Intermediary Metabolism Study Notes

Intermediary Metabolism Overview

  • Energy Requirements: Living cells need energy from external sources to perform work.
  • Most ecosystems acquire energy as sunlight, and it is lost as heat.
  • Photosynthesis:
    • Occurs in chloroplasts, converting CO2 and H2O into organic molecules and O2.
  • Cellular Respiration:
    • Performed in mitochondria, using oxygen to fuel ATP regeneration.
    • Involves three main pathways:
    • Glycolysis
    • Citric acid cycle
    • Oxidative phosphorylation

Catabolic Pathways

  • Catabolic pathways oxidize organic fuels to yield energy:
    • Energy release results from the breakdown of complex molecules to simpler waste products.
    • Some energy is used for cellular work; excess is dissipated as heat.
  • Cellular Respiration:
    • Breakdown of glucose (C6H12O6) is a primary example.
    • Reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP + heat).
    • D G = -686 kcal/mol (exergonic process).

Redox Reactions in Metabolism

  • Redox Reactions:
    • Transfer of electrons between reactants, constituting oxidation-reduction reactions.
    • Oxidation: Loss of electrons; Reduction: Gain of electrons.
    • Example: Na + Cl → Na+ + Cl− (sodium oxidized, chlorine reduced).

Stepwise Electron Transfer

  • Electron Transport Chain: Electrons are not transferred all at once but in steps, primarily using NAD+ as the electron carrier.
  • NAD+ is reduced to NADH during glucose oxidation, serving as an energy carrier.
  • These steps facilitate controlled energy release, preventing heat loss.

Enzymatic Reaction and Activation Energy

  • Enzymes: Catalytic proteins that speed reactions by lowering the activation energy (EA).
  • Example: Hydrolysis of sucrose via sucrase.

Enzyme-Substrate Complex

  • The molecule that an enzyme acts on is the substrate.
  • Enzyme binds substrate forming an enzyme-substrate complex, facilitated by induced fit mechanism enhancing chemical catalytic efficiency.

Cofactors and Inhibitors

  • Cofactors: Non-protein helpers for enzymes.
    • Can be inorganic (like metal ions) or organic (coenzymes, e.g., NAD+).
  • Inhibitors:
    • Competitive inhibitors bind to active sites, while noncompetitive inhibitors bind elsewhere, altering enzyme function.
    • Examples include various toxins and antibiotics.

Summary of Cellular Respiration Stages

  • Stages:
    1. Glycolysis (cytoplasm) - glucose is split, yielding pyruvate; net gain of 2 ATP.
    2. Citric Acid Cycle (mitochondrial matrix) - completes glucose breakdown, yielding CO2, ATP, NADH, FADH2.
    3. Electron Transport Chain & Oxidative Phosphorylation - electrons move down the chain; ATP generated via chemiosmosis.

Glycolysis

  • Glycolysis consists of energy investment (requires 2 ATP) and energy payoff phases (produces 4 ATP, net gain of 2 ATP and 2 NADH).
  • Process is anaerobic (can function without oxygen).

Citric Acid Cycle

  • Converts pyruvate to acetyl CoA, continuing the oxidation process to yield CO2, ATP, NADH, FADH2.
  • Named after Hans Krebs; each turn generates 1 GTP (converted to ATP) and releases high-energy electrons to carriers.

Electron Transport Chain

  • Location: Inner mitochondrial membrane generating ATP through chemiosmosis.
  • Oxygen is essential for regenerating NADH to NAD+, allowing ATP production via oxidative phosphorylation.

Fermentation

  • Anaerobic respiration mechanism providing ATP without oxygen; includes alcohol fermentation (yeast) and lactic acid fermentation (muscles).
  • Axes glycolysis but relies on NAD+ recycling for continued ATP production.

Metabolic Pathway Flexibility

  • Glycolysis and citric acid cycle interconnect with various pathways, allowing catabolism of carbohydrates, fats, and proteins into usable ATP.
  • Regulation through feedback mechanisms: if ATP is high, glycolysis slows; if low, it speeds up to produce ATP.
  • Phosphofructokinase example: regulated by ATP and citrate to ensure metabolic balance.