Bioenergetics

Definition: The quantitative study of energy relationships and energy transformations (conversions) in biological systems.

Main Processes: Photosynthesis (Energy Capture) and Respiration (Energy Release).

ATP: Adenosine Triphosphate is the 'Energy Currency' of the cell.

Pigments absorb visible light (380-750nm).

• Chlorophylls: Main pigments. Absorb Blue and Red light, reflect Green.
  • Chlorophyll 'a': Universal pigment. Methyl group (-CH₃). Blue-green color.
  • Chlorophyll 'b': Accessory pigment. Aldehyde/Formyl group (-CHO). Yellow-green color.
• Carotenoids: Accessory pigments (Yellow/Orange/Red). Absorb blue-violet light. Protect chlorophyll from photo-oxidation.
• Spectra: Absorption Spectrum shows absorption at different wavelengths. Action Spectrum shows photosynthetic rate at different wavelengths.

Occurs in Thylakoid membranes. Generates ATP and NADPH.

1. Excitation: Light hits Photosystem II (P680), exciting 2 electrons.
2. Photolysis: Water splits (H₂O → 2H⁺ + 2e^- + ½O₂) to replace electrons in P680. O₂ is released.
3. ETC: Electrons pass through Plastoquinone (PQ) → Cytochrome complex → Plastocyanin (PC) to PS-I. ATP is produced via Chemiosmosis (Photophosphorylation).
4. Re-excitation: Light hits Photosystem I (P700), re-exciting electrons.
5. Reduction: Electrons pass to Ferredoxin (Fd) and then reduce NADP⁺ to NADPH.

Occurs in Stroma. Uses ATP and NADPH to reduce CO₂ to Glucose. (C3 Pathway).

• Phase 1: Carbon Fixation: CO₂ reacts with RuBP (5-Carbon) catalyzed by Rubisco to form unstable 6-C compound, which splits into two 3-PGA molecules.
• Phase 2: Reduction: 3-PGA is reduced to G3P (Glyceraldehyde-3-Phosphate) utilizing ATP and NADPH. G3P is the precursor for Glucose.
• Phase 3: Regeneration: Most G3P used to regenerate RuBP (requires ATP) to continue the cycle.

Breakdown of Glucose (6C) into two Pyruvates (3C). Occurs in Cytoplasm. Anaerobic.

• Preparatory Phase (Investment): Uses 2 ATP to convert Glucose → Fructose-1,6-Bisphosphate. Splits into G3P.
• Oxidative Phase (Payoff): G3P oxidized to Pyruvate. Produces 4 ATP (Substrate Level Phosphorylation) and 2 NADH.
• Net Yield: 2 ATP + 2 NADH per Glucose.

Occurs in Mitochondrial Matrix. Aerobic.

• Pyruvate Oxidation (Link Rxn): Pyruvate → Acetyl-CoA + CO₂ + NADH.
• Cycle Steps: Acetyl-CoA (2C) combines with Oxaloacetate (4C) → Citrate (6C). Through decarboxylation and oxidation, Oxaloacetate is regenerated.
• Yield (per Glucose/2 turns): 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂.

Occurs in Inner Mitochondrial Membrane.

• NADH and FADH₂ donate electrons to carriers (Cytochromes).
• Energy released pumps protons (H⁺) out, creating gradient.
• Protons flow back via ATP Synthase to generate ATP (Oxidative Phosphorylation).
• Oxygen is the final electron acceptor, forming Water.

Breakdown of glucose in absence of oxygen.

• Alcoholic Fermentation: Pyruvate → Ethyl Alcohol + CO₂ (Yeast).
• Lactic Acid Fermentation: Pyruvate → Lactic Acid (Muscles, Bacteria).
• Yields only 2 ATP (from Glycolysis).