Cellular Respiration & ATP Synthesis

ATP Synthesis: Big Picture

  • Two fundamental phosphorylation strategies that create adenosine triphosphate (ATP)

    • Oxidative phosphorylation

    • Location: mitochondrial inner membrane

    • Yield: ≈34 ATP per glucose

    • Substrate-level phosphorylation

    • Location: cytoplasm (and a small amount in the mitochondrial matrix during the Krebs cycle)

    • Yield in glycolysis: 2 net ATP per glucose

    • Yield in the Krebs cycle: 1 ATP per acetyl unit (2 per glucose)

  • Cellular-respiration roadmap

    1. Glycolysis (cytoplasm)

    2. Krebs / Citric-acid / Tricarboxylic-acid (TCA) cycle (mitochondrial matrix)

    3. Electron-transport chain (ETC) + oxidative phosphorylation (mitochondrial inner membrane)

Mitochondrial Architecture & Nomenclature

  • Outer membrane: permeable to small molecules/ions

  • Inner membrane: highly folded (cristae) and impermeable; houses ETC complexes and ATP-synthase

  • Intermembrane space (IMS): between the two membranes; H⁺ reservoir

  • Matrix: interior to inner membrane; site of Krebs cycle and β-oxidation

  • Proton distribution snapshot

    • High [H⁺] in IMS, low [H⁺] in matrix → electrochemical gradient that stores potential + electrical energy

  • Key protein players embedded in the inner membrane

    • Complex I, II, III, IV = electron-transport chain (ETC)

    • ATP synthase (purple in diagram) = H⁺ channel + rotary enzyme that makes ATP

Mechanistic Sequence: Oxidative Phosphorylation

  • L Energy source: kinetic energy of protons diffusing down gradient through ATP synthase

  • Coupling reaction: ADP+Pi→ATP SynthaseH⁺ flowATP\text{ADP} + P_i \xrightarrow[\text{ATP Synthase}]{\text{H⁺ flow}} \text{ATP}

  • Establishing the gradient

    • ETC uses energetic electrons supplied by NADH & FADH₂

    • Each electron transfer releases free energy → pumps H⁺ from matrix → IMS (primary active transport)

    • Oxygen is the terminal electron acceptor: 12O<em>2+2e−+2H+→H</em>2O\frac12 O<em>2 + 2e^- + 2H^+ \to H</em>2O

    • Without O₂, ETC stalls, gradient collapses, oxidative phosphorylation ceases

  • Functional analogy

    • ETC = Na⁺/K⁺ pump (primary) … ATP synthase = Na⁺/glucose cotransporter (secondary). Both rely on gradients set up by a separate energy-consuming step.

Glycolysis (Phase 1)

  • Venue: cytoplasm (only source of ATP for red blood cells—no mitochondria)

  • Fuel: mostly glucose; other monosaccharides first converted to glucose in the liver

  • Energetics of glucose: ≈686 kcal mol−1\approx 686\,\text{kcal mol}^{-1} of potential energy

  • Net outputs per glucose

    • 2 pyruvate (3-C each)

    • 2 ATP (net; 4 produced − 2 consumed)

    • 2 NADH

  • O₂ requirement: none (anaerobic); pyruvate → lactate when O₂ absent (lactic-acid fermentation)

  • Substrate-level phosphorylation example

    • Phosphocreatine + ADP → creatine + ATP

    • Demonstrates direct phosphate transfer from a high-energy substrate

Pyruvate Oxidation (Link Step)

  • Location: mitochondrial matrix (requires O₂ indirectly)

  • Reaction (per pyruvate):

    • Pyruvate (3 C) + CoA + NAD+NAD^+ → Acetyl-CoA (2 C) + NADHNADH + CO2CO_2

  • Happens twice per glucose → 2 NADH + 2 CO₂ + 2 acetyl-CoA

Krebs / Citric-Acid Cycle (Phase 2)

  • Occurs in matrix; cyclical pathway

  • Entry molecule: acetyl unit (2 C) that condenses with oxaloacetate (4 C) → citrate (6 C)

  • Oxaloacetate is both first reactant and final product → cycle nature

  • Products per turn (per acetyl unit)

    • 3 NADH

    • 1 FADH₂

    • 1 ATP (substrate-level)

    • 2 CO₂

  • Products per glucose (two turns)

    • 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂

Electron Carriers: Accounting After One Glucose

  • From glycolysis: 2 NADH

  • From pyruvate → Acetyl-CoA: 2 NADH

  • From Krebs: 6 NADH + 2 FADH₂

  • Total: 10 NADH, 2 FADH₂ → feed ETC to pump H⁺

Stoichiometric & Energetic Summary

  • Global cellular-respiration equation
    C<em>6H</em>12O<em>6+6O</em>2→6CO<em>2+6H</em>2O+ATP+heatC<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + \text{ATP} + \text{heat}

  • Source of the six CO2CO_2

    • 4 from Krebs (2 × 2)

    • 2 from pyruvate → acetyl-CoA (2 × 1)

  • ATP yield per glucose (typical textbook numbers)

    • Glycolysis: 2 ATP (substrate-level)

    • Krebs: 2 ATP total (substrate-level)

    • Oxidative phosphorylation: ≈34 ATP (depends on shuttle efficiency + proton-leak)

    • Grand total: ~38 ATP (ideal), often ~30-32 in vivo

  • Heat: inevitable by-product; maintains body temperature but represents lost usable energy

Conceptual & Clinical Connections

  • Red-blood-cell metabolism: obligatory glycolysis → explains vulnerability to hypoxia & glucose-level disturbances

  • Importance of O₂ breathing: sole role in aerobic respiration is to serve as final electron sink; any impairment (respiratory, circulatory) rapidly halts ETC → ATP crisis

  • Creatine phosphate shuttle: quick ATP buffer in muscle during sudden exertion (substrate-level)

  • Pharmacological/poison considerations

    • ETC inhibitors (cyanide, CO) bind complex IV → prevent electron transfer to O₂ → collapse gradient → fatal ATP shortage

    • Uncouplers (e.g., DNP) shuttle H⁺ across membrane, dissipating gradient as heat → drastic hyperthermia, weight loss

Key Take-Home Points

  • Energy begets energy: proton-motive force must first be built (ETC) before ATP synthase can harvest it.

  • Oxidative phosphorylation is the primary ATP generator; glycolysis and Krebs chiefly supply reduced coenzymes to drive it.

  • Substrate-level phosphorylation is smaller-scale but vital for anaerobic contexts and rapid energy buffering.

  • Oxygen’s only biochemical duty in respiration: accept spent electrons and form water, yet its absence shuts down the entire high-yield pathway.

  • Cellular respiration disassembles glucose gradually, capturing manageable energy packets instead of releasing large unharvestable bursts.