Respiration in Plants MCQ Quiz - Objective Question with Answer for Respiration in Plants - Download Free PDF

Key Points

• Statement I is true: During glycolysis, Nicotinamide Adenine Dinucleotide, reduced form (NADH) is produced in the cytoplasm. Since the mitochondrial inner membrane is impermeable to NADH, the electrons from NADH are transferred to the electron transport chain in the mitochondria using shuttle systems, which subsequently undergo oxidative phosphorylation to produce ATP.
• NADH plays a key role in energy production: It donates electrons to the electron transport chain, which generates a proton gradient that drives ATP synthesis.
• Statement II is false: In fermentation, NADH is oxidized back to NAD+ quickly to sustain glycolysis in the absence of oxygen. However, in aerobic respiration, NADH oxidation via oxidative phosphorylation is a highly efficient process, producing much more ATP.
• Fermentation is less efficient: It regenerates NAD+ but produces significantly less ATP compared to oxidative phosphorylation in aerobic respiration.

Additional Information

• Aerobic Respiration: This process occurs in the presence of oxygen, where glucose is completely oxidized into carbon dioxide and water. It involves glycolysis, the Krebs cycle, and oxidative phosphorylation. It is highly efficient, producing approximately 36-38 ATP molecules per glucose molecule.
• Fermentation: In the absence of oxygen, cells rely on fermentation to regenerate NAD+ from NADH to sustain glycolysis. Two common types are lactic acid fermentation (in muscle cells) and alcoholic fermentation (in yeast). Only 2 ATP molecules are produced per glucose molecule.
• Shuttle Systems: The NADH produced during glycolysis cannot directly enter the mitochondria. Instead, shuttle systems like the glycerol-phosphate shuttle or the malate-aspartate shuttle transfer electrons from cytosolic NADH to mitochondrial carriers, facilitating its entry into oxidative phosphorylation.
• Oxidative Phosphorylation: This is the process by which ATP is synthesized using the energy released by the electron transport chain. It occurs in the inner mitochondrial membrane and utilizes oxygen as the final electron acceptor.
• NADH Function: NADH serves as an essential electron donor in metabolic reactions. It transfers high-energy electrons to the electron transport chain, aiding in the production of ATP.
• Comparison of Energy Efficiency: Aerobic respiration is far more efficient than fermentation. While aerobic respiration yields 36-38 ATP molecules per glucose molecule, fermentation yields only 2 ATP molecules.

The correct answer is Succinate dehydrogenase

Concept:

• The mitochondrial electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons derived from nutrients to oxygen, producing ATP through oxidative phosphorylation.
• Complex II of the ETC is also known as Succinate dehydrogenase. It plays a dual role in both the ETC and the citric acid cycle.
• Unlike other ETC complexes, Complex II does not pump protons across the mitochondrial membrane. Instead, it directly transfers electrons to ubiquinone (coenzyme Q).

Explanation:

• Complex I (NADH dehydrogenase): This complex transfers electrons from NADH to ubiquinone (coenzyme Q), creating ubiquinol and pumping protons into the intermembrane space.
• Complex II (succinate dehydrogenase): This complex oxidizes FADH2 to FAD and transfers electrons to ubiquinone, which is then reduced to ubiquinol.
• Complex III (cytochrome bc1 complex): This complex transfers electrons from reduced ubiquinone (ubiquinol) to cytochrome c, coupled with the translocation of protons across the membrane.
• Complex IV (cytochrome c oxidase): This complex transfers electrons from cytochrome c to oxygen, reducing it to water and helping to pump protons across the membrane.

Fig. ETS.

The correct answer is succinyl-CoA to succinic acid

Concept:

• The Krebs cycle, also known as the citric acid cycle or TCA (tricarboxylic acid) cycle, is a series of enzymatic reactions in the mitochondria that generates ATP through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.
• One molecule of GTP (or ATP in certain cells) is produced directly in the Krebs cycle.
• GTP synthesis involves substrate-level phosphorylation, a process where a phosphate group is directly transferred from a substrate to ADP or GDP.
• Succinyl-CoA to succinic acid: This step involves the conversion of succinyl-CoA to succinate. The enzyme succinyl-CoA synthetase catalyzes a substrate-level phosphorylation reaction, resulting in the formation of GTP (or ATP) from GDP and Pi. This is the only step in the Krebs cycle where GTP is directly synthesized.

Concept:

• Glycolysis is the metabolic pathway that converts glucose (C₆H₁₂O₆) into pyruvate, releasing energy and forming ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).
• It is the first step in cellular respiration and occurs in the cytoplasm of the cell.
• Glycolysis does not require oxygen (anaerobic process), making it essential for both aerobic and anaerobic organisms.

Explanation:

• PGA (Phosphoglycerate): This is an intermediate compound formed during glycolysis, not the end product. Specifically, 3-phosphoglycerate is formed in the middle stages of glycolysis.
• Pyruvic Acid (PA): The end product of glycolysis. Each molecule of glucose is converted into two molecules of pyruvic acid. This pyruvate can then be used in the Krebs cycle (aerobic respiration) or fermentation (anaerobic processes).
• Acetyl CoA: This is not a direct product of glycolysis. Pyruvic acid undergoes oxidative decarboxylation to form Acetyl CoA before entering the Krebs cycle.
• Citric Acid: This is a product of the Krebs cycle, not glycolysis. It is formed when Acetyl CoA combines with oxaloacetic acid in the first step of the Krebs cycle.

Fig: Steps in Glycolysis

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

Explanation:

• Complex I: Also known as NADH dehydrogenase, this complex transfers electrons from NADH to ubiquinone (coenzyme Q), initiating the electron transport chain. It is the first enzyme in the mitochondrial respiratory chain.
• Complex II: Known as succinate dehydrogenase, this complex transfers electrons from FADH2 to ubiquinone. It is the only complex that is part of both the citric acid cycle and the electron transport chain.
• Complex III: Also known as the cytochrome bc1 complex, it transfers electrons from ubiquinol to cytochrome c. It plays a critical role in creating the proton gradient used for ATP synthesis.
• Complex IV: Known as cytochrome c oxidase, this complex facilitates the transfer of electrons from cytochrome c to oxygen, forming water. It is the final electron acceptor in the chain and is crucial for the reduction of oxygen to water.
• ATP Synthase: Also known as Complex V, although not always counted among the electron transport chain complexes, it uses the proton gradient generated by the ETC to synthesize ATP from ADP and inorganic phosphate.

The correct answer is Zero.

Key Points

• The term glycolysis has originated from the Greek words, glycos for ''sugar'' and lysis for ''splitting''.
• The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway.
• In anaerobic organisms, it is the only process in respiration.
• Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms.
• In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.

​Explanation:

• The glycolysis is a common pathway for aerobic and anaerobic respiration and no oxygen O₂ is consumed during this process.
• It is a universal pathway that occurs in every living organism be it be aerobic or anaerobic.
• Therefore, the number of oxygen molecules required during glycolysis of one glucose molecule is ''Zero''

Additional Information 

• It involves a series of 10 biochemical reactions where 1 molecule of glucose is degraded to 2 molecules of pyruvic acid.
• Each step is governed by some enzyme.
• In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates.
• Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway.
• Glucose and fructose are phosphorylated to give rise to glucose-6- phosphate by the activity of the enzyme hexokinase.
• This phosphorylated form of glucose then isomerizes to produce fructose-6- phosphate.
• Subsequent steps of the metabolism of glucose and fructose are the same.
• In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose

The correct answer is glucose.

Key Points

• The breakdown of glucose into pyruvate within a cell's cytoplasm is known as glycolysis.
• During aerobic conditions, pyruvate can diffuse into mitochondria where it joins the citric acid cycle and produces reducing equivalents in the form of NADH and FADH2.
• The electron transport chain then receives these reducing equivalents, which results in the creation of 32 ATP per molecule of glucose.
• Because the electron transport chain requires oxygen as the final electron acceptor, inadequate tissue oxygenation inhibits the process of oxidative phosphorylation.
• Under anaerobic conditions, pyruvate has a different fate.
• Instead of entering mitochondria, the cytosolic enzyme lactate dehydrogenase converts pyruvate to lactate.
• The regeneration of NAD+ from NADH is also made possible by this process.
• An oxidizing cofactor called NAD+ is required to keep the flow of glucose through glycolysis going.
• Glycolysis produces 2 ATP per glucose molecule, and thus provides a direct means of producing energy in the absence of oxygen.
• This process of breaking down glucose in the absence of oxygen is aptly named anaerobic glycolysis.

Additional Information

• Lactic acid:
  • Lactic acid is mainly produced in muscle cells and red blood cells.
  • It forms when the body breaks down carbohydrates to use for energy when oxygen levels are low. 
• Fructose:
  • Fructose is a type of sugar known as a monosaccharide.
• Citric acid:
  • Citric acid is a weak acid that is found naturally in all citrus fruits.

Mistake Points

• Please note that the correct answer is "b, c₁, c, a, a₃", which is not given in the options.
• So, the nearest possible option has been marked correct and the correct explanation has been provided in the following solution.

Concept-

• Aerobic respiration is the complete oxidation of food with the use of food.

C₆H₁₂O₆ + 6 O₂ + 6 H₂O → 6 CO₂ + 12 H₂O + Energy

• Steps of aerobic respiration- Glycolysis, Krebs cycle, ETS.

Explanation-

• The metabolic pathway through which the electron passes from one carrier to another is called the electron transport system (ETS).
• It is present in the inner mitochondrial membrane. 
• Cytochromes are the proteins that help in electrons transfer.

Hence the correct sequence is Cyt b, c₁, c, a, a₃.

Note- You can easily get the answer from the above diagram.

Additional Information

• Molecular oxygen act as the final electron acceptor.
• Oxygen act as the final H2 acceptor. \

The correct answer is more than 1.

Hint

Respiratory substrate & Respiratory Quotient (RQ) 

• The respiratory substrate is broken down to release energy in the process of respiration. Aerobic and anaerobic respiration are the main two types of respiration that occur in the presence or absence of molecular oxygen. Glucose is the most common respiratory substrate present in plants.
• During aerobic respiration, O₂ is consumed and CO₂ is released.
• "The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called the Respiratory Quotient (RQ) or Respiratory Ratio."
• Respirometer is an instrument used for measuring the rate of respiration as well as R.Q. 

RQ Value is calculated using the given formula : 

\(RQ = \frac{Volume\ of \ CO_2 \ evolved }{Volume\ of \ O_2 \ consumed}\)

The respiratory quotient depends upon the type of respiratory substrate used during respiration.

Explanation:

RQ of Organic acids (More than 1)

•    Organic acids like - Malic acid (Refer below reaction), Tartaric acid, Oxalic acid are rich in O₂ and require less O₂  for their oxidation, so when the organic acids are oxidized in respiration, the value of R.Q. is greater than one. The reaction & RQ is given below : 

​C₄H₆O₅ (Malic acid) + 3O₂  → 4CO₂ + 3H₂O + Energy

\(RQ = \frac{Volume\ of \ CO_2 \ evolved }{Volume\ of \ O_2 \ consumed} = \frac{4}{3} = 1.33 \)

Important Points

Significance of RQ:

•   Helps in determining the type of respiration being performed.
• Determine the nature of the respiratory substrate that oxidized the transformation of the substrate and the biochemical mechanism of respiration.
• Knowing the metabolic rate.
• Determination of various pathological conditions like acidosis, alkalosis, etc.  

Additional Information

1. RQ of Succulent plants  [Zero]

• In succulent plants like - Bryophyllum, Opuntia, etc., stomata remain open during the nighttime. So, at night when oxygen is absorbed, the carbohydrates are partially oxidized to organic acids without the formation of CO₂. So, the value of R.Q. remains zero. 
• During the day, the value of RQ remains zero because the CO₂ released during respiration is used up as a substrate for photosynthesis

​2C6H12O6 (Glucose) + 3O2  → 3C₄H₆O₅ (Malic acid) + 3H2O + Energy

\(RQ = \frac{Volume\ of \ CO_2 \ evolved }{Volume\ of \ O_2 \ consumed} = \frac{0}{3} = 0\)

      2. RQ of Carbohydrates [1]

• When carbohydrates are used as substrate and are completely oxidized, the RQ will be 1, because equal amounts of CO₂ and O₂ are evolved and consumed, respectively.

​ C₆H₁₂O₆ + 6O2  (Glucose) → 6CO₂ + 6H2O + Energy

\(RQ = \frac{Volume\ of \ CO_2 \ evolved }{Volume\ of \ O_2 \ consumed} = \frac{6}{6} = 1 \)

     3. RQ of Fats [Less than 1]

• Fats need more amount of O₂ for their oxidation as compared to other substrates. Fats are usually present in seeds as storage food in plants.
• Fats need more amount of O₂ for their oxidation as compared to other substrates.
• R.Q. value is less than one or unity in the aerobic respiration of fats.

​2C5 H98O6 (Tripalmitin)+ 145O2  → 102CO2 + 98H2O + Energy

\(RQ = \frac{Volume\ of \ CO_2 \ evolved }{Volume\ of \ O_2 \ consumed} = \frac{102}{145} = 0.7 \) (Less than 1)

     4. RQ of Proteins & their derivatives [Less than 1]

• Proteins are building blocks of the living body and are made up of amino acids. 
• When the cells are going to starvation, then proteins act as a respiratory substrate.
• When proteins are respiratory substrates the ratio would be about 0.9.
• Protein structures vary with changes in amino acid sequence.
•  So, the value of R.Q. also varies from 0.8 - 0.9 depending upon the structure of different amino acids.

​2C3H₇O₂ N(Alanine)+ 6O2  → CO(NH₂)₂  (Urea) + 5CO2 + 5H2O + Energy

     5. RQ of Aerobic respiration [More than 1]

• In aerobic respiration, when respiratory substrate is fat or protein, R.Q is less than 1.
• When carbohydrate is the substrate, R.Q is 1.

Explanation:

• Pyruvate or pyruvic acid, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters the mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalyzed by pyruvic dehydrogenase.
• The reactions catalyzed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD⁺ and Coenzyme A. This process is called Link reaction or gateway reaction.
• During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis)
• The acetyl CoA then enters a cyclic pathway, the citric acid cycle more commonly called Krebs’ cycle after the scientist Hans Krebs who first elucidated it.

• Thus, Pyruvic acid before combining with oxaloacetic acid of the citric acid cycle becomes Acetyl CoA​

Additional Information

• Under anaerobic conditions, the pyruvic acid undergoes partial oxidation to form lactic acid. This process is catalyzed by the enzyme lactate dehydrogenase
• During the Kreb's cycle, Acetyl CoA condenses with Oxaloacetic acid to form Citric acid. This Citric acid is then converted into Cis Aconitic acid in the presence of enzyme Aconitase.

Concept:

• Glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway. 

• Glycolysis is a step in respiration that occurs in aerobic and anaerobic processes.
• It occurs in the cytoplasm of the cell and is present in all living organisms.
• During the process of glycolysis, glucose undergoes partial oxidation to form two molecules of pyruvic acid.

Explanation:

Option 1: 

• Phosphoglucomutase is an enzyme that transfers a phosphate group in D-glucose monomer from 1-to 6 positions of carbon in a forward direction (changes glucose 1-phosphate to glucose-6 phosphate).
• Therefore, this is an incorrect option.​

Option 2: 

• Phosphoglucoisomerase catalyzes the conversion of glucose- 6- phosphate to fructose -6 -phosphate.
• Therefore, this is an incorrect option.​

Option 3:

• In the process of glycolysis, the first step involves the phosphorylation of glucose/fructose, which results in the formation of Glucose-6-phosphate.
• The reaction is catalyzed with the help of the enzyme hexokinase in the presence of an Mg⁺² ion. The reaction is:
• Glucose(6C) + ATP → Glucose-6-phosphate + ADP
• Therefore, this is the correct answer.​

Option 4: 

• Phosphorylase is an enzyme that catalyzes the addition of phosphate groups from inorganic phosphate to an acceptor.
• Therefore, this is an incorrect option.

​ 

Concept:

• Lipids are polymers made up of fatty acids and glycerol.
• Fatty acids are organic acids with hydrocarbon side chains ending in a carboxylic group (-COOH).
• The hydrocarbon side chain may be straight or have a ring structure.
• Fatty acids are obtained from the hydrolysis of fats.
• There are two types of fatty acids - 

1. Saturated:
   • ​Do not possess double bonds in the carbon chain.
   • E.g.: Palmitic acid [C₁₆H₃₂O₂]
2. Unsaturated:
   • Presence of one or more double bonds in the carbon chains.
   • E.g.: Linoleic acid [C₁₈H₃₂O₂ - two double bond]

Important Points

Biosynthesis of fatty acids:

• Fatty acid synthesis takes place in the endoplasmic reticulum and cytoplasm.
• Fatty acids are synthesized in most living organisms including humans from non-lipid substances.
• The building material from which fatty acids are synthesized is Acetyl CoA.
• Carbohydrates and proteins can be metabolically degraded to Acetyl CoA. Thus they can be precursors for fatty acid formation.
• From the Krebs cycle which is a part of the respiratory mechanism in aerobic organisms, acetyl CoA is used as a precursor for the synthesis of fatty acid.
• Fatty acid synthesis involves joining together acetyl CoA units to form long carbon chain molecules.
• It employs different enzymes, ATP, NADPH, coenzyme A, vitamin B₁₂ and biotin.
• Thus from the above-given information, the compound that will be withdrawn from the respiratory pathway for the synthesis of fatty acid is Acetyl CoA.

So the correct answer is option 2.

Confusion Points

• TCA cycle takes place in the mitochondria and hence, Acetyl CoA is also produced in the mitochondria.
• Though acetyl CoA is the molecule required for fatty acid synthesis, it is first converted to citric acid in the mitochondria.
• This citric acid is then exported from mitochondria to the cytoplasm and converted back to Acetyl CoA by the cytosolic enzyme ATP-citrate lyase.
• However, the answer has been given according to official answer key as well as NCERT text.

Key Points

• Substrate level phosphorylation is defined as the process of formation of ATP by the physical addition of phosphate group to ADP.
• It occurs during both glycolysis and Kreb's cycle. 
• Oxidative phosphorylation refers to the oxidation of reducing substances NADH and FADH₂ by the transfer of their electrons through an electron transport chain to oxygen, which releases energy for the synthesis of ATP by ADP and Pi.
• In oxidative phosphorylation, ATP produces by chemiosmosis and it takes place in the inner membrane of mitochondria.
• EMP pathway is commonly known as glycolysis.
• It takes place in the cytoplasm of all living cells, both aerobically as well as anaerobically.

Explanation:

• In the EMP pathway, 4 ATP molecules are formed as a result of substrate-level phosphorylation. 
  • When a phosphate group is lost, 1,3-diphosphoglycerate transforms into 3-phosphoglycerate, forming the first two ATP molecules.
  • In the final stage of glycolysis, 2-phosphoenol pyruvic acid transforms into pyruvic acid by transferring its high-energy phosphate bond from ADP and forming two more ATP.
• In the EMP pathway, there are 6 ATP molecules are formed during oxidative phosphorylation.
  • Glycolysis proceeds by phosphorylation of glyceraldehyde-3-phosphate to 1,3- diphosphoglyceric acid at carbon 1 by inorganic phosphoric acid and simultaneously oxidation takes by dehydrogenation.
  • NAD-dependent glyceraldehyde-3-phosphate dehydrogenase enzyme is responsible for oxidation.
  • The resulting hydrogen is carried by NAD as NADH + H⁺.
  • Forms six ATP molecules in this step.

Additional Information

• During Kreb's cycle, succinyl coenzyme A transfers its high energy to a phosphate group, which combines GDP/ADP to produce GTP/ATP and proceeds to phosphorylate two ATP molecules at the substrate level.
• In oxidative phosphorylation, the final electron acceptor is oxygen, and the resulting proton gradient provides energy for the ATP synthase enzyme to produce ATP.

Hence, the correct option is (3) 4 and 6.

Concept:

• The respiratory quotient is also known as the respiratory ratio and is denoted by RQ.
• It is defined as the volume of carbon dioxide released over the volume of oxygen absorbed during the process of respiration.
• \({RQ} = {Volume \space of \space CO_2 \space released \over Volume \space of \space O_2 \space absorbed}\)​

Important Points

• The respiratory quotient depends on the type of respiratory substrate used during respiration, the amount of oxygen present in the respiratory substrate, the extent to which respiratory substrate is broken down.

Carbohydrates -

• When carbohydrate is the substrate, following three cases can be there:

1. In aerobic respiration:
   • C₆H₁₂O₆+ 6O₂ → 6CO₂ + 6H₂O
   • Equal amounts of carbon dioxide and oxygen are evolved and consumed, respectively. So, RQ = 1.
2. In anaerobic respiration:
   • As this process occurs in absence of oxygen, so total amount of oxygen consumed/absorbed is 0 and only carbon dioxide production is there.
   • RQ = ∞
3. When Carbohydrate is incompletely oxidized, there is no production of CO₂. RQ = 0

​Proteins -

• RQ for protein is approximately 0.8.
• We can consider the reaction for albumin metabolism here,

C₇₂H₁₁₂N₁₈O₂₂S + 77 O₂ → 63 CO₂ + 38 H₂O + SO₃ + 9 CO(NH₂)₂

• \({RQ} = {63 \over 77} = 0.8\)

Fats -

• Fats are not completely oxidized so, RQ is 0.7.
• We can consider the chemical equation of triglyceride here,

C₅₅H₁₀₄O₆ + 78 O₂ → 55 CO₂ + 52 H₂O

• \({RQ} = {55 \over 78} = 0.7\)

​Organic acids -

• Organic acids (malic acid , tartaric acid, oxalic acid) contain a higher amount of oxygen in them, so they require less O₂ for respiration.
• This makes the respiratory quotient to be more than 1.

C₄H₆O₅ (Malic acid) + 3 O₂ → 4 CO₂ + 3 H₂O + Energy

• \({RQ} = {4 \over 3} = 1.33\)

​​Explanation:

• A RQ of 0.7 can be achieved when fats or lipids are used as substrate.
• On the other hand, incomplete oxidation of carbohydrates will result in a RQ = 0.
• Thus, carbohydrates and lipids can together give a RQ of 0.7.
• For all the other given combinations, the RQ will be more than 0.7.

Therefore, the correct answer is option 4 - (b) + (d).

Concept:

• Cellular respiration can be defined as the metabolic process by which complex organic substances are broken down into simple products. 
• During this process, energy is released that is stored in energy-carrying biomolecules called ATP. This energy is later utilized to carry out other metabolic activities.
• In eukaryotic cells, mitochondria play an important role in ATP synthesis.
• The enzymes required for ATP synthesis are present in the mitochondria.
• However, the generation of ATP also takes place in certain processes without involving mitochondria such as in glycolysis.

Explanation:

Option 1: Krebs cycle - INCORRECT

• ​Krebs cycle also known as the TCA cycle is part of aerobic respiration.
• It provides energy to cells in the form of ATP.
• It takes place in the matrix of mitochondria.

Option 2: Electron Transport Chain - INCORRECT

• Electron Transport Chain is the third step during aerobic respiration and involves the formation of the water molecule and ATP.
• ETS takes place in the inner mitochondrial membrane.
• NADH₂ and FADH₂ produced during the initial steps of aerobic respiration - glycolysis, connecting link reaction, and Kreb's cycle- are oxidized in ETC with the help of various electron carriers and enzymes.

Option 3: Substrate-level Phosphorylation - CORRECT

• ​Substrate-level Phosphorylation is a chemical reaction in which ATP is synthesized by direct transfer of phosphate group from an intermediate molecule to ADP.
• Substrate-level Phosphorylation is seen during the process of glycolysis.
• Glycolysis is the first step in both aerobic and anaerobic respiration.
• It takes place in the cytoplasm without the use of oxygen.
• During glycolysis 2 ATP molecules are produced.
• These ATP molecules are formed without the involvement of mitochondria.

Option 4: Photorespiration - INCORRECT

• ​Photorespiration occurs in photosynthetic plants.
• Photorespiration can be defined as a light-dependent process in which the photosynthetic organs of a plant utilize oxygen and release carbon dioxide.
• Photorespiration is considered to be a wasteful process. This is because 25% of photosynthetically fixed carbon dioxide is lost in this process.
• It takes place in several organelles - chloroplast, peroxisome and mitochondria.
• At the end of the photorespiration CO₂ and ammonia are released. No ATP is produced in this process.

So the correct answer is option 3 (Substrate-level phosphorylation).