Class 11 Biology NCERT Notes- Chapter 12: Respiration in Plants

Detailed Study Notes – Chapter 12: Respiration in Plants (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. Introduction to Cellular Respiration

• Core Purpose: All living organisms, including plants, require energy for life processes such as absorption, transport, movement, and reproduction. This energy is obtained by the oxidation of macromolecules, referred to as “food.”
• Energy Source: The ultimate source of all food on Earth is photosynthesis. Green plants and cyanobacteria trap light energy and convert it into chemical energy stored in the bonds of carbohydrates like glucose, sucrose, and starch.
• Cellular Respiration Defined: The process of breaking the C-C bonds of complex compounds (respiratory substrates) through oxidation within the cells to release a considerable amount of energy.
• Respiratory Substrates: These are the compounds that are oxidized during respiration.
  • Primary Substrate: Carbohydrates (like glucose) are the usual substrates.
  • Other Substrates: Proteins, fats, and even organic acids can be used under certain conditions.
• Energy Trapping: The energy released during respiration is not released all at once as heat. It is released in a series of slow, enzyme-controlled, step-wise reactions. This energy is trapped in the form of chemical energy as ATP (Adenosine Triphosphate).
• ATP as Energy Currency: ATP acts as the “energy currency of the cell.” It is broken down whenever and wherever energy is needed. The carbon skeletons produced during respiration are also used as precursors for the biosynthesis of other molecules.
• Locations in Eukaryotes:
  • Photosynthesis: Occurs in the chloroplasts.
  • Respiration (Breakdown of molecules): Begins in the cytoplasm (Glycolysis) and is completed in the mitochondria (Krebs’ cycle and ETS).

2. Plant Respiration specifics (Do Plants Breathe?)

• Gas Exchange: Plants require O₂ for respiration and release CO₂. They possess stomata (on leaves) and lenticels (on woody stems) for this purpose, but they lack specialized respiratory organs like animals.
• Reasons Plants Don’t Need Respiratory Organs:
  1. Localized Gas Exchange: Each plant part (roots, stems, leaves) takes care of its own gas exchange needs with very little transport of gases between parts.
  2. Lower Demand: The rate of respiration in roots, stems, and leaves is far lower than in animals. High volumes of gas exchange occur only during photosynthesis, for which leaves are well-adapted.
  3. Proximity to Surface: Most living cells in a plant are located close to the surface. In woody stems, living cells are organized in thin layers beneath the bark, and lenticels provide openings. Interior cells are dead and provide only mechanical support.
  4. Interconnected Air Spaces: Loose packing of parenchyma cells in leaves, stems, and roots creates a network of air spaces, facilitating contact with air.

3. Glycolysis (EMP Pathway)

• Definition: The breakdown of glucose into pyruvic acid. The term originates from Greek glycos (sugar) and lysis (splitting).
• Alternative Name: Known as the EMP pathway, named after its discoverers Gustav Embden, Otto Meyerhof, and J. Parnas.
• Location: Occurs in the cytoplasm of the cell. It is a universal pathway present in all living organisms.
• Oxygen Requirement: This process does not require oxygen and is the sole process of respiration in anaerobic organisms.
• Process Overview:
  1. Starting Material: One molecule of glucose (6-carbon). In plants, glucose is derived from sucrose (the end product of photosynthesis) or storage carbohydrates. Sucrose is broken into glucose and fructose by the enzyme invertase.
  2. Key Steps (A 10-reaction chain):
     • Energy Investment Phase: ATP is used twice. First, to convert glucose to glucose-6-phosphate (catalyzed by hexokinase). Second, to convert fructose-6-phosphate to fructose-1,6-bisphosphate.
     • Splitting: Fructose-1,6-bisphosphate (6C) is split into two 3-carbon molecules: dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
     • Energy Generation Phase:
       • One molecule of NADH + H⁺ is formed from NAD⁺ when PGAL is converted to 1,3-bisphosphoglycerate (BPGA). Since this happens for each 3C molecule, a total of 2 NADH + H⁺ are formed per glucose.
       • ATP is synthesized at two steps (substrate-level phosphorylation): when BPGA is converted to 3-phosphoglyceric acid (PGA), and when phosphoenolpyruvate (PEP) is converted to pyruvic acid. This results in 4 ATP molecules being synthesized per glucose.
  3. End Product: Two molecules of pyruvic acid (a 3-carbon compound).
• Net Yield from One Glucose Molecule:
  • ATP: 4 synthesized – 2 utilized = 2 ATP
  • NADH: 2 NADH + H⁺
  • Pyruvic Acid: 2 molecules

4. Fermentation (Anaerobic Respiration)

• Condition: Occurs under anaerobic (oxygen-deficient) conditions in many prokaryotes and unicellular eukaryotes.
• Purpose: To partially oxidize glucose and regenerate NAD⁺ from NADH+H⁺, allowing glycolysis to continue.
• Energy Yield: Very low. Less than 7% of the energy in glucose is released. The net gain is only 2 ATP (from glycolysis).
• Types of Fermentation:
  1. Alcoholic Fermentation:
     • Organisms: Yeast.
     • Process: Pyruvic acid is converted to ethanol and CO₂.
     • Enzymes: Pyruvic acid decarboxylase and alcohol dehydrogenase.
     • Toxicity: Yeast poisons itself to death when the alcohol concentration reaches about 13%.
  2. Lactic Acid Fermentation:
     • Organisms: Some bacteria; animal muscle cells during strenuous exercise.
     • Process: Pyruvic acid is reduced to lactic acid.
     • Enzyme: Lactate dehydrogenase.
     • Key Step: The reducing agent is NADH+H⁺, which is reoxidized to NAD⁺.

5. Aerobic Respiration

This is the process of complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water, and a large amount of energy. It is common in higher organisms.

5.1. The Link Reaction: Oxidative Decarboxylation

• Location: Mitochondrial matrix.
• Process: Pyruvic acid from the cytoplasm is transported into the mitochondria. It is then converted into acetyl CoA (a 2-carbon molecule).
• Enzyme Complex: Pyruvic dehydrogenase.
• Coenzymes Required: NAD⁺ and Coenzyme A.
• Products (per glucose molecule, i.e., 2 pyruvic acids):
  • 2 molecules of Acetyl CoA
  • 2 molecules of CO₂
  • 2 molecules of NADH + H⁺

5.2. Tricarboxylic Acid (TCA) Cycle / Krebs’ Cycle

• Discoverer: Hans Krebs.
• Location: Mitochondrial matrix.
• Process: A cyclic pathway where acetyl CoA is completely oxidized.
  1. Step 1: Acetyl CoA (2C) condenses with oxaloacetic acid (OAA) (4C) to form citric acid (6C). The enzyme is citrate synthase.
  2. Subsequent Steps: Citric acid undergoes a series of reactions, including two decarboxylation steps (release of CO₂), leading to the regeneration of OAA.
• Products (per turn of the cycle, i.e., per acetyl CoA):
  • 2 molecules of CO₂
  • 3 molecules of NADH + H⁺
  • 1 molecule of FADH₂
  • 1 molecule of GTP (which is converted to ATP, an example of substrate-level phosphorylation).
• Total Products (per glucose molecule, i.e., 2 acetyl CoA):
  • 4 molecules of CO₂
  • 6 molecules of NADH + H⁺
  • 2 molecules of FADH₂
  • 2 molecules of ATP (from GTP)

5.3. Electron Transport System (ETS) and Oxidative Phosphorylation

• Purpose: To release and utilize the energy stored in NADH+H⁺ and FADH₂ to synthesize ATP.
• Location: Inner mitochondrial membrane.
• Process:
  1. Electron Donation: Electrons from NADH and FADH₂ are passed down a chain of electron carriers.
  2. Oxygen’s Role: Oxygen is the final electron acceptor. It accepts electrons and protons (H⁺) to form water (H₂O). This drives the entire process by removing hydrogen from the system.
  3. Proton Pumping: As electrons move through the carrier complexes, energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient.
  4. ATP Synthesis (Oxidative Phosphorylation): The energy from this proton gradient is used by ATP synthase (Complex V) to synthesize ATP from ADP and inorganic phosphate. The process is explained by the chemiosmotic hypothesis.
• Electron Carrier Complexes:
  • Complex I: NADH dehydrogenase (oxidizes NADH).
  • Complex II: Succinate dehydrogenase (oxidizes FADH₂).
  • Ubiquinone: A mobile carrier receiving electrons from Complex I and II.
  • Complex III: Cytochrome bc₁ complex.
  • Cytochrome c: A small, mobile protein that transfers electrons between Complex III and IV.
  • Complex IV: Cytochrome c oxidase (contains cytochromes a and a₃, and two copper centers).
  • Complex V: ATP synthase. It has two components: F₀ (an integral protein forming a proton channel) and F₁ (a peripheral protein headpiece that synthesizes ATP). For each ATP produced, 4H⁺ pass through F₀.
• Energy Yield from ETS:
  • 1 NADH → 3 ATP
  • 1 FADH₂ → 2 ATP

6. The Respiratory Balance Sheet

• Theoretical Yield: A net gain of 38 ATP molecules can be calculated for the complete aerobic respiration of one glucose molecule.
• Calculation Breakdown:
  • Glycolysis: 2 ATP (direct) + 6 ATP (from 2 NADH) = 8 ATP
  • Link Reaction: 6 ATP (from 2 NADH) = 6 ATP
  • Krebs’ Cycle: 2 ATP (from GTP) + 18 ATP (from 6 NADH) + 4 ATP (from 2 FADH₂) = 24 ATP
  • Total: 8 + 6 + 24 = 38 ATP
• Real-world Caveats: This calculation is theoretical and based on assumptions that are not entirely valid in a living system (e.g., pathways are not perfectly sequential, intermediates are used for other syntheses).

7. Amphibolic Pathway

• Definition: A pathway that is involved in both catabolism (breakdown) and anabolism (synthesis).
• Respiration as Amphibolic:
  • Catabolic Aspect: It breaks down complex substrates like carbohydrates, fats, and proteins to release energy.
  • Anabolic Aspect: Intermediates from the respiratory pathway can be withdrawn to be used as precursors for synthesis. For example, acetyl CoA can be withdrawn to synthesize fatty acids. Amino acids can enter the Krebs’ cycle at various points after deamination, and likewise, intermediates can be withdrawn to synthesize amino acids.
• Conclusion: It is more accurate to consider the respiratory pathway as amphibolic rather than purely catabolic.

8. Respiratory Quotient (RQ)

• Definition: The ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.
• Formula: RQ = Volume of CO₂ evolved / Volume of O₂ consumed
• Significance: The RQ value depends on the type of respiratory substrate being used.
• RQ Values:
  • Carbohydrates (complete oxidation): RQ = 1.0 (e.g., 6 CO₂ / 6 O₂)
  • Fats (e.g., tripalmitin): RQ = < 1 (approx. 0.7) because they require more O₂ for their oxidation.
  • Proteins: RQ = ~0.9.
• In Living Organisms, Respiratory substrates are often a mix, not pure fats or proteins.

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Short-Answer Quiz (25 Questions)

Answer each question in 2-3 sentences.

1. What is the primary function of ATP in a cell?
2. Define cellular respiration in the context of energy release from food.
3. Name the two primary structures plants use for gaseous exchange and where they are typically located.
4. Explain the first of the three reasons why plants can survive without specialized respiratory organs.
5. What is the origin of the term “glycolysis,” and what is its alternative name?
6. Where does glycolysis occur in a eukaryotic cell, and is oxygen required for this process?
7. What are the net products of glycolysis from a single molecule of glucose?
8. Describe what happens to sucrose before it can enter the glycolytic pathway in plants.
9. Name the two steps in glycolysis where ATP is consumed.
10. What is the metabolic fate of pyruvic acid under anaerobic conditions in yeast?
11. How is lactic acid produced in animal muscle cells?
12. What is the main limitation of fermentation in terms of energy yield?
13. What crucial event happens to pyruvate before it enters the Krebs’ cycle?
14. What are the roles of NAD⁺ and Coenzyme A in the oxidative decarboxylation of pyruvic acid?
15. Name the molecule that acetyl CoA combines with to start the Krebs’ cycle and the initial product formed.
16. For every one molecule of glucose, how many molecules of NADH + H⁺ and FADH₂ are produced in the Krebs’ cycle?
17. What is the primary purpose of the Electron Transport System (ETS)?
18. What is the ultimate role of oxygen in aerobic respiration?
19. Differentiate between substrate-level phosphorylation and oxidative phosphorylation.
20. Describe the two main components of ATP synthase (Complex V) and their functions.
21. What are two of the key assumptions made when calculating the theoretical net gain of 38 ATP molecules per glucose?
22. Explain why the respiratory pathway is considered amphibolic rather than purely catabolic.
23. How do fats and proteins enter the respiratory pathway?
24. Define the Respiratory Quotient (RQ).
25. Why is the RQ for fats less than 1?

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Multiple-Choice Quiz (20 Questions)

Select the best answer for each question.

1. The complete oxidation of glucose produces which of the following as end products? a) Pyruvic acid and ATP b) Ethanol and CO₂ c) CO₂ and H₂O d) Lactic acid and H₂O
2. In eukaryotes, the Krebs’ cycle takes place in the: a) Cytoplasm b) Chloroplast stroma c) Inner mitochondrial membrane d) Mitochondrial matrix
3. The EMP pathway is another name for: a) The Krebs’ cycle b) Glycolysis c) Fermentation d) The Electron Transport System
4. How many net ATP molecules are produced directly from glycolysis of one glucose molecule? a) 1 b) 2 c) 4 d) 38
5. The conversion of pyruvic acid to acetyl CoA involves: a) Oxidative phosphorylation b) Substrate-level phosphorylation c) Oxidative decarboxylation d) Isomerisation
6. During alcoholic fermentation by yeast, pyruvic acid is converted to: a) Lactic acid only b) Acetyl CoA and CO₂ c) Ethanol and CO₂ d) Glucose and O₂
7. Which of the following is the final electron acceptor in the ETS? a) NAD⁺ b) Cytochrome c c) H₂O d) O₂
8. Oxidation of one molecule of FADH₂ via the ETS yields how many molecules of ATP? a) 1 b) 2 c) 3 d) 4
9. The respiratory quotient (RQ) for carbohydrates is: a) 0.7 b) 0.9 c) 1.0 d) Greater than 1
10. Which complex in the ETS is also known as NADH dehydrogenase? a) Complex I b) Complex II c) Complex III d) Complex IV
11. The enzyme that converts sucrose into glucose and fructose is: a) Hexokinase b) Invertase c) Pyruvic dehydrogenase d) Citrate synthase
12. If fatty acids are used as a respiratory substrate, they are first degraded to: a) Pyruvic acid b) Glycerol c) PGAL d) Acetyl CoA
13. The synthesis of ATP by ATP synthase is directly driven by: a) The flow of electrons b) The reduction of NAD⁺ c) A proton gradient d) The formation of water
14. How many molecules of CO₂ are released during the Krebs’ cycle for each molecule of glucose respired? a) 2 b) 3 c) 4 d) 6
15. In woody stems, gaseous exchange occurs through openings called: a) Stomata b) Parenchyma c) Lenticels d) Chloroplasts
16. Which of the following is a 6-carbon compound formed in the Krebs’ cycle? a) Oxaloacetic acid b) Citric acid c) α-ketoglutaric acid d) Malic acid
17. Yeast poisons itself to death when the alcohol concentration reaches approximately: a) 7% b) 13% c) 25% d) 50%
18. Cytochrome c acts as a mobile carrier for the transfer of electrons between: a) Complex I and Complex II b) Complex II and Complex III c) Complex III and Complex IV d) Complex IV and Oxygen
19. The F₁ headpiece of ATP synthase is a: a) Peripheral membrane protein complex b) Integral membrane protein complex c) Mobile electron carrier d) Proton donor
20. A respiratory substrate with an RQ of approximately 0.7 is likely a: a) Carbohydrate b) Protein c) Fat d) Organic acid

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Essay Questions (10 Questions with Answers)

1. Explain in detail the process of glycolysis, including the energy-investing and energy-yielding phases, and its final products. Answer: Glycolysis, also known as the EMP pathway, is the process of partially oxidizing one molecule of glucose to form two molecules of pyruvic acid. It occurs in the cytoplasm of all living cells and consists of a chain of ten enzyme-controlled reactions. The process begins with the phosphorylation of glucose to glucose-6-phosphate, using one ATP molecule. After isomerizing to fructose-6-phosphate, another ATP is used to form fructose-1,6-bisphosphate. This concludes the energy-investing phase. The fructose-1,6-bisphosphate is then split into two 3-carbon molecules, which eventually become 3-phosphoglyceraldehyde (PGAL). The energy-yielding phase begins as each PGAL is converted to 1,3-bisphosphoglycerate, reducing one molecule of NAD⁺ to NADH + H⁺. Subsequently, two substrate-level phosphorylation events occur for each 3-carbon chain: one when 1,3-bisphosphoglycerate is converted to 3-phosphoglyceric acid, and another when phosphoenolpyruvate is converted to pyruvic acid, yielding ATP at each step. Because one glucose yields two 3-carbon chains, the total output is 4 ATP and 2 NADH + H⁺. The final net products of glycolysis from one glucose molecule are 2 molecules of pyruvic acid, a net gain of 2 ATP, and 2 molecules of NADH + H⁺.
2. Compare and contrast aerobic respiration and fermentation.Answer: Aerobic respiration and fermentation are both processes that break down glucose to produce ATP, but they differ significantly in their mechanisms, oxygen requirement, location, and efficiency.
   • Oxygen Requirement: Aerobic respiration is strictly dependent on the presence of oxygen, which acts as the final electron acceptor. Fermentation is an anaerobic process that occurs in the absence of oxygen.
   • Extent of Breakdown: In aerobic respiration, glucose is completely broken down into carbon dioxide and water. In fermentation, glucose undergoes only a partial breakdown, resulting in products like ethanol and CO₂ or lactic acid.
   • Location: In eukaryotes, aerobic respiration’s main stages (Krebs’ cycle, ETS) occur within the mitochondria. Fermentation occurs entirely in the cytoplasm.
   • ATP Yield: Aerobic respiration is far more efficient, with a theoretical maximum yield of 38 ATP per glucose molecule. Fermentation has a net gain of only two molecules of ATP per glucose, produced during the initial glycolysis stage.
   • NADH Oxidation: In aerobic respiration, NADH is oxidized to NAD⁺ vigorously by the electron transport system, generating a large amount of ATP. In fermentation, NADH is oxidized back to NAD⁺ much more slowly by transferring its electrons to pyruvic acid or its derivatives, yielding no additional ATP.
3. Describe the sequence of events in the Tricarboxylic Acid (TCA) Cycle for one molecule of acetyl CoA. What is the total yield from one glucose molecule? Answer: The Tricarboxylic Acid (TCA) Cycle, or Krebs’ cycle, is a cyclic pathway in the mitochondrial matrix that completes the oxidation of acetyl CoA. The cycle begins when a two-carbon acetyl group from acetyl CoA condenses with the four-carbon molecule oxaloacetic acid (OAA) to form the six-carbon citric acid. Citrate is then isomerized to isocitrate. This is followed by two successive steps of decarboxylation, where two molecules of CO₂ are released, first forming α-ketoglutaric acid (5C) and then succinyl-CoA (4C). During these steps, NAD⁺ is reduced to NADH + H⁺. Succinyl-CoA is then converted to succinic acid, a step that involves a substrate-level phosphorylation, producing one molecule of GTP (which is then converted to ATP). In the remaining steps, succinic acid is oxidized back to OAA, generating one molecule of FADH₂ and one more molecule of NADH + H⁺. Since one glucose molecule produces two molecules of acetyl CoA, the cycle turns twice. Therefore, the total yield from one glucose molecule via the TCA cycle is 4 CO₂, 6 NADH + H⁺, 2 FADH₂, and 2 ATP (from GTP).
4. Explain the functioning of the Electron Transport System (ETS) in mitochondria, detailing the roles of the five complexes.Answer: The Electron Transport System (ETS) is a metabolic pathway located on the inner mitochondrial membrane whose purpose is to generate ATP through oxidative phosphorylation. It utilizes the energy stored in NADH+H⁺ and FADH₂.
   • Complex I (NADH dehydrogenase): Oxidizes NADH produced in the mitochondrial matrix, transferring electrons to ubiquinone. This process pumps protons from the matrix to the intermembrane space.
   • Complex II (Succinate dehydrogenase): Receives reducing equivalents from the oxidation of succinate to fumarate in the Krebs’ cycle via FADH₂ and transfers them to ubiquinone. This complex does not pump protons.
   • Complex III (Cytochrome bc₁ complex): Receives electrons from reduced ubiquinone (ubiquinol) and passes them to cytochrome c, a mobile carrier. This step also pumps protons across the membrane.
   • Complex IV (Cytochrome c oxidase): Receives electrons from cytochrome c and transfers them to the final electron acceptor, molecular oxygen. Oxygen combines with protons from the matrix to form water. This complex also pumps protons.
   • Complex V (ATP synthase): Is not an electron carrier but uses the energy from the electrochemical proton gradient created by Complexes I, III, and IV. Protons flow back into the matrix through its F₀ channel, driving the F₁ component to synthesize ATP from ADP and inorganic phosphate.
5. Discuss the statement: “The respiratory pathway is an amphibolic pathway.” Answer: The statement that the respiratory pathway is amphibolic is more accurate than calling it purely catabolic because it participates in both catabolism (breakdown) and anabolism (synthesis). Traditionally, respiration is seen as catabolic because it involves the breakdown of substrates like glucose, fats, and proteins to release energy. For instance, fats are broken into fatty acids and glycerol, and fatty acids are degraded to acetyl CoA to enter the pathway. However, the intermediates of this pathway also serve as precursors for biosynthesis. When the cell needs to synthesize molecules, it can withdraw these intermediates. For example, acetyl CoA, a product of glucose and fatty acid breakdown, can be withdrawn from the pathway to synthesize new fatty acids. Similarly, intermediates of the Krebs’ cycle, like α-ketoglutaric acid, can be used to synthesize amino acids (after amination). Because the respiratory pathway’s components are involved in both the breakdown and synthesis of key biomolecules, it functions as a central metabolic hub, justifying its classification as an amphibolic pathway.
6. Plants lack specialized respiratory organs. Justify this statement with the three main reasons provided in the text. Answer: Plants can thrive without specialized respiratory organs like lungs for three primary reasons. First, each plant part—roots, stems, and leaves—is largely self-sufficient and takes care of its own gas-exchange needs. There is very little transport of respiratory gases from one part of the plant to another. Second, plants generally do not have great demands for gas exchange; their overall metabolic rate is far lower than that of animals. The only period of high gas exchange is during photosynthesis, and leaves are perfectly adapted for this. Third, the physical structure of a plant ensures that gases do not have to diffuse over long distances. In large, bulky plants, most living cells are located quite close to the surface, such as in thin layers beneath the bark of stems, and are serviced by openings called lenticels. Furthermore, the loose packing of parenchyma cells in most tissues creates an interconnected network of air spaces, ensuring that most cells have at least part of their surface in contact with air.
7. Describe the process of oxidative phosphorylation, including the roles of the proton gradient and ATP synthase. Answer: Oxidative phosphorylation is the metabolic process that produces the vast majority of ATP during aerobic respiration. It occurs on the inner mitochondrial membrane and is powered by the energy of oxidation-reduction reactions from the electron transport system (ETS). As electrons from NADH and FADH₂ are passed along the ETS carrier complexes, the energy released is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This action creates a steep electrochemical proton gradient, with a higher concentration of H⁺ in the intermembrane space. This proton gradient represents a form of stored potential energy. This energy is harnessed by ATP synthase (Complex V). ATP synthase consists of two major components: F₀, an integral protein that forms a channel for protons to pass through, and F₁, a peripheral protein headpiece that contains the catalytic site for ATP synthesis. The protons flow down their concentration gradient, from the intermembrane space back into the matrix, through the F₀ channel. This passage of protons is coupled to the catalytic site in the F₁ component, driving the synthesis of ATP from ADP and inorganic phosphate.
8. What is the Respiratory Quotient (RQ)? Explain how its value differs for carbohydrates, fats, and proteins.Answer: The Respiratory Quotient (RQ) is defined as the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration. The value of RQ depends on the chemical nature of the respiratory substrate being oxidized.
   • Carbohydrates: When carbohydrates like glucose are completely oxidized, the amount of CO₂ released is equal to the amount of O₂ consumed (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Therefore, the RQ is 6 CO₂ / 6 O₂ = 1.0.
   • Fats: Fats are richer in hydrogen and poorer in oxygen compared to carbohydrates. Consequently, they require more oxygen for their complete oxidation relative to the amount of carbon dioxide produced. For example, the oxidation of tripalmitin requires 145 molecules of O₂ to produce 102 molecules of CO₂, resulting in an RQ of 102/145, which is approximately 0.7. Thus, the RQ for fats is always less than 1.
   • Proteins: The oxidation of proteins is more complex, but they also require more oxygen than the carbon dioxide they produce. The RQ for proteins is approximately 0.9.
9. Outline the journey of a glucose molecule from the cytoplasm to its complete oxidation into CO₂ in the mitochondria.Answer: The journey of a glucose molecule to complete oxidation is a multi-stage process.
   1. Glycolysis (Cytoplasm): The process begins in the cytoplasm where the 6-carbon glucose molecule is broken down into two 3-carbon molecules of pyruvic acid. This stage yields a net of 2 ATP and 2 NADH.
   2. Transport into Mitochondria: The two pyruvic acid molecules are then transported from the cytoplasm into the mitochondrial matrix.
   3. Link Reaction (Mitochondrial Matrix): Inside the matrix, each pyruvic acid molecule undergoes oxidative decarboxylation, catalyzed by the pyruvate dehydrogenase complex. This reaction removes one carbon atom as CO₂, reduces NAD⁺ to NADH, and attaches the remaining 2-carbon acetyl group to Coenzyme A, forming acetyl CoA. From the original glucose, this yields 2 acetyl CoA, 2 NADH, and 2 CO₂.
   4. Krebs’ Cycle (Mitochondrial Matrix): Each acetyl CoA molecule enters the Krebs’ cycle by combining with oxaloacetic acid. Through a series of reactions, the two carbons from the acetyl group are completely oxidized and released as two molecules of CO₂. For the two acetyl CoA from one glucose, the cycle turns twice, producing a total of 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP (via GTP). At this point, all six carbons from the original glucose molecule have been released as CO₂. The energy is now primarily stored in the electron carriers NADH and FADH₂.
10. What are the major metabolic fates of pyruvic acid produced during glycolysis? Explain the conditions under which each pathway is followed.Answer: Pyruvic acid, the key product of glycolysis, can be handled by cells in three major ways, depending on the cellular need and the availability of oxygen.
    1. Lactic Acid Fermentation: This pathway is followed under anaerobic conditions, for example, in some bacteria and in animal muscle cells during intense exercise when oxygen supply is inadequate. Pyruvic acid is directly reduced to lactic acid by the enzyme lactate dehydrogenase. This process oxidizes NADH + H⁺ to NAD⁺, allowing glycolysis to continue producing ATP.
    2. Alcoholic Fermentation: This also occurs under anaerobic conditions, typically in organisms like yeast. In a two-step process, pyruvic acid is first decarboxylated to release CO₂ and form an intermediate, which is then reduced by NADH + H⁺ to form ethanol. The enzymes involved are pyruvic acid decarboxylase and alcohol dehydrogenase. This also regenerates NAD⁺.
    3. Aerobic Respiration: This is the pathway taken when oxygen is available. Pyruvic acid is transported from the cytoplasm into the mitochondria. There, it is completely oxidized to CO₂ and water through the Krebs’ cycle and the electron transport system. This pathway yields a much larger number of ATP molecules compared to fermentation.

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Answer Keys

Short-Answer Quiz Answer Key

1. ATP acts as the energy currency of the cell. It traps the chemical energy released during respiration and is broken down whenever and wherever energy needs to be utilized for various life processes.
2. Cellular respiration is the breaking of C-C bonds of complex compounds (respiratory substrates) through oxidation within cells. This process leads to the release of a considerable amount of energy that is trapped in the form of ATP.
3. Plants use stomata, typically located on leaves, and lenticels, which are openings found on thick, woody stems.
4. First, each plant part (root, stem, leaf) takes care of its own gas-exchange needs. There is very little transport of gases between different parts of the plant.
5. The term originates from the Greek words glycos (sugar) and lysis (splitting). Its alternative name is the EMP pathway, after its discoverers Embden, Meyerhof, and Parnas.
6. Glycolysis occurs in the cytoplasm of the cell. It is an anaerobic process, meaning it does not require oxygen to proceed.
7. From one molecule of glucose, glycolysis produces 2 molecules of pyruvic acid, a net gain of 2 ATP, and 2 molecules of NADH + H⁺.
8. Sucrose, a disaccharide, must first be converted into its monosaccharide components, glucose and fructose. This conversion is catalyzed by the enzyme invertase, and both glucose and fructose can then readily enter the glycolytic pathway.
9. ATP is used first in the conversion of glucose into glucose-6-phosphate, and second in the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
10. Under anaerobic conditions, yeast converts pyruvic acid into CO₂ and ethanol through alcoholic fermentation. This process regenerates NAD⁺ from NADH, allowing glycolysis to continue.
11. In animal muscle cells, during periods of inadequate oxygen supply, pyruvic acid is reduced to lactic acid by the enzyme lactate dehydrogenase. The reducing agent for this reaction is NADH+H⁺.
12. Fermentation releases less than seven percent of the energy stored in glucose. The net gain is only two ATP molecules per glucose molecule, which is far less than the yield from aerobic respiration.
13. Before entering the Krebs’ cycle, pyruvate is transported into the mitochondrial matrix where it undergoes oxidative decarboxylation. It is converted into acetyl CoA, releasing one molecule of CO₂ and producing one molecule of NADH.
14. In this reaction, NAD⁺ acts as an oxidizing agent, accepting hydrogen atoms to become NADH + H⁺. Coenzyme A is a carrier molecule that attaches to the 2-carbon acetyl group, forming acetyl CoA, which can then enter the Krebs’ cycle.
15. Acetyl CoA combines with the 4-carbon molecule oxaloacetic acid (OAA). The initial product formed from this condensation reaction is the 6-carbon molecule citric acid.
16. For one glucose molecule (which yields two acetyl CoA), the Krebs’ cycle produces a total of 6 molecules of NADH + H⁺ and 2 molecules of FADH₂.
17. The primary purpose of the ETS is to release and utilize the energy stored in the electron carriers NADH+H⁺ and FADH₂. This energy is used to synthesize a large number of ATP molecules.
18. Oxygen acts as the final hydrogen (and electron) acceptor in the electron transport chain. By accepting electrons and protons, it forms water, which drives the entire respiratory process by removing hydrogen from the system.
19. Substrate-level phosphorylation is the direct synthesis of ATP from ADP by transferring a phosphate group from a substrate molecule, as seen in glycolysis and the Krebs’ cycle. Oxidative phosphorylation involves the synthesis of ATP using the energy released from the oxidation of NADH and FADH₂ via the ETS and the creation of a proton gradient.
20. ATP synthase consists of F₀, an integral membrane protein complex that forms a channel for protons to cross the inner membrane, and F₁, a peripheral membrane protein headpiece that contains the catalytic site for ATP synthesis.
21. Two key assumptions are: 1) There is a sequential, orderly pathway where glycolysis, TCA cycle, and ETS follow one another. 2) None of the intermediates in the pathway are utilized to synthesize any other compound.
22. It is considered amphibolic because it is involved in both catabolism (the breakdown of substrates for energy) and anabolism (the synthesis of complex molecules). Intermediates from the respiratory pathway can be withdrawn to act as precursors for synthesizing compounds like fatty acids and amino acids.
23. Fats are broken down into glycerol (which enters as PGAL) and fatty acids (which are degraded to acetyl CoA). Proteins are degraded to amino acids, which, after deamination, can enter the pathway at various stages, such as pyruvate, acetyl CoA, or as an intermediate in the Krebs’ cycle.
24. The Respiratory Quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.
25. Fats contain more hydrogen atoms and fewer oxygen atoms relative to carbon than carbohydrates do. Therefore, their complete oxidation requires a larger volume of O₂ to be consumed compared to the volume of CO₂ that is evolved, resulting in a ratio less than 1.

Multiple-Choice Quiz Answer Key

1. c) CO₂ and H₂O
2. d) Mitochondrial matrix
3. b) Glycolysis
4. b) 2
5. c) Oxidative decarboxylation
6. c) Ethanol and CO₂
7. d) O₂
8. b) 2
9. c) 1.0
10. a) Complex I
11. b) Invertase
12. d) Acetyl CoA
13. c) A proton gradient
14. c) 4
15. c) Lenticels
16. b) Citric acid
17. b) 13%
18. c) Complex III and Complex IV
19. a) Peripheral membrane protein complex
20. c) Fat

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Glossary of Key Terms

• Aerobic Respiration: The process that leads to a complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water, and a large amount of energy.
• Amphibolic Pathway: A metabolic pathway that is involved in both catabolism (breakdown) and anabolism (synthesis).
• Anaerobic Respiration: Respiration that occurs in the absence of oxygen, such as fermentation.
• ATP (Adenosine Triphosphate): The energy currency of the cell; a molecule that stores and transports chemical energy within cells.
• ATP Synthase (Complex V): An enzyme complex in the inner mitochondrial membrane that synthesizes ATP from ADP and inorganic phosphate using the energy of a proton gradient.
• Catabolism: The set of metabolic pathways that breaks down molecules into smaller units to release energy.
• Cellular Respiration: The mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for the synthesis of ATP.
• Electron Transport System (ETS): A series of protein complexes and electron carriers in the inner mitochondrial membrane through which electrons pass, releasing energy to form a proton gradient.
• EMP Pathway: Another name for glycolysis, after its discoverers Gustav Embden, Otto Meyerhof, and J. Parnas.
• FADH₂ (Flavin Adenine Dinucleotide): A redox cofactor that is created during the Krebs cycle and utilized during the last part of respiration, the electron transport chain.
• Fermentation: The incomplete oxidation of glucose achieved under anaerobic conditions where pyruvic acid is converted to CO₂ and ethanol or to lactic acid.
• Glycolysis: The breakdown of glucose by enzymes, releasing energy and pyruvic acid. It occurs in the cytoplasm.
• Krebs’ Cycle (TCA Cycle): A cyclic series of reactions in the mitochondrial matrix that completes the breakdown of glucose by oxidizing acetyl CoA to carbon dioxide.
• Lenticels: Openings in the bark of woody stems that allow for gaseous exchange.
• NADH (Nicotinamide Adenine Dinucleotide): An electron carrier that stores energy used to make ATP.
• Oxidative Decarboxylation: A chemical reaction in which a carboxyl group is removed from a molecule, forming carbon dioxide, while the molecule itself is oxidized.
• Oxidative Phosphorylation: The process in which ATP is formed as a result of the transfer of electrons from NADH or FADH₂ to O₂ by a series of electron carriers.
• Pyruvic Acid: The three-carbon compound that is the end product of glycolysis.
• Respiratory Quotient (RQ): The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration.
• Respiratory Substrates: The compounds that are oxidised during the process of respiration to release energy.
• Stomata: Pores, typically on the underside of leaves, that allow for gaseous exchange.
• Substrate-level Phosphorylation: The direct formation of ATP by the transfer of a phosphate group from a substrate to ADP.