Cellular Respiration & ATP – Comprehensive Lecture Notes

Fun Fact – How Much ATP Do We Burn?

• Baseline cellular ATP turnover
  • Each cell (at absolute minimum, i.e., “at rest”) hydrolyzes $68{,}000{,}000$ ATP · s⁻¹.
  • Daily use per cell: $68{,}000{,}000\;\text{ATP s}^{-1}\times3600\;\text{s h}^{-1}\times24\;\text{h d}^{-1}=5.88\times10^{12}\;\text{ATP d}^{-1}$.
  • Whole-body scale
  • Approx. $10{,}000{,}000{,}000{,}000$ (10¹³) cells ≈ reference human.
  • Total ATP/day ≈ $5.88\times10^{12}\times10^{13}=5.88\times10^{25}\;\text{ATP}$.
  • Mass perspective
  • One ATP ≈ $505\;\text{g mol}^{-1}$; calculation → ≈ 70 kg of ATP synthesized/hydrolyzed per day.
  • Key implication – ATP is recycled many times per second; you do not consume your body weight in raw food every day!

ATP – The Universal Energy Currency

• Structure recap
  • Adenosine (adenine + ribose) + triphosphate (ATP) ↔ adenosine diphosphate (ADP) + Pi.
  • Bond between β- and γ-phosphates is “spring-loaded”: highly energetic & unstable.
• Function cycle
  • Phosphorylation: ATP donates γ-phosphate to a target molecule → work accomplished (e.g., pump conformational change, myosin reset).
  • Re-phosphorylation (ADP + Pi → ATP) uses energy harvested from catabolic pathways.

Why We Need So Much ATP – Cellular Jobs

• Metabolic reactions (catabolic & anabolic).
• Active transport: Na⁺/K⁺ pumps, Ca²⁺ pumps, proton pumps.
• Muscle contraction: myosin head detachment & cocking.
• Action potentials: restoring ion gradients.
• Anabolism: DNA replication, RNA transcription, protein synthesis, growth & repair.
• Mitosis/meiosis: chromosome duplication, spindle operation.

Fuel Sources & Nutrient Context

• Preferred substrate: glucose → yields 30–34 ATP per molecule.
• Other monosaccharides (galactose, fructose) are readily converted to glucose.
• Insufficient glucose → switch to lipid catabolism (β-oxidation, ketone formation).
• Prolonged starvation → protein catabolism (last resort; tissue wasting).
• Links to prior course content
  • Digestive enzymes (studied on midterm) break polymers to absorbable monomers.
  • Nutrition worksheet introduced vitamin-derived cofactors (B₂ & B₃) essential for redox carriers.

Two Modes of ATP Synthesis

• Substrate-level phosphorylation
  • Direct transfer of Pi from an intermediate to ADP.
  • Occurs in glycolysis & Krebs cycle.
• Chemiosmosis / oxidative phosphorylation
  • Proton-motive force across inner mitochondrial membrane drives ATP synthase.
  • Responsible for the bulk (~28 ATP) of output.

Mitochondrion: Anatomy Meets Biochemistry

• Double membrane; inner membrane folded into cristae → high surface area for electron-transport chains (ETC).
• Spatial organization of pathways
  • Cytoplasm: glycolysis.
  • Mitochondrial matrix: pyruvate → acetyl-CoA processing & Krebs cycle.
  • Inner membrane (cristae): ETC & chemiosmosis.
• More cristae surface ⇒ more ETC complexes ⇒ greater ATP production potential.

Big-Picture Chemical Equations

• Oversimplified global view
  $\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\,\text{O}</em>2 \rightarrow 6\,\text{CO}<em>2 + 6\,\text{H}</em>2\text{O} + \text{Energy (≈30–34 ATP)}$
• Actual energetic core $2\,\text{H}^+ + 2\,e^- + \tfrac12\,\text{O}<em>2 \rightarrow \text{H}</em>2\text{O} + \text{Energy}$
  • Pure H₂ + O₂ reaction is explosive (rocket fuel, Hindenburg disaster).
  • Biology prevents “explodey” chemistry by feeding electrons/protons to O₂ incrementally via carriers in discrete enzyme complexes.

Electron Carriers – The Cellular "Transport Trucks"

• NAD⁺ / NADH (derived from vitamin B₃ – niacin).
• FAD / FADH₂ (derived from vitamin B₂ – riboflavin).
• Cycle
  • Reduction: NAD⁺ + 2e⁻ + H⁺ → NADH (loaded truck).
  • Oxidation: NADH → NAD⁺ + 2e⁻ + H⁺ (unloaded truck) at ETC.

Reaction Classifications

• Catabolic: break bonds, release energy (exergonic). e.g., overall cellular respiration.
• Anabolic: build bonds, require energy (endergonic). e.g., protein synthesis.
• Redox (always paired)
  • Oxidation = loss of electrons.
  • Reduction = gain of electrons.
  • Mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain.

Pathway Overview – “Forest Before Trees”

• Total ≈ $30–34\;\text{ATP per glucose}$ (variation due to shuttle systems & proton leak).
• Only high-level intermediates, enzymes, and quantities required for this course → avoid deeper rabbit holes unless curious.

Course & Administrative Notes

• Tuesday & Thursday structure shifted (holiday):
  • Tuesdays: review videos & group work.
  • Thursdays: quiz + worksheet.
• Final exam: Aug 14, 12:00–15:00, campus gym.
• Lab-final sign-up posted; short practical slots available.
• Reminder: massive detail exists (e.g., 10 glycolytic enzymes, full Krebs intermediates). For exam prep, stick to objectives & instructor videos.

Metaphors, Analogies & Humor Recap

• ATP’s γ-phosphate likened to a compressed spring – eager to pop off.
• Controlled electron flow vs. rocket fuel – prevents cellular “explodey” reactions.
• NADH/FADH₂ compared to transport trucks making delivery runs between Krebs and ETC.
• Cell counting joke – avoiding “468… what? 1 2 3…” errors when tallying 10¹³ cells.
• Chocolate incentive: recall NADH/FADH₂ appearance on nutrition worksheet earns confectionery reward.