Questions Answers Class 11 Chapter 13 Biology Maharashtra Board

Respiration and Energy Transfer

1. Choose correct option

A. The reactions of the TCA cycle occur in
a. ribosomes
b. grana
c. mitochondria
d. endoplasmic reticulum

Answer: c. mitochondria

• Explanation: The document states that the TCA (Krebs) cycle occurs in the mitochondrial matrix.

B. In eucaryotes the complete oxidation of a molecule of glucose results in the net gain of
a. 2 molecules of ATP
b. 36 molecules of ATP
c. 4 molecules of ATP
d. 38 molecules of ATP

Answer: d. 38 molecules of ATP

• Explanation: The document mentions that the complete aerobic oxidation of one glucose molecule yields 38 ATP molecules in eukaryotes.

C. Which step of Krebs cycle operates substrate-level phosphorylation?
a. α-ketoglutarate → succinyl CoA.
b. Succinyl CoA → succinate
c. Succinate → fumarate
d. Fumarate → malate

Answer: b. Succinyl CoA → succinate

• Explanation: The document indicates that substrate-level phosphorylation in the Krebs cycle occurs when Succinyl CoA is converted to succinate, producing GTP (equivalent to ATP).

2. Fill in the blanks with suitable words

A. Acetyl CoA is formed from …………….. and co-enzyme A.

Answer: pyruvic acid

B. In the prokaryotes …………….. molecules of ATP are formed per molecule of glucose oxidised.

Answer: 38

C. Glycolysis takes place in ………………

Answer: Cytoplasm

D. F1 – F0 particles participate in the synthesis of ………………

Answer: ATP

E. During glycolysis ……………… molecules of NADH+H+ are formed.

Answer: Two

3. Answer the following questions

A. When and where does anaerobic respiration occur in man and yeast?

Answer:

• In man: Anaerobic respiration occurs in skeletal muscles during vigorous exercise when oxygen supply is limited, leading to lactic acid accumulation.
• In yeast: Anaerobic respiration (alcoholic fermentation) occurs in the cytoplasm when oxygen is absent, converting pyruvate to ethanol and CO₂.

B. Why is less energy produced during anaerobic respiration than in aerobic respiration?

Answer: Anaerobic respiration produces less energy because it only involves glycolysis, yielding 2 ATP molecules per glucose molecule via substrate-level phosphorylation. Aerobic respiration, however, includes glycolysis, Krebs cycle, and electron transport chain (ETC), producing up to 38 ATP by fully oxidizing glucose using oxygen.

C. Which is the site of ETS in mitochondrial respiration?

Answer: The electron transport system (ETS) is located on the inner mitochondrial membrane.

D. Which is the terminal electron acceptor in aerobic respiration?

Answer: The terminal electron acceptor in aerobic respiration is molecular oxygen (O₂), which is reduced to form water.

E. What is RQ? What is its value for fats?

Answer:

• Respiratory Quotient (RQ): is the ratio of the volume of CO₂ released to the volume of O₂ consumed during respiration.
• Value for fats: Approximately 0.7, as fats require more oxygen for oxidation, resulting in less CO₂ produced compared to O₂ consumed.

F. What are respiratory substrates? Name the most common respiratory substrate.

Answer:

• Respiratory substrates are organic molecules oxidized during respiration to produce energy (e.g., carbohydrates, fats, proteins).
• Most common: Glucose, as it is the primary substrate for glycolysis and aerobic respiration.

G. Write explanatory notes on:

i. Glycolysis

Answer: Glycolysis is the metabolic pathway that breaks down one glucose molecule (6-carbon) into two pyruvic acid molecules (3-carbon) in the cytoplasm. It is common to both aerobic and anaerobic respiration and occurs in two phases:

• Preparatory phase: Glucose is phosphorylated twice (using 2 ATP) to form fructose-1,6-bisphosphate, which splits into two 3-carbon molecules (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate).
• Pay-off phase: Each glyceraldehyde-3-phosphate is oxidized and phosphorylated, producing 2 NADH+H⁺ and 4 ATP via substrate-level phosphorylation. The net gain is 2 ATP and 2 NADH+H⁺ per glucose. Glycolysis is tightly regulated by enzymes like hexokinase and phosphofructokinase.

ii. Fermentation by yeast

Answer: Fermentation in yeast is an anaerobic process where glucose is partially oxidized to produce energy without oxygen. After glycolysis, pyruvate is decarboxylated to acetaldehyde, which is then reduced by NADH+H⁺ to ethanol, regenerating NAD⁺. CO₂ is released during decarboxylation. This process, called alcoholic fermentation, allows yeast to produce energy (2 ATP from glycolysis) in oxygen-scarce conditions. Ethanol accumulation can be toxic to yeast, halting growth.

iii. Electron transport chain

Answer: The electron transport chain (ETC) is a series of protein complexes (I-IV) and electron carriers (ubiquinone, cytochrome c) on the inner mitochondrial membrane. It oxidizes NADH+H⁺ and FADH₂ produced in glycolysis, pyruvate oxidation, and Krebs cycle. Electrons from NADH+H⁺ (via complex I) and FADH₂ (via complex II) are transferred through complexes, ultimately reducing O₂ to water at complex IV (terminal oxidation). Proton movement across the membrane drives ATP synthesis via chemiosmosis at complex V (F0-F1). The ETC produces 34 of the 38 ATP molecules in aerobic respiration.

H. How are glycolysis, TCA cycle and electron transport chain linked? Explain.

Answer: Glycolysis, TCA (Krebs) cycle, and ETC are interconnected in aerobic respiration:

• Glycolysis (cytoplasm) converts glucose to two pyruvate molecules, producing 2 ATP and 2 NADH+H⁺.
• Pyruvate oxidation (mitochondrial matrix) converts pyruvate to Acetyl CoA, producing 2 NADH+H⁺ per glucose.
• TCA cycle (mitochondrial matrix) oxidizes Acetyl CoA, generating 2 GTP (ATP), 6 NADH+H⁺, and 2 FADH₂ per glucose.
• ETC (inner mitochondrial membrane) oxidizes NADH+H⁺ and FADH₂, transferring electrons to O₂ and driving proton pumping. This powers ATP synthesis via chemiosmosis, producing up to 34 ATP. The NADH+H⁺ and FADH₂ from glycolysis and TCA cycle fuel the ETC, linking all stages to maximize energy yield.

I. How would you demonstrate that yeast can respire both aerobically and anaerobically?

Answer:

• Anaerobic respiration: Suspend yeast in a 10% glucose solution in a test tube, cover with oil to exclude air, and connect to a tube with lime water. Place in warm water (37-38°C). Lime water turns milky due to CO₂ production, and no O₂ is consumed (no change in gas volume). After a day, the smell of ethanol confirms alcoholic fermentation.
• Aerobic respiration: Expose yeast to oxygen with glucose. Measure O₂ consumption (e.g., via respirometer) and CO₂ production. No ethanol is produced, and more ATP (up to 38) is generated, indicating aerobic respiration via glycolysis, Krebs cycle, and ETC.

J. What is the advantage of step wise energy release in respiration?

Answer:

Step-wise energy release in respiration:

• Maximizes ATP synthesis by capturing energy gradually, improving efficiency.
• Allows regulation of energy output via enzyme control, meeting cellular needs.
• Provides intermediates (e.g., from Krebs cycle) for biosynthesis of other molecules like amino acids.

K. Explain ETS.

Answer: The Electron Transport System (ETS) is a series of complexes (I-IV) and carriers on the inner mitochondrial membrane that transfers electrons from NADH+H⁺ and FADH₂ to O₂, producing water.

• Complex I (NADH dehydrogenase) oxidizes NADH+H⁺, transferring electrons to ubiquinone.
• Complex II (succinate dehydrogenase) oxidizes FADH₂, also feeding electrons to ubiquinone.
• Complex III (cytochrome bc₁) transfers electrons to cytochrome c.
• Complex IV (cytochrome c oxidase) reduces O₂ to water.
• Proton pumping during electron transfer creates a gradient, driving ATP synthesis via chemiosmosis at complex V (F0-F1).
• The ETS generates 34 ATP per glucose, regenerates NAD⁺/FAD⁺, and produces water.

L. Discuss. “The respiratory pathway is an amphibolic pathway”.

Answer: The respiratory pathway is amphibolic because it involves both catabolism (breaking down molecules for energy) and anabolism (synthesizing molecules). While glycolysis and Krebs cycle oxidize glucose to produce ATP and CO₂, their intermediates serve as precursors for biosynthesis:

• Glycolysis: Provides pyruvate for amino acid synthesis.
• Krebs cycle: Intermediates like α-ketoglutarate and oxaloacetate are used to synthesize glutamate, aspartate, and fatty acids.
  This dual role allows cells to balance energy production and biomolecule synthesis, making respiration amphibolic.

M. Why is Krebs cycle reffered as amphibolic pathway?

Answer: The Krebs cycle is amphibolic because it serves both catabolic and anabolic functions. Catabolically, it oxidizes Acetyl CoA to produce ATP, NADH+H⁺, FADH₂, and CO₂. Anabolically, its intermediates (e.g., α-ketoglutarate, oxaloacetate) are used to synthesize amino acids (glutamate, aspartate) and other molecules like fatty acids. This dual role makes it a central hub for energy production and biosynthesis.

N. Which of the following step of aerobic respiration would be omitted when fatty acids are used as respiratory substrate?
a. Glycolysis
b. Krebs cycle
c. Electron transfer chain reaction
d. Terminal oxidation.

Answer: a. Glycolysis

• Explanation: Fatty acids are oxidized via β-oxidation to produce Acetyl CoA, which enters the Krebs cycle. Glycolysis, which breaks down glucose, is not required when fatty acids are the substrate.

4. Compare

A. Photosynthesis and Respiration

Answer:

B. Anaerobic and Anaerobic respiration.

Answer:

5. Differentiate between

A. Respiration and Combustion

Answer:

B. Glycolysis and Krebs Cycle

Answer:

C. Aerobic respiration and fermentation

Answer:

6. Identify the cycle given below. Correct it and fill in the blanks and write discription of it in your own words

Answer: