Sample Notes: Energy

A2 Level Biology – Chapter 2.1 Energy

1. The Need for Energy in Living Organisms

• All living organisms require energy to perform various metabolic processes necessary for life. Energy allows organisms to maintain internal conditions, respond to their environment, and reproduce.
• Essential processes requiring energy:
  • Active Transport: Movement of ions and molecules across cell membranes against their concentration gradients (e.g. sodium-potassium pump in neurons).
  • Anabolic Reactions: Biosynthesis of large complex molecules from smaller ones.
    • Examples:
      • DNA replication – formation of DNA strands from nucleotides.
      • Protein synthesis – forming polypeptides from amino acids in ribosomes.
  • Cell Division: Energy is required during all stages of mitosis and meiosis.
  • Secretion: Packaging and transport of materials via the Golgi apparatus and vesicles.
  • Homeostasis: Energy is needed for maintaining a stable internal environment (e.g. body temperature regulation in endotherms).
  • Movement:
    • Muscle contraction in animals.
    • Flagella or cilia movement in unicellular organisms.
    • Cytoplasmic streaming in plant cells.
  • Growth and repair: Energy fuels the synthesis of new cell materials and the replacement of damaged cells.

2. Adenosine Triphosphate (ATP) – The Universal Energy Currency

• ATP Structure:
  • Consists of three main components:
    • Adenine: A nitrogenous base.
    • Ribose: A five-carbon sugar.
    • Three phosphate groups – the terminal phosphate bond is high-energy.
• Properties that make ATP suitable as a universal energy carrier:
  • Immediate energy release: Hydrolysis of ATP into ADP + Pi releases 30.5 kJ/mol.
  • Reversible reaction: ATP can be regenerated from ADP and Pi by phosphorylation.
  • Soluble and mobile: Easily diffuses through the cytoplasm to deliver energy.
  • Universal usability: Used in almost all metabolic reactions across all forms of life.
• Hydrolysis reaction:
  • ATP + H₂O → ADP + Pi + energy
  • Enzyme: ATPase
• ATP regeneration:
  • Occurs during:
    • Respiration (in mitochondria).
    • Photosynthesis (in chloroplasts).
    • Substrate-level phosphorylation (in cytoplasm during glycolysis).

3. ATP Synthesis Methods

3.1 Substrate-Level Phosphorylation

• A direct transfer of phosphate from a donor molecule to ADP.
• Occurs during:
  • Glycolysis (cytoplasm)
  • Krebs cycle (mitochondrial matrix)

3.2 Oxidative Phosphorylation (Chemiosmosis)

• Involves:
  • Electron transport chain (ETC)
  • Proton gradient
  • ATP synthase
• Location: Mitochondrial inner membrane (cristae)
• Steps:
  1. Electrons are passed along the ETC.
  2. H⁺ ions are pumped into the intermembrane space.
  3. ATP synthase allows H⁺ ions to flow back into the matrix.
  4. This flow provides energy to phosphorylate ADP to ATP.

3.3 Photophosphorylation

• Occurs during photosynthesis in chloroplasts.
• Similar chemiosmotic principles as oxidative phosphorylation.

4. Energy Values of Respiratory Substrates

• Glucose (Carbohydrate):
  • RQ = 1.0
  • Energy yield ≈ 2870 kJ/mol
  • Each molecule yields ~30–32 ATP under aerobic conditions.
• Lipids:
  • Contain more C–H bonds than carbohydrates → more reduced → more ATP.
  • Triglycerides are hydrolyzed to glycerol and fatty acids.
  • Fatty acids enter β-oxidation → converted to acetyl CoA → Krebs cycle.
  • RQ ≈ 0.7
  • Energy yield ≈ 39 kJ/g
• Proteins:
  • Only used when carbohydrate and fat stores are depleted.
  • Deaminated in the liver → amino group converted to urea.
  • Carbon skeleton enters respiration (as pyruvate or Krebs intermediates).
  • RQ ≈ 0.8–0.9
  • Moderate energy yield.

Energy comparison table:

5. Respiratory Quotient (RQ)

• Formula:
  RQ = CO₂ produced / O₂ consumed
• Used to:
  • Identify the type of respiratory substrate.
  • Assess anaerobic vs aerobic conditions.
• Typical RQ values:
  • Glucose (aerobic): RQ = 6 CO₂ / 6 O₂ = 1.0
  • Fatty acids: RQ = ~0.7 (more oxygen needed)
  • Proteins: RQ = ~0.8–0.9
  • Anaerobic respiration: RQ > 1 (no oxygen used, CO₂ still produced)

Example: Palmitic Acid (C₁₆H₃₂O₂)
Equation: C₁₆H₃₂O₂ + 23 O₂ → 16 CO₂ + 16 H₂O
RQ = 16 / 23 ≈ 0.7

6. Determining RQ Experimentally (Using a Respirometer)

• Respirometer Components:
  • Sealed chamber with organism.
  • CO₂ absorber (soda lime or KOH).
  • Manometer or fluid-filled capillary tube to measure O₂ uptake.
• Procedure:
  • Organism consumes O₂ → pressure drops → fluid moves.
  • Measure change over time → calculate O₂ consumption.
  • Without CO₂ absorber: measure both CO₂ & O₂ → determine RQ.
• Use cases:
  • Comparing respiration rates of seeds, insects, yeast.
  • Studying effect of temperature or substrate.

7. Definitions and Key Terms

8. Applications and Additional Notes

• Anaerobic vs Aerobic:
  • Anaerobic respiration in animals produces lactate → less ATP (2 per glucose).
  • In yeast, produces ethanol + CO₂.
  • ATP production is lower, but faster.
• Industrial and research use of RQ:
  • Monitoring fermentation.
  • Studying metabolism in exercise physiology.
  • Assessing ripening in fruit storage.
• Metabolic disorders and ATP:
  • Diseases like mitochondrial myopathy or enzyme deficiencies affect ATP production.
• Importance in cells:
  • Cells with high energy needs (neurons, muscle cells, sperm) contain many mitochondria.
  • Red blood cells lack mitochondria – rely on glycolysis.