Respiration in Plants

Complex II of the mitochondrial electron transport chain is referred to as succinate dehydrogenase. It is distinct from the other complexes in the chain, each of which has its specific role:

Cytochrome c oxidase is known as Complex IV.

NADH dehydrogenase refers to Complex I.

Cytochrome bc$_1$ is Complex III.

The correct answer is Option B: A-I, B-IV, C-II, D-III.

Here's why:

The Electron Transport System (ETS) in cellular respiration involves a series of protein complexes embedded in the inner mitochondrial membrane. Each complex plays a specific role in transferring electrons and pumping protons across the membrane, generating a proton gradient that drives ATP synthesis.

Let's break down the matching:

1. ETS Complex I (NADH Dehydrogenase): This complex is responsible for accepting electrons from NADH, a reduced electron carrier generated in glycolysis and the Krebs cycle. It then pumps protons into the intermembrane space.
2. ETS Complex II (Succinate Dehydrogenase): This complex is directly involved in the Krebs cycle, accepting electrons from succinate and transferring them to the electron transport chain. It does not pump protons.
3. ETS Complex III (Cytochrome bC$_1$): This complex receives electrons from Complex I and II and passes them to cytochrome c. It also pumps protons across the membrane.
4. ETS Complex IV (Cytochrome C Oxidase): This complex is the final electron acceptor in the chain, receiving electrons from cytochrome c and transferring them to molecular oxygen (O$_2$) to form water (H$_2$O). It also pumps protons.

Therefore, the correct match is:

• (A) ETS Complex I - (I) NADH Dehydrogenase
• (B) ETS Complex II - (IV) Succinate Dehydrogenase
• (C) ETS Complex III - (II) Cytochrome bC$_1$
• (D) ETS Complex IV - (III) Cytochrome C oxidase

To determine the correct statements about cellular respiration, let's analyze each statement one by one:

A. Cellular respiration is the breaking of C-C bonds of complex organic molecules by oxidation.

This statement is correct. Cellular respiration involves the oxidation of organic molecules to break the carbon-carbon (C-C) bonds and release energy.

B. The entire cellular respiration takes place in Mitochondria.

This statement is incorrect. While the majority of cellular respiration (Krebs cycle and oxidative phosphorylation) occurs in the mitochondria, glycolysis (the first step) takes place in the cytoplasm.

C. Fermentation takes place under anaerobic condition in germinating seeds.

This statement is correct. Fermentation is an anaerobic process that converts sugars to acids, gases, or alcohol, and it can occur in germinating seeds when oxygen is scarce.

D. The fate of pyruvate formed during glycolysis depends on the type of organism also.

This statement is correct. The fate of pyruvate can vary depending on the type of organism and the presence of oxygen. For example, in aerobic conditions, pyruvate enters the mitochondria for the Krebs cycle, whereas in anaerobic conditions, it can be converted to lactate or ethanol, depending on the organism.

E. Water is formed during respiration as a result of O$_2$ accepting electrons and getting reduced.

This statement is correct. During the electron transport chain in oxidative phosphorylation, oxygen ($O_2$) acts as the final electron acceptor and gets reduced to form water ($H_2O$).

Based on the analysis, the correct statements are A, C, D, and E. Hence, the correct option is:

Option A: A, C, D, E only

To correctly match the given processes in List I with their locations in List II, let's go through each item:

• Citric Acid Cycle (A) occurs in the mitochondrial matrix. This process, also known as the Krebs cycle, takes place in the matrix of mitochondria where enzymes for the cycle are located.


• Glycolysis (B) takes place in the cytoplasm of the cell. It is the process of breaking down glucose into pyruvate, generating small amounts of energy (ATP) and does not involve the mitochondria.


• Electron Transport System (C) occurs at the inner mitochondrial membrane. This is where the electron transport chains are located and where ATP is produced by oxidative phosphorylation.


• Proton Gradient (D) is involved in the creation of ATP and is formed across the intermembrane space of the mitochondria. Protons (H+ ions) are pumped from the mitochondrial matrix to the intermembrane space, creating a gradient that drives ATP synthesis.

Now using the above information, let's match these with the correct options:

• A. Citric Acid Cycle - II. Mitochondrial Matrix
• B. Glycolysis - I. Cytoplasm
• C. Electron Transport System - IV. Inner Mitochondrial Membrane
• D. Proton Gradient - III. Intermembrane Space of Mitochondria

Comparing these to the options provided:

• Option A: A-I, B-II, C-III, D-IV - Incorrect
• Option B: A-II, B-I, C-IV, D-III - Correct
• Option C: A-III, B-IV, C-I, D-II - Incorrect
• Option D: A-IV, B-III, C-II, D-I - Incorrect

Therefore, Option B is the correct match:

A-II, B-I, C-IV, D-III

The tricarboxylic acid cycle (TCA cycle), also known as the Krebs cycle or citric acid cycle, is a series of enzyme-catalyzed chemical reactions that form a key part of aerobic respiration in cells. This cycle is primarily used to generate high-energy electron carriers and carbon dioxide. Most of the steps in the TCA cycle involve the oxidation of the substrate, where electrons are transferred to NAD+ to form NADH or to FAD to form FADH2.

Each option provided corresponds to a step in the TCA cycle:

• Option A: Malic acid $\rightarrow$ Oxaloacetic acid involves the enzyme malate dehydrogenase, where malate is oxidized to oxaloacetate. NAD+ is reduced to NADH in this process, indicating an oxidation reaction.


• Option B: Succinic acid $\rightarrow$ Malic acid is not a direct step in the TCA cycle. Instead, succinic acid is first converted to fumarate by succinate dehydrogenase, involving the reduction of FAD to FADH2, then fumarate is hydrated to malate by fumarase. Both involve transformations but particularly the conversion of succinate to fumarate involves oxidation.


• Option C: Succinyl-CoA $\rightarrow$ Succinic acid involves the conversion of Succinyl-CoA to Succinic acid catalyzed by succinyl-CoA synthetase. In this process, a high-energy thioester bond in Succinyl-CoA is broken to release a molecule of coenzyme A (CoA) and produce succinate. This step does not involve the transfer of electrons or the reduction of NAD+ or FAD; rather, it's coupled with the phosphorylation of GDP to GTP (or ADP to ATP in some organisms), which is a substrate-level phosphorylation.


• Option D: Isocitrate $\rightarrow \alpha$-ketoglutaric acid involves the enzyme isocitrate dehydrogenase that catalyzes the oxidative decarboxylation of isocitrate into alpha-ketoglutarate. NAD+ is reduced to NADH, showing that this step involves oxidation.

Therefore, the correct answer is Option C (Succinyl-CoA $\rightarrow$ Succinic acid). This is the step in the TCA cycle that does not involve the oxidation of the substrate but instead involves substrate-level phosphorylation, which is associated with the energy conversion of GTP formation rather than an electron transfer process.

The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, involves the oxidative decarboxylation of malate to oxaloacetate and of isocitrate to alpha-ketoglutarate. However, the latter reaction is followed by another decarboxylation when alpha-ketoglutarate is converted to succinyl-CoA. So in total, there are two decarboxylation reactions per TCA cycle.

So, the correct answer is Option D : Twice.

Yes, that's correct. Fatty acids are broken down through a process called beta-oxidation, which occurs in the mitochondria. During beta-oxidation, fatty acids are broken down two carbon atoms at a time, resulting in the formation of acetyl CoA. This acetyl CoA can then enter the Krebs cycle (also known as the citric acid cycle or TCA cycle) to be further oxidized, producing NADH and FADH2, which can be used in the electron transport chain to produce ATP, the cell's main form of energy.

Hence, fatty acids are connected with the respiratory pathway through Acetyl CoA. So, the correct answer is Option A : Acetyl CoA.

Chemiosmosis is a process by which ATP (adenosine triphosphate) is produced in the cell. It relies on a concentration gradient of protons (H+ ions) across a membrane. The proton gradient is created by a proton pump. As protons flow back across the membrane, down their concentration gradient, they pass through a protein complex called ATP synthase, which uses the energy of the proton flow to produce ATP.

So, the correct answer is :

Option D : Membrane, proton pump, proton gradient, ATP synthase

Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalyzed by pyruvate dehydrogenase.

The scheme of glycolysis was given by Gustav Embden, Otto Meyrhof and J. Parnas, and is often referred to as the EMP pathway.

In electron transport system, the energy of oxidation-reduction is utilized for the production of proton gradient required for phosphorylation, thus, this process is also called oxidative phosphorylation.

The TCA (tricarboxylic acid cycle) starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. The reaction is catalysed by the enzyme citrate synthase. Thus, option (C) is correct.

Melonate is a known inhibitor of the enzyme succinic dehydrogenase, which is involved in the citric acid cycle (also known as the Krebs cycle or TCA cycle), an essential metabolic pathway in many organisms, including bacteria.

By inhibiting succinic dehydrogenase, melonate can disrupt the normal metabolism of pathogenic bacteria and inhibit their growth.

So, the correct answer is :

Option D : Succinic dehydrogenase

During TCA cycle, 6-C compound isocitrate is converted into succinyl CoA, a 4-C compound by removing two CO₂ molecules.

The steps are as follows -

During glycolysis, total 4 ATPs are produced from one glucose molecule with a net gain of 2 ATPs.

Less than seven percent of the energy in glucose is released during lactic acid fermentation and not all of it is trapped as high energy bonds of ATP.

The energy-releasing metabolic process in which the substrate is oxidized without an external electron acceptor is called :

Option B : Fermentation.

Fermentation is a metabolic process that occurs in the absence of an external electron acceptor, like oxygen. In fermentation, the organic substrate itself acts as the electron donor and acceptor. This process allows for the partial oxidation of glucose (or other substrates) under anaerobic conditions, resulting in the production of energy in the form of ATP, along with various end products like ethanol or lactic acid, depending on the type of fermentation.

To clarify the other options :

• Option A : Glycolysis is the process of breaking down glucose into pyruvate, yielding a small amount of energy. It occurs in both aerobic and anaerobic conditions and is not dependent on an external electron acceptor. However, it is not synonymous with fermentation.


• Option C : Aerobic respiration is a metabolic process that requires an external electron acceptor, typically oxygen, for the complete oxidation of substrates like glucose to produce energy.


• Option D : Photorespiration is a process in plants that occurs in light and consumes oxygen while producing carbon dioxide, but it is not primarily an energy-releasing process like fermentation.

All the given mechanisms in options leads to ATP formation. The type of method used depends on the type of organism and their energy transfer mechanisms.

$\bullet$ Photophosphorylation is the enzymatic formation of ATP from ADP coupled to the light dependent transfer of electrons in photosynthetic cells.

$\bullet$ Substrate level phosphorylation is the phosphorylation of ADP or some other nucleoside 5-diphosphate coupled to the dehydrogenation of an organic substrate, independent of the electron transfer chain.

$\bullet$ Oxidative phosphorylation is the enzymatic phosphorylation of ADP to ATP coupled to electron transfer from a substrate to molecular oxygen.

If mitochondria is absent in mature RBC then the source of energy for cell will be glycolysis. Mature mammalian RBC lack nucleus, mitochondria and other cell organelles in order to increase their oxygen carrying capacity. In such case, they cannot use none of the oxygen they transport for their own energy requirements. Thus, they use glycolysis and lactic acid fermentation as a source of energy.

Both assertion and reason are true and reason is correct explanation of assertion.

Respiratory pathway is considered as an amphibolic pathway as it involves both anabolic and catabolic reactions. For example, fatty acids are broken down to acetyl Co-A before entering respiratory pathway and many other carbonic acids are synthesised as an intermediate compounds.

The final stage of respiratory chain involves cytochrome oxidase, which contains copper. This stage can be specifically inhibited by cyanide or carbon monoxide. Cyanide combines with the copper and prevents oxygen from combining with it.

Glucose is converted into two molecules of pyruvic acid so, two cycles of Krebs’ cycle occur for one per glucose molecule. Each turn of Krebs’ cycle produces 3 NADH, 1 FADH$_2$ and 1 ATP molecule. On oxidation, NADH produces 3 ATP molecules and 1 FADH$_2$, which in turn produces 2 ATP molecules. Thus, in each turn of Krebs’ cycle 12 ATP molecules are produced. Two Krebs’ cycles occur per glucose molecule because glycolysis results in the formation of 2 pyruvic acid molecules by glucose breakdown.