Notes Class 11 Chapter 13 Biology Maharashtra Board

Respiration and Energy Transfer

Introduction

• Respiration is a catabolic process where complex organic substrates are oxidized to simpler components, releasing biological energy in the form of ATP.
• It is essential for maintaining life by fulfilling the continuous need for energy.
• Key Processes: Cellular respiration occurs in two forms: aerobic (requires oxygen) and anaerobic (does not require oxygen).

13.1 Formation of ATP

• ATP Formation (Phosphorylation): The process of adding a phosphate group to ADP to form ATP. It occurs in three ways:
  1. Photophosphorylation: Occurs during photosynthesis (not detailed here).
  2. Substrate-Level Phosphorylation:
     • Direct transfer of a phosphate group from a substrate to ADP.
     • Occurs in the cytoplasm (glycolysis) and mitochondrial matrix (Krebs cycle).
  3. Oxidative Phosphorylation:
     • Phosphorylation of ADP using energy from the oxidation of NADH+H⁺ and FADH₂.
     • Occurs on the inner mitochondrial membrane via the electron transport chain.
• ATP Hydrolysis: When ATP is hydrolyzed, it releases energy for metabolic processes.

13.2 Anaerobic Respiration

• Definition: Cellular respiration without atmospheric oxygen, also called fermentation.
• Steps:
  1. Glycolysis: Breakdown of glucose into pyruvate.
  2. Fermentation: Incomplete conversion of pyruvate into lactic acid (muscles) or ethanol + CO₂ (yeast).
• Energy Yield: Only 2 ATP per glucose (from glycolysis).

Glycolysis

• Overview: A common step in both aerobic and anaerobic respiration, occurring in the cytoplasm.
• Process: Breaks down one glucose (6C) into two pyruvic acid (3C) molecules.
• Phases:
  1. Preparatory Phase (Steps 1-5):
     • Glucose is phosphorylated twice, consuming 2 ATP.
     • Forms fructose-1,6-bisphosphate, which splits into two 3-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
     • DHAP is isomerized to G3P, yielding two G3P molecules.
  2. Pay-Off Phase (Steps 6-10):
     • Each G3P is oxidized and phosphorylated, producing 2 NADH+H⁺ and 2 molecules of 1,3-bisphosphoglycerate.
     • 1,3-bisphosphoglycerate is converted to pyruvate, generating 4 ATP via substrate-level phosphorylation.
• Overall Reaction: Glucose + 2 ATP + 2 Pi + 4 ADP + 2 NAD⁺ → 2 Pyruvate + 2 ADP + 4 ATP + 2 NADH + 2 H⁺ + 2 H₂O
• Net Gain: 2 ATP, 2 NADH+H⁺ per glucose.
• Regulation: Controlled by enzymes (hexokinase, phosphofructokinase-1, pyruvate kinase), ATP levels, NADH regeneration, and hormones (glucagon, epinephrine, insulin).

Fermentation

• In Muscles:
  • Pyruvate is reduced to lactic acid by NADH+H⁺, regenerating NAD⁺.
  • Occurs during vigorous exercise, leading to lactic acid accumulation and muscle fatigue.
  • During rest, lactic acid is converted back to pyruvate for aerobic respiration.
• In Yeast:
  • Pyruvate is decarboxylated to acetaldehyde, then reduced to ethanol by NADH+H⁺, releasing CO₂.
  • Called alcoholic fermentation.
  • Ethanol accumulation can be toxic, halting yeast growth.

13.3 Aerobic Respiration

• Definition: Complete oxidation of glucose using molecular oxygen, yielding up to 38 ATP.
• Steps:
  1. Glycolysis (cytoplasm).
  2. Pyruvate oxidation (mitochondrial matrix).
  3. Krebs cycle (mitochondrial matrix).
  4. Electron transport chain and terminal oxidation (inner mitochondrial membrane).

Pyruvate to Acetyl CoA (Connecting Link Reaction)

• Location: Mitochondria (eukaryotes), cytoplasm (prokaryotes).
• Process: Oxidative decarboxylation of pyruvate (3C) to Acetyl CoA (2C), catalyzed by the pyruvate dehydrogenase complex (PDH).
• Coenzyme: Requires thiamin (vitamin B₁). Deficiency leads to pyruvic and lactic acidosis.
• Reaction: Pyruvate + NAD⁺ + CoA → Acetyl CoA + NADH + H⁺ + CO₂
• Significance: Links glycolysis to the Krebs cycle.

Krebs Cycle (TCA/Citric Acid Cycle)

• Location: Mitochondrial matrix.
• Overview: Oxidizes Acetyl CoA to CO₂, producing energy carriers (NADH, FADH₂) and GTP.
• Steps:
  1. Acetyl CoA (2C) combines with oxaloacetate (4C) to form citric acid (6C).
  2. Citric acid is oxidized step-wise, releasing 2 CO₂ and regenerating oxaloacetate.
  3. Four oxidation steps produce 3 NADH+H⁺ and 1 FADH₂ per Acetyl CoA.
  4. One GTP (equivalent to ATP) is produced via substrate-level phosphorylation (Succinyl CoA → Succinate).
• Per Glucose (two Acetyl CoA molecules):
  • • 6 NADH+H⁺, 2 FADH₂, 2 GTP, 4 CO₂.
  • Significance:
    • Common pathway for carbohydrates, fats, and proteins (via Acetyl CoA).
    • Intermediates (e.g., α-ketoglutarate, oxaloacetate) are used for synthesizing amino acids and fatty acids.
    • Amphibolic Pathway: Combines catabolism (energy release) and anabolism (biosynthesis).

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• Location: Inner mitochondrial membrane.
• Components: Four complexes (I-IV), ubiquinone (CoQ), cytochrome c, and complex V (F0-F1 ATP synthase).
• Process:
  1. Complex I (NADH dehydrogenase): Oxidizes NADH+H⁺, transferring electrons to ubiquinone (CoQ), forming ubiquinol.
  2. Complex II (Succinate dehydrogenase): Oxidizes FADH₂, also feeding electrons to CoQ.
  3. Complex III (Cytochrome bc₁): Transfers electrons from ubiquinol to cytochrome c.
  4. Complex IV (Cytochrome c oxidase): Transfers electrons to O₂, forming water (terminal oxidation).
  5. Proton Gradient: Electron transfer pumps protons (H⁺) into the intermembrane space, creating a gradient.
  6. Chemiosmosis: Protons flow back into the matrix via F0-F1 ATP synthase, driving ATP synthesis (oxidative phosphorylation).
• ATP Yield:
  • 1 NADH+H⁺ → 3 ATP.
  • 1 FADH₂ → 2 ATP.
  • Total from ETC: ~34 ATP per glucose.
• Significance:
  • Produces the majority of ATP (34/38).
  • Regenerates NAD⁺ and FAD⁺ for glycolysis and Krebs cycle.
  • Produces water as a byproduct.
  • Step-wise energy release prevents cellular damage.

ATP Balance Sheet (Aerobic Respiration)

13.4 Utility of Step-Wise Oxidation

• Advantages:
  1. Efficient Energy Capture: Gradual release of energy maximizes ATP synthesis.
  2. Regulation: Enzyme activities can be controlled to adjust energy output based on cellular needs.
  3. Intermediates for Biosynthesis: Provides molecules (e.g., pyruvate, α-ketoglutarate) for synthesizing amino acids, fatty acids, etc.

Respiratory Quotient (RQ)

• Definition: Ratio of CO₂ released to O₂ consumed during respiration.
• Values:
  • Carbohydrates: RQ = 1 (equal CO₂ and O₂ volumes).
  • Fats: RQ ≈ 0.7 (more O₂ consumed, less CO₂ produced).
  • Proteins: RQ ≈ 0.9.
  • Anaerobic Respiration: RQ = ∞ (CO₂ produced without O₂ consumption).

Significance of Respiration

1. Provides energy for biomolecule synthesis, cell division, growth, and locomotion.
2. Supplies intermediates for synthesizing complex compounds (e.g., amino acids, fatty acids).
3. Maintains atmospheric CO₂ and O₂ balance with photosynthesis.
4. Anaerobic Respiration:
   • Used in industries (e.g., bakeries, distilleries) for producing alcohol, organic acids, antibiotics, etc.