Respiration

Definition

• Respiration is the enzymatic process of oxidation of organic substances (mainly carbohydrates) within cells to release energy (ATP) for various metabolic activities.
• It is catabolic (breakdown) and exergonic (energy-releasing).

General Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Types of Respiration

1. Aerobic Respiration – Occurs in the presence of O₂; complete oxidation of glucose.
2. Anaerobic Respiration – Occurs in the absence of O₂; partial oxidation producing alcohol or lactic acid.

Examples:

• Aerobic: Most plant cells.
• Anaerobic: Yeast (fermentation → ethanol) or oxygen-deficient tissues.

Site of Respiration

Major Steps of Respiration

1. Glycolysis (EMP Pathway)

• Discovered by Embden, Meyerhof, and Parnas (1940).
• Occurs in the cytoplasm of all living cells.
• Anaerobic process – does not require oxygen.
• One molecule of glucose (6C) splits into two molecules of pyruvic acid (3C).

Net Reaction: Glucose + 2ADP + 2Pi + 2NAD ⁺ → 2Pyruvate + 2ATP + 2NADH + 2H⁺

Energy Yield: → 4 ATP formed − 2 ATP used = Net gain 2 ATP.

🔁 II. Link Reaction (Oxidative Decarboxylation of Pyruvate)

• Occurs in mitochondrial matrix.
• Converts Pyruvate (3C) → Acetyl-CoA (2C) + CO₂ + NADH.
• Enzyme: Pyruvate dehydrogenase complex.

2Pyruvate + 2CoA + 2NAD⁺ →2Acetyl – CoA + 2CO₂ + 2NADH

🔄 III. Krebs Cycle (Citric Acid Cycle / TCA Cycle)

• Discovered by Hans Krebs (1937).
• Occurs in the mitochondrial matrix.
• Acetyl-CoA (2C) combines with oxaloacetic acid (4C) → citric acid (6C), which is then oxidized to release CO₂.

End Products (per glucose molecule):

• 6 NADH
• 2 FADH₂
• 2 ATP (or GTP)
• 4 CO₂

Significance: Central metabolic hub; provides intermediates for amino acids, fatty acids, etc.

2. Electron Transport Chain (ETC)

• Takes place in inner mitochondrial membrane (cristae).
• NADH and FADH₂ donate electrons to ETC components:
  → NADH → FMN → CoQ → Cytochromes (b, c₁, c, a, a₃).
• Final electron acceptor: Oxygen (O₂), forming H₂O.

ATP Synthesis via Oxidative Phosphorylation:

• Each NADH → 3 ATP
• Each FADH₂ → 2 ATP

Total ATP yield (per glucose): → 38 ATP (in prokaryotes) or 36 ATP (in eukaryotes)

Anaerobic Respiration (Fermentation)

• Occurs in absence of oxygen (e.g., in roots under waterlogged conditions, yeast).
• End products:
  • In yeast: Ethanol + CO₂
  • In muscles: Lactic acid
• Equation (Yeast): C6H₁₂O₆ → 2C₂H₅OH +2CO₂ + 2ATP
• Energy Yield: → Only 2 ATP per glucose (very low efficiency).

Energy Accounting in Aerobic Respiration

(Using P/O ratio: NADH = 3 ATP, FADH₂ = 2 ATP)

Respiratory Substrates

• Substances used for respiration.
• Main substrate: Glucose.
• Others: organic acids, fats, proteins.

Types:

• Carbohydrate respiration: Normal (R.Q. = 1).
• Fat respiration: R.Q. < 1
• Protein respiration: R.Q. ≈ 0.8
• Organic acid respiration: R.Q. > 1

Respiratory Quotient (R.Q.); RQ = CO2 evolved / O2 consumed

Factors Affecting Respiration

• Temperature: Increases rate up to optimum (~35°C).
• Oxygen concentration: Required for aerobic respiration.
• Substrate availability: More carbohydrate → higher rate.
• Tissue age: Young, metabolically active tissues respire more.
• Light: Indirect effect (influences carbohydrate supply).

Significance of Respiration

• Provides ATP – universal energy currency for metabolism.
• Generates reducing power (NADH, FADH₂).
• Provides carbon skeletons for synthesis of amino acids, fats, etc.
• Releases CO₂ for photosynthetic use.
• Maintains temperature in certain plants (e.g., Arum lily).

Differences Between Aerobic and Anaerobic Respiration