylation is the process of ATP formation due to the transfer of electrons from NADH or $\mathrm{FADH}_2$ to oxygen molecule $\left(\mathrm{O}_2\right)$ by a series of  electron carriers. This process occurs in the inner mitochondrial membrane because of its less permeability, presence of ETC proteins and ATP synthase. The rest three statements are correct.

Question

 What is the role of $\mathrm{NAD}^{+}$in cellular respiration? [NEET 2018]

(a) It is a nucleotide source of ATP synthesis

(b) It functions as an electron carrier

(c) It functions as an enzyme

(d) It is the final electron acceptor for anaerobic respiration

Answer/Explanation

Ans. (b)

$\mathrm{NAD}^{+}$functions as an electron carrier in cellular respiration. NAD is an oxidising agent which accept electrons and then transfer them to the Electron Transport System(ETS). As a result, 3ATP molecules are formed.

Question

 Which statement is wrong for Krebs’ cycle? [NEET 2017]

(a) There are three points in the cycle where $\mathrm{NAD}^{+}$is reduced to NADH $+\mathrm{H}^{+}$

(b) There is one point in the cycle where $\mathrm{FAD}^{+}$is reduced to $\mathrm{FADH}_2$

(c) During conversion of succinyl Co-A to succinic acid, a molecule of GTP is synthesised

(d) The cycle starts with condensation of acetyl group (acetyl Co-A) with pyruvic acid to yield citric acid

Answer/Explanation

Ans. (d)
Option (d) is incorrect, which can be corrected as Krebs’ cycle starts with the condensation of acetyl group with oxaloacetic acid and water to yield citric acid. During conversion of succinic acid to fumaric acid $\mathrm{FAD}^{+}$is reduced to $\mathrm{FADH}_2$. During conversion of pyruvic acid to acetyl Co-A, isocitrate to oxalosuccinic acid and $\alpha$-ketoglutaric acid to succinyl Co-ANAD ${ }^{+}$is reduced to $\mathrm{NADPH}+\mathrm{H}^{+}$.

Question

Oxidative phosphorylation is [NEET 2016, Phase II]

(a) formation of ATP by transfer of phosphate group from a substrate to ADP

(b) oxidation of phosphate group in ATP

(c) addition of phosphate group to ATP

(d) formation of ATP by energy released from electrons removed during substrate oxidation

Answer/Explanation

Ans. (a)

Oxidative phosphorylation is the process of formation of ATP from ADP and inorganic phosphate $(P)$ in the presence of oxygen. It occurs mainly in the Electron Transport Chain (ETC) of cellular respiration.

Question

Which of the metabolites is common to respiration mediated breakdown of fats, carbohydrates and proteins? [NEET 2013]

(a) Glucose-6-phosphate

(b) Fructose 1, 6-bisphosphate

(c) Pyruvic acid

(d) Acetyl Co-A

Answer/Explanation

Ans. (d)

Acetyl Co-A is common to respiration mediated breakdown of fats, carbohydrates and proteins. Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme hexokinase. Glucose-6-phosphate is then converted into fructose-6-phosphate and further to fructose 1-6-bisphosphate. Pyruvic acid is the end product of glycolysis.

Question

 The three boxes in this diagram represent the three major biosynthetic pathways in aerobic respiration. Arrows represent net reactants or products [NEET 2013]

Arrows numbered 4,8 and 12 can all be
(a) $\mathrm{NADH}$

(b) ATP

(c) $\mathrm{H}_2 \mathrm{O}$

(d)$FAD^+$ or $FADH_2$

Answer/Explanation

Ans. (b)

Pathway $A$ is glycolysis, pathway $B$ is the Krebs’ cycle and pathway $C$ is oxidative phosphorylation
Arrow $1-\mathrm{ADP}^{-}$or $\mathrm{NAD}^{+}$
Arrow 2 – Pyruvate
Arrow $3 \quad-\mathrm{NADH}$
Arrow $4 \quad-$ ATP
Arrow $5-A D P, \mathrm{NAD}^{+}$or FAD
Arrow 6 and $7 \quad-\mathrm{FADH}_2$ and NADH
(either one can be 6 or 7)
Arrow 8 – ATP or $\mathrm{CO}_2$
Arrow 9 and $10-\mathrm{O}_2$ and $\mathrm{ADP}$ (either one can be 9 or 10)
Arrow 11 and $12-\mathrm{H}_2 \mathrm{O}$ and ATP
(either onecan be 11 or 12 )

Question

Aerobic respiratory pathway is appropriately termed [CBSE AIPMT 2009]

(a) catabolic

(b) parabolic

(c) amphibolic

(d) anabolic

Answer/Explanation

Ans. (c)

An amphibolic pathway is a biochemical pathway that serves both anabolic and catabolic processes. An important example of an amphibolic pathway is the Krebs’ cycle, which involves both the catabolism of carbohydrates and fatty acid and the synthesis of anabolic precursors for amino acid synthesis, eg, $\alpha$-ketogluturate and oxalo acetate.

Question

 The chemiosmotic coupling hypothesis of oxidative phosphorylation proposes that Adenosine Triphosphate (ATP) is formed because [CBSE AIPMT 2008]

(a) high energy bonds are formed in mitochondrial proteins

(b) ADP is pumped out of the matrix into the intermembrane space

(c) a proton gradient forms across the inner membrane

(d) there is a change in the permeability of the inner mitochondrial membrane toward Adenosine Diphosphate (ADP)

Answer/Explanation

Ans. (c)
The production of ATP with the help of energy liberated during oxidation of reduced coenzymes and terminal oxidation is called oxidative phosphorylation. Peter Mitchell(1961) gave a hypothesis known as chemiosmotic hypothesis for ATP synthesis. According to this when electrons flow from dual proton, electron carrier to a non-hydrogen carrier the $\mathrm{H}^{+}$are released and expelled into the intermembrane space and thus creates a proton gradient with higher concentration of $\mathrm{H}^*$ in the inter membranous space than the matrix. Due to the proton motive force the protons flow back and energy liberated during this back flow of protons activate ATPase present in $\mathrm{F}_1$ head to synthesize ATP.

Question

 The overall goal of glycolysis, Krebs’ cycle and the electron transport system is the formation of [CBSE AIPMT 2007]

(a) ATP in small stepwise units

(b) ATP in one large oxidation reaction

(c) sugars

(d) nucleic acids

Answer/Explanation

Ans. (a)

Glycolysis, Krebs’ cycle and electron transport system are meant for ATP synthesis in different steps. ATP is the energy currency of cell.

Question

All enzymes of TCA cycle are located in the mitochondrial matrix except one which is located in inner mitochondrial membranes in eukaryotes and in cytosol in prokaryotes. This enzyme is [CBSE AIPMT 2007]

(a) lactate dehydrogenase

(b) isocitrate dehydrogenase

(c) malate dehydrogenase

(d) succinate dehydrogenase

Answer/Explanation

Ans. (d)

Succinate dehydrogenase enzyme is present on inner membrane of mitochondria and catalyses the oxidation of succinate to fumarate.

Question

 During which stage, in the complete oxidation of glucose are the greatest number of ATP molecules formed from ADP [CBSE AIPMT 2005]

(a) glycolysis
(b) Krebs’ cycle

(c) conversion of pyruvic acid to acetyl Co-A

(d) electron transport chain

Answer/Explanation

Ans. (d)

The last step of aerobic respiration is the axidation 2 of reduced coenzymes, i.e., $\mathrm{NADH}_2$ and $\mathrm{FADH} \mathrm{F}_2$ by molecular oxygen through FAD, ubiquinone, cyt.-b, cyt. $-c_2$ cyt- $c_1$, cyt. $-a$ and cyt. $\sigma_2$. By oxidation of 1 molecule of $\mathrm{NADH}_2, 3$ ATP molecules are produced and by oxidation of 1 molecule of $\mathrm{FADH}_2$ 2ATP molecules are Produced. In glycolysis 2 ATP molecules are produced from ADP. Further $2 \mathrm{NADH}_2$ produced, give $2 \times 3=6$ ATP, on oxidative phosphorylation. Similarly in Krebs’ cycle 2 ATP molecules are produced. So the greatest number of ATP molecules are produced in the electron transport chain.

Question

Chemiosmotic theory of ATP synthesis in the chloroplast and mitochondria is based on [CBSE AIPMT 2005]

(a) membrane potential

(b) accumulation of $\mathrm{Na}^{+}$ions

(c) accumulation of $\mathrm{K}^{+}$ions

(d) protan gradient.

Answer/Explanation

Ans. (d)
Chemiosmotic hypothesis for oxidative phosphorylation (ATP synthesis) was proposed by Peter Mitchell in 1961, for this he was awarded Nobel Prize in 1978. This theory is based on proton gradient.

Question

 Which one of the following concerns photophosphorylation? [CBSE AIPMT 2003]

(a) AMP + inorganic $\mathrm{PO}_4$
$\stackrel{\text { Light energy }}{\longrightarrow}$ ATP

(b) $A D P+A M P \stackrel{\text { Light energy }}{\longrightarrow}$ ATP

(c) $A D P+$ inorganic $\mathrm{PO}_4$

$\stackrel{\text { Light energy }}{\longrightarrow}$ ATP

(d) $\mathrm{ADP}+$ inorganic $\mathrm{PO}_4 \longrightarrow$ ATP

Answer/Explanation

Ans. (c)

Phosphorylation refers to the process in which ATP is made when energy is used to bind another phosphate to ADP. Photophosphorylation reactions are part of both respiration and photosynthesis.

Question

 In which one of the following do the two names refer to one and the same thing? [CBSE AIPMT 2003]

(a) Tricarboxylic acid cycle and urea cycle

(b) Krebs’ cycle and Calvin cycle

(c) Tricarboxylic acid cycle and citric acid cycle

(d) Citric acid cycle and Calvin cycle

Answer/Explanation

Ans. (c)
Tricarboxylic acid cycle is also known as citric acid cycle. This is an aerobic process, that takes place in the matrix of mitochondria. Kreb that discovered this cycle in 1937. So, this is also known as Krebs’ cycle.

Question

 The mechanism of ATP formation both in chloroplast and mitochondria is explained by [CBSE AIPMT 1997]

(a) relay pump theory of Godlewski

(b) Munch’s pressure/mass flow model

(c) chemiosmotic theory of Mitchell

(d) Cholondy-Went’s model

Answer/Explanation

Ans. (c)

In chemiosmotic-coupling hypothesis, outward pumping of protons across the inner chloroplast or mitochondrial membrane results in accumulation of protons between auter membrane and inner membrane. A proton gradient is thus established. As protons now flow back passively down the gradient, the proton mative force is utilised to synthesise ATP.

Question

 In Krebs’ cycle FAD participates as electron acceptor during the conversion of [CBSE AIPMT 1997]

(a) succinyl Co-A to succinic acid

(b) $\alpha$-ketoglutarate to succinyl Co-A

(c) succinic acid to fumaric acid

(d) fumaric acid to malic acid

Answer/Explanation

Ans. (c)

Electrons and H-ions during oxidation. of succinic acid to fumaric acid, are taken up by FAD which gets reduced to $\mathrm{FADH}_2$ –

Question

 Oxidative phosphorylation involves simultaneous oxidation and phosphorylation to finally form [CBSE AIPMT 1996]

(a) pyrunate

(b) NADP

(c) DPN

(d) ATP

Answer/Explanation

Ans. (d)

In coxidative phosphorylatian ATP is formed as the electrons are transferred from NADH or $\mathrm{FADH} \mathrm{A}_4$ to $\mathrm{O}_2$ by a series of electron carriers, located in the inner membrane of mitochondria.

Question

 Krebs’ cycle occurs in [CBSE AIPMT 1996]

(a) mitochandria

(b)cytoplasm

(c) chlaroplast

(d) ribosomes

Answer/Explanation

Ans. (a)

Krebs’ cycle occurs inside the matrox of mitochandria. The cycle is also named as citric acid cylce or tricarboxylic acid cycle. It includes stepwise oxidative and cyclic degradation of activated acetate derived from pyruvic acid.

Question

 Which of the following is essential for conversion of pyruvic acid into acetyl Co-A? [CBSE AIPMT 1995]

(a) LAA

(b) $\mathrm{NAD}^{+}$

(c) TPP

(d) All of these

Answer/Explanation

Ans. (d)

The axidative decarboxylation of pyruvate into acetyl Co-A involves the presence of atleast five essential co-factors and an enzyme complex. The co-factors imolved are Mg ions. Thiamine Pyrophosphate/TPPI NAD’, Coenzyme-A (Co-A) and lipoic acid.

Question

ATP is injected in cyanide poisoning because it is [CBSE AIPMT 1994]

(a) necessary for cellular functions

(b) necessary for $\mathrm{Na}^{+}-\mathrm{K}^{+}$pump

(c) $\mathrm{Na}^{+}-\mathrm{K}+$ pump operates at the cell membranes

(d) ATP breaks down cyanide

Answer/Explanation

Ans. (a)

Cyanide is a deadly poisan. It stops respiration by inhibiting electron flow tram cyt.-b tocyy. – $c$. ATP is the energy currency of cell is injected in cyanide poisoning because, it is necessary for cellular functions.

Question

 Out of 38 ATP molecules produced per glucose, 32 ATP molecules are formed from $\mathrm{NADH} / \mathrm{FADH}_2$ in [CBSE AIPMT 1993]

(a) respiratory chain

(b) Krebs’ cycle

(c) oxidative decarboxylation

(d) EMP

Answer/Explanation

Ans. (a)

Respiratory chain helps in forming 32 ATP molecules from NADH/FADH molecules. In which $0 x i d a t i v e$ phosphorylation is the synthesis of energy rich ATP molecules with the help of energy liberated during oxidation of reduced coenzymes $\left(\mathrm{NADH}_2, \mathrm{FADH}_2\right)$. produced in glycolysis and Krebs’ cycle. A total of $10 \mathrm{NADH}_2$ and $2 \mathrm{FADH}_2$ molecules are formed in aerobic respiration. They help in formation of 32 or 34ATP molecules.

Question

 End product of citric acid/Krebs’ cycle is [CBSE AIPMT 1993]

(a) citric acid

(b) lactic acid

(c) pyruvic acid

(d) $\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}$

Answer/Explanation

Ans. (d)

Krebs’ cycle or citric acid cycle that takes place in the matrix of mitochondrion begins by linking acetyl Co-A to oxaloacetic acid forming citric acid. In the presence of various enzymes, cycle continues through the formation of various intermediates and release of $\mathrm{CO}_2$ and $\mathrm{H}_2 \mathrm{O}$ as end-products.

Question

 Link between glycolysis, Krebs’ cycle and $\beta$-oxidation of fatty acid or carbohydrate and fat metabolism is [CBSE AIPMT 1992, 90]

(a) oxaloacetic acid

(b)succinic acid

(c) citric acid

(d) acetyl Co-A

Answer/Explanation

Ans. (d)

The pyruvic acid synthesised from glycolysis enters into mitochondria and undergoes oxidative decarboxylation to produce $\mathrm{CO}_2$ and $\mathrm{NADH}_2$. The product combines with coenzyme-A to form acetyl Co-A. It is the connecting link between glycolysis, Krebs’ cycle and fat oxidation.

Question

 Oxidative phosphorylation is production of [CBSE AIPMT 1992]

(a) ATP in photosynthesis

(b) NADPH in photosynthesis

(c) ATP in respiration

(d) NADH in respiration

Answer/Explanation

Ans. (c)

Oxidative phosphorylation is the synthesis of ATP from ADP and inorganic phosphate which occurs with the help of energy obtained from oxidation of reduced coenzymes formed in cellular respiration.

Question

 Terminal cytochrome of respiratory chain which donates electrons to oxygen is [CBSE AIPMT 1992]

(a) cyt-b

(b) cyt-c

(c) cyt- $a_1$

(d) $\mathrm{cyt}-\mathrm{a}_3$

Answer/Explanation

Ans. (d)

The ETS system contains various electron carriers such as cytochromes. The correct sequence of electron carrier/acceptor in ATP synthesis is cyt-b, cyt- $c_1, c y t-c$, cyt ( $a$ and cyt $-a_3$ ). Cyt $-a_3$ is the terminal cytochrome, it possess two copper centers, which help in transfer of electron to oxygen.

Question

$\mathrm{NADP}^{+}$is reduced to $\mathrm{NADPH}$ in [CBSE AIPMT 1988]

(a) HMP

(b) Calvin cycle

(c) glycolysis

(d) EMP

Answer/Explanation

Ans. (a)

Pentose Phosphate Pathway (PPP) or Hexose Monophosphate Shunt (HMP) or phosphogluconate pathway occurs in the cytosol of mammalian cells. It involves oxidation of glucose to $\mathrm{CO}_2$ and water through a series of reactions in which NADP is reduced to NADPH. Complete breakdown of one molecule of glucose forms $12 \mathrm{NADPH}$ equal to 36 ATP molecules.

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