NCERT Solutions: Respiration in Plants

Q1: Differentiate between
(a) Respiration and Combustion
(b) Glycolysis and Krebs' cycle
(c) Aerobic respiration and Fermentation
Ans:
(a) Respiration and combustion

(b) Glycolysis and Krebs cycle

(c) Aerobic respiration and fermentation

Q2: What are respiratory substrates? Name the most common respiratory substrate. 
Ans: Respiratory substrates are compounds that are oxidised in cells to release energy during respiration. The most common respiratory substrate is glucose. Other compounds such as fats (as fatty acids), proteins (as amino acids after deamination) and organic acids can also serve as respiratory substrates.

Q3: Give the schematic representation of glycolysis?
Ans: Glycolysis is the pathway that converts one molecule of glucose into two molecules of pyruvate, producing a small net gain of ATP and reduced NAD (NADH). It consists of ten enzyme-catalysed steps including phosphorylation, cleavage and oxidation steps, and yields ATP by substrate-level phosphorylation.

Q4: What are the main steps in aerobic respiration? Where does it take place?
Ans: The main steps in aerobic respiration are as follows:

• Glycolysis: Occurs in the cytoplasm (cytosol). Glucose (6C) is converted into two molecules of pyruvate (3C), with a small net gain of ATP and formation of NADH.
• Oxidative decarboxylation of pyruvic acid to acetyl coenzyme A: Takes place in the mitochondrial matrix. Pyruvate is decarboxylated and linked to coenzyme A to form acetyl-CoA, producing NADH.
• Tricarboxylic acid (TCA) or Krebs cycle: Takes place in the mitochondrial matrix. Acetyl-CoA is oxidised to CO₂, producing NADH, FADH₂ and a small amount of ATP (or GTP).
• Electron transport chain (ETS) and oxidative phosphorylation: Occur across the inner mitochondrial membrane. Electrons from reduced coenzymes (NADH, FADH₂) pass along carrier complexes and drive proton pumping, creating a proton gradient that powers ATP synthesis by ATP synthase.

Q5: Give the schematic representation of an overall view of Krebs cycle.
Ans: The Krebs cycle is a cyclic series of reactions in the mitochondrial matrix in which acetyl-CoA combines with oxaloacetate to form citrate and is then oxidised step-by-step to regenerate oxaloacetate. The cycle produces CO₂, reduced coenzymes (NADH and FADH₂) and a molecule of ATP (or GTP) per turn; the reduced coenzymes feed electrons into the ETS for further ATP production.

Q6: Explain ETS. 
Ans: The electron transport system (ETS) is a chain of electron carriers located in the inner mitochondrial membrane. It transfers electrons from reduced coenzymes to oxygen and conserves the released energy to synthesise ATP. The process works as follows:

• NADH oxidation: NADH + H⁺ (formed in glycolysis and the Krebs cycle) is oxidised by NADH dehydrogenase (complex I). Electrons are passed first to FMN and then to ubiquinone (coenzyme Q).
• FADH₂ oxidation: FADH₂ (from the Krebs cycle) donates electrons to complex II, which also transfers them to ubiquinone.
• Electron flow: Ubiquinone carries electrons to the cytochrome bc₁ complex (complex III), which passes them to cytochrome c. Cytochrome c then transfers electrons to cytochrome c oxidase (complex IV), which contains cytochromes a and a₃ and copper centres.
• Final acceptor: At complex IV, electrons reduce molecular oxygen, the final electron acceptor, to form water.
• Proton pumping and ATP synthesis: As electrons move through complexes I, III and IV, energy released is used to pump protons from the matrix into the intermembrane space, creating an electrochemical proton gradient. Protons return to the matrix through ATP synthase (complex V), driving ATP synthesis from ADP and inorganic phosphate.

The amount of ATP produced is often summarised as:

• Oxidation of one NADH typically yields about 3 ATP equivalents.
• Oxidation of one FADH₂ typically yields about 2 ATP equivalents.

Q7: Distinguish between the following:
(a) Aerobic respiration and Anaerobic respiration
(b) Glycolysis and Fermentation
(c) Glycolysis and Citric acid Cycle
Ans:
(a) Aerobic respiration and Anaerobic respiration

(b) Glycolysis and Fermentation

(c) Glycolysis and citric acid cycle

Q8: What are the assumptions made during the calculation of the net gain of ATP?
Ans: Common assumptions made when calculating theoretical net ATP gain from one molecule of glucose include:

• All stages of aerobic respiration (glycolysis, pyruvate oxidation, TCA cycle, ETS and oxidative phosphorylation) proceed in sequence and operate fully.
• NADH produced in glycolysis can be transferred into the mitochondrion and oxidised to yield ATP (sometimes assumed without loss).
• Glucose is the only substrate considered; no other molecules enter the pathways or withdraw intermediates for biosynthesis.
• Intermediates formed during respiration are not diverted to other metabolic processes; all reducing equivalents feed the ETS.

These assumptions simplify theoretical calculations but do not fully reflect the dynamic and interconnected pathways in living cells, where intermediates are used for biosynthesis and shuttle systems can alter the effective ATP yield.

Q9: Discuss "The respiratory pathway is an amphibolic pathway."
Ans: The respiratory pathway is called amphibolic because it participates in both catabolic and anabolic processes:

• Catabolic role: It breaks down carbohydrates, fats and proteins to release energy that is conserved as ATP. For example, glucose is catabolised via glycolysis and the Krebs cycle.
• Anabolic role: Intermediates of the respiratory pathway serve as building blocks for biosynthesis. For example, citrate and oxaloacetate can be withdrawn for fatty acid and amino acid synthesis respectively.
• Intermediary connections: Fats are broken down to acetyl-CoA which enters the cycle; amino acids after deamination can feed into various points of the cycle. Conversely, cycle intermediates can be used to synthesise carbohydrates, lipids and amino acids.

Because the same pathway thus supplies energy by breakdown and supplies precursors for synthesis, it functions as an amphibolic (dual-function) pathway.

Q10: Define RQ. What is its value for fats?
Ans: The respiratory quotient (RQ) is the ratio of the volume of CO₂ produced to the volume of O₂ consumed during respiration (RQ = CO₂ produced / O₂ consumed). RQ values vary with the substrate used:

• For carbohydrates, RQ ≈ 1.0 because equal amounts of CO₂ and O₂ are exchanged.
• For fats, RQ is less than 1.0 because fats require relatively more O₂ for oxidation than the CO₂ produced. For example, oxidation of tripalmitin consumes 145 O₂ and produces 102 CO₂, giving an RQ of 0.7.

Q11: What is oxidative phosphorylation?
Ans: Oxidative phosphorylation is the process by which energy released from the transfer of electrons (from NADH and FADH₂ to oxygen) is used to synthesise ATP. Key points are:

• Electron transfer: Electrons flow along the ETS from reduced coenzymes to oxygen, the final acceptor, releasing energy.
• Proton gradient: Energy from electron flow is used to pump protons across the inner mitochondrial membrane, creating an electrochemical gradient.
• ATP synthase: The F₀-F₁ ATP synthase complex uses the proton motive force to produce ATP from ADP and inorganic phosphate. The F₁ part is the catalytic unit where ATP is synthesised; the F₀ part forms the proton channel.
• Proton requirement: Movement of protons through ATP synthase drives ATP formation; in simple terms, a fixed number of protons passing through the complex is associated with synthesis of each ATP molecule.

Q12: What is the significance of step-wise release of energy in respiration? 
Ans: The step-wise release of energy in respiration is important for several reasons:

• It allows gradual extraction of energy from substrates so that much of it can be conserved in the form of ATP rather than being lost as heat.
• Each pathway produces specific intermediates that serve as substrates for the next pathway; this coupling increases overall efficiency.
• Intermediates produced in one stage can be used in other biosynthetic processes, providing metabolic flexibility.
• Control points (regulated enzymes) in successive steps permit fine control of metabolic rate and ATP production according to cellular needs.