Topics to be Learn :

  • Introduction
  • Formation of ATP
  • Anaerobic respiration
  • Aerobic respiration
  • Utility of stepwise oxidation

Introduction
  • Life requires energy for various metabolic activities.
  • Respiration ensures a continuous supply of energy.
  • Key nutrients like carbohydrates, fats, and proteins are used for energy production.
  • At the cellular level, energy is obtained by oxidizing food.
  • Aerobic organisms need oxygen for food oxidation, releasing carbon dioxide as a byproduct.
ATP (Adenosine Triphosphate):
  • Energy-rich organic compound.
  • Synthesized when energy is released, using ADP and Pi.
  • Hydrolysis of ATP releases energy by breaking its high-energy phosphate bond.
  • Known as the energy currency of the cell.
Phosphorylation
  • Definition: Formation of ATP by adding inorganic phosphate (Pi) to ADP.
  • Reaction:
    ADP + Pi ⇌ ATP
Types of Phosphorylation

Metabolic Processes

a) Anabolic Process:

  • Definition: Biosynthetic processes.
  • Example: Photosynthesis.
b) Catabolic Process:
  • Definition: Breakdown processes.
  • Example: Respiration.
Respiration
  • Definition: A catabolic process where complex organic substrates are oxidized into simpler components to produce biological energy (ATP).
Types:
  1. Anaerobic respiration: Does not involve oxygen; also known as fermentation.
  2. Aerobic respiration: Requires oxygen.
Anaerobic Respiration
  • Definition: Cellular respiration occurring without oxygen.
Process:
  • Involves glycolysis.
  • Pyruvate, the product of glycolysis, is converted into: Lactic acid (in some organisms) & Ethanol (in others).
Glycolysis (EMP Pathway)
  • Definition: Breakdown of glucose into two molecules of pyruvic acid (glucose-breaking).
  • Common to both aerobic and anaerobic respiration.
  • Takes place in the cytoplasm.
Consists of 10 steps divided into two phases:
  1. Preparatory Phase (Steps 1–5).
  2. Payoff Phase (Steps 6–10).


Phases of Glycolysis

(i) Preparatory Phase

Steps:
  1. Glucose phosphorylated twice using 2 ATP molecules → Fructose 1,6-bisphosphate.
  2. Fructose 1,6-bisphosphate splits into two 3-carbon isomers: a) Glyceraldehyde-3-phosphate. b) Dihydroxyacetone phosphate (isomerized to glyceraldehyde-3-phosphate).
Result: Two molecules of glyceraldehyde-3-phosphate are formed.
(ii) Payoff Phase
Steps:
  1. Glyceraldehyde-3-phosphate oxidized and phosphorylated (using inorganic phosphate) → 1,3-bisphosphoglycerate.
  2. Conversion of 1,3-bisphosphoglycerate into pyruvic acid via a series of reactions.
Energy released is used to generate 4 ATP molecules through substrate-level phosphorylation.
Overall Reaction of Glycolysis

Glucose + 2ATP + 2Pi + 4ADP + 2NAD⁺ → 2Pyruvate + 2ADP + 4ATP + 2NADH + 2H⁺ + 2H₂O

Chemical Reactions in Glycolysis

1. Irreversible Reactions:

  • Occur in only one direction; products cannot revert to reactants.
  • Catalyzed by specific enzymes: a) Hexokinase, b) Phosphofructokinase c) Phosphoglycerate kinase, d) Pyruvate kinase
  • Involve large negative energy (∆G), making them irreversible.

2. Reversible Reactions:

  • Can occur in both directions; products can revert to reactants under specific conditions.
  • Reversible reactions do not involve large negative ∆G values.
  • Out of 10 steps, 4 are irreversible (involving kinase enzymes), while the rest are reversible.
Key Points to Remember

Products of cleavage:

  • Dihydroxyacetone phosphate (DHAP)
  • 3-Phosphoglyceraldehyde (3-PGAL)
Dehydration:
  • 2-Phosphoglyceric acid loses water (via enolase) to form phosphoenol pyruvic acid.
Glycolysis is the only energy source for: Erythrocytes (red blood cells), Renal medulla, Brain, Sperm, Certain starch-storing plant tissues (e.g., potatoes).
Regulation of Glycolysis
  • Tightly controlled process.
  • Rate depends on: ATP requirement, NADH₂ regeneration.
  • Regulation of enzymes: Hexokinase, Phosphofructokinase-1 (PFK-1), Pyruvate kinase
  • Hormonal regulation: Glucagon, Epinephrine, Insulin
Role of Mg²⁺ and Zn²⁺ in Glycolysis
  • Cofactors tightly bound to enzymes; essential for enzyme function.
  • Regulate key enzymes like: Hexokinase, Phosphofructokinase, Triose phosphate dehydrogenase, Phosphoglycerate kinase, Enolase, Pyruvate kinase
Anaerobic Respiration in Man
  • Occurs: In muscles during oxygen deficiency.
Lactic Acid Fermentation:
  • NADH+H⁺ from glycolysis is reoxidized to NAD⁺ by donating a proton and two electrons to pyruvic acid, forming lactic acid.
  • Skeletal muscles derive energy through anaerobic respiration.
  • After vigorous exercise, lactic acid accumulates, causing muscle fatigue.
  • During rest, lactic acid is reconverted to pyruvic acid and enters the aerobic respiration pathway.
  • White muscle fibers primarily perform anaerobic respiration.
Anaerobic Respiration in Yeast
  • Yeast can perform both aerobic and anaerobic respiration based on oxygen availability.
Anaerobic Respiration:
  • Occurs: In the cytoplasm in the absence of oxygen.
Process:
  1. Pyruvate undergoes decarboxylation to form acetaldehyde.
  2. Acetaldehyde is reduced by NADH+H⁺ to form ethanol and carbon dioxide.
  • This process is termed alcoholic fermentation.
Key Points:
  • Accumulation of ethanol in yeast culture may halt cell multiplication and cause cell death.
  • In the presence of oxygen, yeast switches to aerobic respiration, producing CO₂ and H₂O.
Important Notes on Anaerobic Respiration

Incomplete Oxidation:

  • Leads to the release of less energy.
  • Some anaerobic products can be oxidized further, indicating that not all energy is released.
Energy Yield:
  • NADH₂ does not produce ATP due to the absence of the electron transport chain.
  • Only 2 ATP molecules are generated per glucose molecule in anaerobic respiration.
Definition and Features of Aerobic Respiration
  • Involves Molecular Oxygen: Oxygen acts as the final electron acceptor during glucose oxidation.
Complete Oxidation:
  • Glucose is fully oxidized into CO₂ and H₂O.
  • Results in the release of a large amount of energy.
Processes Involved:
  1. Glycolysis
  2. Acetyl CoA Formation (Connecting Link Reaction)
  3. Krebs Cycle
  4. Electron Transfer Chain Reaction
  5. Terminal Oxidation
Site of Reaction: Occurs in the mitochondria in eukaryotic cells.
Conversion of Pyruvic Acid to Acetyl CoA
  • Reaction Type: Oxidative Decarboxylation.
  • Catalyzed By: Pyruvate Dehydrogenase Complex (PDH). Located in the mitochondria of eukaryotes and cytosol of prokaryotes.
  • Significance: Acts as a connecting link between glycolysis and the Krebs cycle.
Know This

Role of Vitamin B1:

  • Essential for maintaining good health.
  • Thiamin (Vitamin B1) is a co-enzyme for pyruvate dehydrogenase complex.
  • Deficiency of thiamin causes pyruvic acidosis and lactic acidosis, which are life-threatening.
Importance of Diet: A balanced diet is crucial for maintaining overall health.
Krebs Cycle (TCA Cycle/Citric Acid Cycle)
  • The Krebs cycle is the second phase of aerobic respiration.
  • Takes place in the matrix of the mitochondria.
  • Serves as a common oxidative pathway for carbohydrates, fats, and proteins.


Process

1. Formation of Acetyl-CoA:

  • Pyruvic acid is decarboxylated, combining with coenzyme A to form acetyl-CoA.
  • This is an oxidative decarboxylation reaction, producing H⁺ ions, electrons, and CO₂.
  • NAD⁺ is reduced to NADH+H⁺.
2. Sources of Acetyl-CoA:
  • Produced during link reactions.
  • Formed by β-oxidation of fatty acids.
3. Citric Acid Formation:
  • Acetyl-CoA condenses with oxaloacetic acid to form citric acid.
  • Citric acid undergoes stepwise oxidation, evolving CO₂.
4. Regeneration of Oxaloacetic Acid:
  • Completes the cycle and allows continuation.
5. Oxidation Steps:
  • Four oxidation steps, catalyzed by dehydrogenases (oxidoreductases).
  • Use NAD⁺ or FAD⁺ as coenzymes, reducing them to NADH+H⁺ and FADH₂.
6. ATP Production: One molecule of GTP (converted to ATP) is produced per cycle.
Significance of Krebs Cycle
  • Breakdown Pathway: Common for carbohydrates, fats, and proteins.
  • Reduced Coenzymes: Produces NADH₂ and FADH₂, essential for energy production.
  • Energy Yield: Can generate 24 ATP from one glucose molecule.
  • CO₂ Production: CO₂ released is utilized in photosynthesis.
  • Intermediate Products: Provides precursors like α-ketoglutarate and oxaloacetate for synthesizing complex organic compounds.
Amphibolic Pathway

Respiration acts as both catabolic and anabolic:

  • Catabolic: Oxidation of acetyl-CoA for energy release.
  • Anabolic: Intermediates like α-ketoglutarate and oxaloacetate are precursors for: Fatty acids, Glutamic acid, Aspartic acid.
Krebs cycle is amphibolic due to this dual role.
Electron Transport Chain (ETC)

Definition

  • The ETC, or Electron Transfer System, is the final phase of aerobic respiration.
  • NADH₂ (NADH+H⁺) and FADH₂, produced in glycolysis, the link reaction, and the Krebs cycle, are oxidized through a series of electron carriers and enzyme complexes.
  • This process occurs on the inner mitochondrial membrane.
Key Components of ETC

1. Electron Carriers and Complexes:

  • Complex I: NADH dehydrogenase - Oxidizes NADH+H⁺ and transfers electrons to ubiquinone (CoQ).
  • Complex II: Succinate dehydrogenase - Oxidizes FADH₂ and transfers electrons to ubiquinone.
  • Ubiquinone (CoQ): Reduced to ubiquinol, carrying electrons to complex III.
  • Complex III: Cytochrome bc1 complex - Oxidizes ubiquinol and transfers electrons to cytochrome C.
  • Cytochrome C: Iron-containing protein that acts as a mobile electron carrier between complex III and IV.
  • Complex IV: Cytochrome C oxidase (cytochromes a and a3) - Transfers electrons to molecular oxygen (O₂) for terminal oxidation.

2. Terminal Oxidation: Reduced oxygen reacts with protons (H⁺) to form metabolic water.

3. Complex V (Oxysome):

  • F₀ part: Channels protons from the intermembrane space into the mitochondrial matrix.
  • F₁ part: Catalyzes the synthesis of ATP from ADP and inorganic phosphate via oxidative phosphorylation.
Significance of ETS
  1. Major Energy Generation: Produces 34 ATP molecules out of the total 38 ATP from glucose oxidation.
  2. Recycling of Coenzymes: Regenerates NAD⁺ and FAD⁺ from NADH+H⁺ and FADH₂, enabling their reuse.
  3. Metabolic Water Formation: Produces water through the reduction of oxygen.
  4. Stepwise Energy Release: Releases energy gradually to prevent cell damage.
Oxidative Phosphorylation
  • Definition: A metabolic pathway using energy released by oxidation of substrates to produce ATP.
  • Occurs on the inner mitochondrial membrane.
  • Hydrogen atoms released during respiration are trapped by NAD⁺ or FAD⁺.
  • Electrons pass through the ETC, producing ATP and metabolic water.
  • This process was named 'Chemiosmosis' by Peter Mitchell due to the coupling of proton transfer with ATP synthesis.
Balance sheet for ATP by aerobic oxidation of 1 glucose molecule.

 Linking Glycolysis, TCA Cycle, and Electron Transport Chain

Coenzymes:
  • Start as NAD⁺ and FAD⁺ and get reduced to NADH+H⁺ and FADH₂ during glycolysis, the link reaction, and the Krebs cycle.
  • Reduced coenzymes are reoxidized in the Electron Transport Chain (ETC), generating ATP.
Process Breakdown

1. Glycolysis:

  • Glucose → 2 Pyruvic acid.
  • Net gain: 2 NADH+H⁺ and 2 ATP (substrate-level phosphorylation).

2. Link Reaction:

  • Pyruvic acid → 2 Acetyl CoA.
  • Net gain: 2 NADH+H⁺.

3. Krebs Cycle (TCA Cycle):

  • Acetyl CoA → CO₂ + H₂O.
  • Net gain: 6 NADH+H⁺, 2 FADH₂, 2 ATP (substrate-level phosphorylation).

4. Electron Transport Chain (ETC):

  • Reoxidation of NADH+H⁺ and FADH₂.
  • Net gain: 34 ATP (via oxidative phosphorylation).
Key Links
  • The ATP generated in ETC powers glycolysis and other processes.
  • Oxidized forms of coenzymes (NAD⁺, FAD⁺) are recycled to sustain cellular respiration.
Respiration Experiments

1. Anaerobic Respiration in Yeast

  • Materials: Baker’s yeast, glucose solution, lime water, oil, test tubes, rubber stoppers, glass tubes, warm water (37°C-38°C).
Steps:
  1. Suspend yeast in glucose solution (test tube A). Cover the surface with oil and seal with a stopper.
  2. Connect to a delivery tube leading to lime water (test tube B).
  3. Place test tube A in warm water.
Observations:
  • Lime water turns milky → CO₂ release.
  • Alcohol smell after 1-2 days → Ethanol formation.
Inference: Yeast ferments glucose to ethanol and CO₂ under anaerobic conditions.
2. Aerobic Respiration in Yeast
  • Lime water level rises in test tube B → Oxygen consumption.
  • No alcohol smell → Yeast respires aerobically.
3. Respiration in Germinating Seeds
  • Materials: Germinating seeds, test tube, mercury, trough, potassium hydroxide (KOH).
Steps:
  1. Remove seed coats and place seeds in a mercury-filled test tube.
  2. Invert the test tube into a mercury trough.
  3. Observe gas accumulation at the top.
  4. Introduce KOH to absorb CO₂.
Observations:
  • CO₂ production → Germinating seeds undergo anaerobic respiration in mercury.
  • KOH absorption of CO₂ restores mercury levels.
Inference: Germinating seeds release CO₂ through respiration.
Utility of Stepwise Energy Release in Respiration
  1. Cell Protection: Gradual energy release in ETS prevents cell damage due to uncontrolled heat generation.
  2. Efficient ATP Synthesis: Stepwise release allows for higher energy capture in ATP synthesis.
  3. Regulation and Control: Enzymatic activities are regulated by specific compounds, ensuring energy release matches the cell's needs.
  4. Biosynthesis Support: Pathways provide intermediates for synthesizing biomolecules like amino acids.
Role of Hydrogen Removal in Respiration

Primary Process:

  • Hydrogen removal (not direct oxygen reaction) is the key to respiration.
  • Enzymes like dehydrogenases catalyze hydrogen removal.

Hydrogen Acceptors:

  • Free hydrogen is captured by acceptors like NAD⁺ and FAD⁺.
  • In aerobic respiration, hydrogen combines with oxygen to form water.
Respiratory Substrates
  • Molecules oxidized during respiration to release energy for ATP synthesis.
  • Common substrates: Carbohydrates, Fats, Proteins
Respiratory Quotient (RQ)
  • Definition: The ratio of the volume of CO2 released to the volume of O2 consumed during respiration.
  • Formula: R.Q. = Volume of CO2 released / Volume of O2 consumed
  • Depends on: The type of respiratory substrate.
RQ for Different Respiratory Substrates
Key Points to Remember
  • The RQ value varies depending on the respiratory substrate used.
  • Higher RQ values (close to 1) indicate carbohydrate usage, while lower RQ values (< 1) indicate the usage of fats or proteins.
Significance of Respiration
  1. Energy Supply: Provides energy for synthesis, cell division, repair, and movement.
  2. Building Blocks: Intermediates of the Krebs cycle are used to synthesize complex compounds.
  3. Atmospheric Balance: Along with photosynthesis, maintains CO₂-O₂ balance in the atmosphere.
  4. Industrial Applications: Anaerobic Respiration (Fermentation): Used in industries like dairies, bakeries, distilleries, leather, and paper. Produces alcohol, organic acids, vitamins, and antibiotics.