Question 7.
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

(b) Glycolysis and Fermentation

(c) Glycolysis and Citric Acid Cycle (Krebs Cycle)

Question 8.
What are the assumptions made during the calculation of net gain of ATP?
Answer:

The calculation of net gain of ATP for every glucose molecule oxidised is based on the following assumptions:

1. There is a sequential and orderly pathway functioning, where one substrate forms the next, and glycolysis, TCA cycle, and ETS operate one after another without any deviation.
2. The NADH synthesized during glycolysis in the cytoplasm is transported into the mitochondria and undergoes oxidative phosphorylation to produce ATP.
3. The respiratory pathway operates efficiently, with no intermediate compounds being used for other biosynthetic purposes.
4. Complete oxidation of one molecule of glucose takes place under aerobic conditions.

Note:
In actual conditions, the net gain of ATP may be slightly less than the theoretical 38 ATP, due to energy loss during transport of NADH into mitochondria and other cellular activities.

Question 9.
Discuss “The respiratory pathway is an amphibolic pathway.”
Answer:

Glucose is the most favoured substrate for respiration, and all other carbohydrates are usually first converted into glucose before being used in respiration.

Respiration involves both the breakdown (catabolism) and the synthesis (anabolism) of substrates; hence, it performs dual functions.

• Catabolic role:
  During respiration, glucose is broken down to release energy. This degradative process is called catabolism.
• Anabolic role:
  Many intermediates of the respiratory pathway are withdrawn and used for the synthesis of new compounds. For example, acetyl-CoA can be used to synthesize fatty acids when required. This constructive process is called anabolism.

Thus, since the respiratory pathway involves both catabolic and anabolic activities, it is known as an amphibolic pathway.

Question 10.
Define RQ. What is its value for fats?
Answer:

Respiratory Quotient (RQ):
Respiratory Quotient or Respiratory Ratio is defined as the ratio of the volume of carbon dioxide (CO₂) liberated to the volume of oxygen (O₂) consumed during respiration.

The value of the respiratory quotient depends on the type of respiratory substrate used:

• For carbohydrates, the RQ is 1.
• For fats, the RQ is less than 1 because fats require more oxygen for complete oxidation than the amount of CO₂

Hence, the RQ value for fats is approximately 0.7.

Question 11.
What is oxidative phosphorylation?
Answer:

Oxidative phosphorylation is a metabolic pathway in which ATP is synthesized using the energy released during the oxidation of nutrients.

• Electrons are transferred from electron donors (NADH and FADH₂) to electron acceptors such as oxygen through a series of redox reactions in the electron transport chain (Complexes I–IV) located in the inner mitochondrial membrane.
• The energy released during electron transfer is used to pump protons (H⁺) across the membrane, creating a proton gradient.
• Protons flow back into the matrix via ATP synthase (Complex V), driving the production of ATP from ADP and inorganic phosphate (Pi).
• ATP yield: Oxidation of 1 NADH → 3 ATP; oxidation of 1 FADH₂ → 2 ATP.
• Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

Unlike photophosphorylation, where light energy generates the proton gradient, in oxidative phosphorylation, it is the energy from oxidation-reduction reactions that drives ATP synthesis, hence the name.

Site: Inner mitochondrial membrane

Question 12.
What is the significance of step-wise release of energy in respiration?

Answer:
Respiration is the process by which energy is released from food molecules (respiratory substrates) such as carbohydrates, fats, and proteins through oxidation.

Significance of step-wise energy release:

1. Controlled energy release:
   • Energy in food is not released all at once; it is released gradually in a series of enzyme-controlled steps.
2. Efficient ATP production:
   • The energy released in small steps is trapped in the form of ATP, which acts as the energy currency of the cell.
3. Safety for the cell:
   • Sudden release of all energy at once would be wasted as heat and could damage cellular components.
4. Support for biosynthesis:
   • Carbon skeletons and intermediates produced during respiration can be used as precursors for the synthesis of other molecules in the cell.
5. Energy availability on demand:
   • ATP can be broken down wherever and whenever energy is needed for various cellular processes, making energy use efficient and controlled.

Conclusion:
Step-wise release of energy ensures safe, efficient, and regulated energy production, while simultaneously providing intermediates for biosynthesis.

Additional Questions and Answers

Question 1.
What are respiratory substrates? Name the most common one.
Answer:
Respiratory substrates are organic compounds that are oxidized to release energy during respiration.

• Most common respiratory substrate: Glucose
• Other substrates: Fats, proteins, and organic acids.

Question 2.
Give the schematic representation of glycolysis.
Answer:
Glycolysis (in cytoplasm):

Glucose (6C) → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate → Glyceraldehyde-3-phosphate → 2 Pyruvate (3C each)

• Net ATP: 2
• NADH formed: 2

Question 3.
Differentiate between:

Answer:
(a) Respiration and Combustion

(b) Aerobic respiration and Fermentation

Question 4.
Name the end products of:
Answer:

• Aerobic respiration: CO₂ and H₂O
• Anaerobic respiration in yeast: Ethanol + CO₂
• Anaerobic respiration in muscles: Lactic acid

Question 5.
Why is oxygen called the final electron acceptor?
Answer:
In aerobic respiration, electrons are transferred through the electron transport chain to oxygen, which combines with protons to form water. Without oxygen, electrons cannot flow, and ATP production stops.

Question 6.
What is the net ATP yield from one glucose molecule?
Answer:

• Glycolysis: 2 ATP
• Krebs cycle: 2 ATP
• Electron Transport Chain (oxidative phosphorylation): 34 ATP
  Total: 36–38 ATP per glucose molecule

Question 7.
Explain the term “oxidative phosphorylation.”
Answer:
ATP synthesis that occurs when energy released from electron transfer in the ETS is used to phosphorylate ADP to ATP. Oxygen acts as the final electron acceptor.

Question 8.
What is the Respiratory Quotient (RQ)? Give its value for carbohydrates, fats, and proteins.
Answer:

Question 9.
Why is respiration called an amphibolic pathway?
Answer:
Because it serves both catabolic (breakdown of substrates for energy) and anabolic (providing intermediates for biosynthesis) functions.

Question 10.
Where do the following processes occur?
Answer: