EXERCISES

Q1. Differentiate between (a) Respiration and Combustion
Answer:
(a) Respiration and Combustion.
Answer: The difference between respiration and combustion. are-

Q2. What are respiratory substrates? Name the most common respiratory substrate.
Answer:

Respiratory Substrates: The compounds that are oxidised during the process of respiration, leading to the release of considerable amounts of energy, are known as respiratory substrates.
Most Common Respiratory

Common Substrate: Carbohydrates are usually the most common respiratory substrate used to release energy. Other substrates like proteins, fats, and organic acids can also be used under certain conditions.

Q3. Give the schematic representation of glycolysis.
Answer: Glycolysis (EMP Pathway) is the partial oxidation of glucose that occurs in the cytoplasm of all living organisms, forming two molecules of pyruvic acid$$\begin{array}{c}\\ \end{array}$$
Q4. What are the main steps in aerobic respiration? Where does it take place?
Answer: Aerobic respiration is the process of complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water, and a large amount of energy.

Main Steps in Aerobic Respiration:

1. Glycolysis: Breakdown of glucose into two molecules of pyruvic acid (Pyruvate).
2. Oxidative Decarboxylation: Pyruvic acid is converted into Acetyl CoA with the release of CO₂.
3. Tricarboxylic Acid Cycle (TCA or Krebs’ Cycle): Acetyl CoA undergoes oxidation, releasing CO₂ and generating NADH and FADH₂.
4. Electron Transport System (ETS) and Oxidative Phosphorylation: The energy from NADH and FADH₂ is used to form ATP.

Location of Aerobic Respiration:

1. Glycolysis: Cytoplasm
2. Oxidative Decarboxylation and TCA Cycle: Mitochondrial Matrix
3. ETS and Oxidative Phosphorylation: Inner Mitochondrial Membrane

Q5. Give the schematic representation of an overall view of Krebs’ cycle.
Answer:

$$\begin{array}{c}\text{Pyruvate }(3\text{C})\\ \downarrow {\text{NAD}}^{+}\to \text{NADH}+{\text{H}}^{+},\text{CoA}\to {\text{CO}}_2\\ \text{Acetyl coenzyme A }(2\text{C})\\ \downarrow \\ \text{Oxaloacetic acid }(4\text{C})+\text{Acetyl CoA}\\ \downarrow \text{ Citrate synthase}\\ \text{Citric acid }(6\text{C})\\ \downarrow \\ \text{Isocitrate}\\ \downarrow {\text{NAD}}^{+}\to \text{NADH}+{\text{H}}^{+},{\text{CO}}_2\\ \text{a-ketoglutaric acid }(5\text{C})\\ \downarrow {\text{NAD}}^{+}\to \text{NADH}+{\text{H}}^{+},{\text{CO}}_2,\text{CoA}\\ \text{Succinyl-CoA}\\ \downarrow \text{ GDP}\to \text{GTP}(\to \text{ATP})\\ \text{Succinic acid }(4\text{C})\\ \downarrow \text{ FAD}\to {\text{FADH}}_2\\ \text{Fumarate}\\ \downarrow \\ \text{Malic acid }(4\text{C})\\ \downarrow {\text{NAD}}^{+}\to \text{NADH}+{\text{H}}^{+}\\ \text{Oxaloacetic acid }(4\text{C})\end{array}$$

Q6. Explain ETS.
Answer: ETS (Electron Transport System) is a series of protein complexes located on the inner mitochondrial membrane. It releases and utilizes the energy from NADH and FADH₂ produced during respiration. Electrons move through Complexes I–IV, pumping protons into the intermembrane space. Oxygen acts as the final electron acceptor, forming water. The proton gradient drives ATP synthase (Complex V) to generate ATP.
Output: 1 NADH = 3 ATP; 1 FADH₂ = 2 ATP.

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

(a) Aerobic respiration and Anaerobic respiration
Answer:

(b) Glycolysis and Fermentation:  The difference between glycolysis and fermentation are-

1. Glycolysis is the process of partial oxidation of glucose to pyruvic acid, whereas fermentation converts pyruvic acid into ethanol and CO₂ or lactic acid.
2. Glycolysis is common to both aerobic and anaerobic respiration, whereas fermentation occurs only under anaerobic conditions.
3. The end product of glycolysis is pyruvic acid, whereas in fermentation it is alcohol or lactic acid.
4. Both processes yield 2 ATP molecules, but glycolysis prepares pyruvate for further oxidation, whereas fermentation regenerates NAD⁺ to allow glycolysis to continue.

(c) Glycolysis and Citric Acid Cycle : The difference between glycolysis and fermentation are-

1. Glycolysis occurs in the cytoplasm, whereas the citric acid cycle (Krebs’ cycle) occurs in the mitochondrial matrix.
2. Glycolysis starts with glucose (6-carbon), whereas the citric acid cycle starts with acetyl CoA (2-carbon).
3. Glycolysis produces pyruvic acid as its end product, whereas the citric acid cycle regenerates oxaloacetic acid (OAA).
4. Glycolysis can occur without oxygen (anaerobic), whereas the citric acid cycle requires oxygen (aerobic).
5. No CO₂ is released during glycolysis, whereas CO₂ is released in the citric acid cycle during complete oxidation of pyruvate.

Q8. What are the assumptions made during the calculation of net gain of ATP?
Answer: The assumptions made during the calculation of net gain of ATP are- 

1. Pathways occur sequentially and completely.
2. NADH from glycolysis enters mitochondria for oxidative phosphorylation.
3. Intermediates are not diverted for synthesis.
4. Only glucose is respired; no other substrates interfere.

Q9. Discuss “The respiratory pathway is an amphibolic pathway.”
Answer: The respiratory pathway is both catabolic and anabolic:

1. Catabolic: Breaks down carbohydrates, fats, and proteins for energy.
2. Anabolic: Intermediates are used for synthesis of fatty acids, amino acids, etc.

Hence, it is termed an amphibolic pathway (dual role in metabolism).

Q10. Define RQ. What is its value for fats?
Answer: RQ (Respiratory Quotient) = Volume of CO₂ evolved / Volume of O₂ consumed. It depends on the substrate used.

For fats (e.g., tripalmitin), RQ = 0.7 (less than 1).

Q11. What is oxidative phosphorylation?
Answer: It is the synthesis of ATP using the energy from oxidation-reduction reactions in the Electron Transport System. It occurs on the inner mitochondrial membrane. The ATP synthase uses the proton gradient to convert ADP + Pi → ATP.

Q12. What is the significance of step-wise release of energy in respiration?
Answer: Step-wise release of energy in respiration are-

1. Energy is released in controlled steps, not explosively.
2. Enables ATP formation efficiently.
3. Prevents excess heat generation.
4. Allows energy to be stored and used when needed.