Cellular Respiration Overview:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

(a) Four stages:

1. Glycolysis
2. Pyruvate Oxidation (transition reaction)
3. Krebs Cycle (Citric Acid Cycle)
4. Electron Transport Chain + Oxidative Phosphorylation

(b) Locations:

1. Glycolysis:

• Location: Cytoplasm (cytosol)
• Anaerobic (doesn't require O₂)

2. Pyruvate Oxidation:

• Location: Mitochondrial matrix
• Entry into mitochondria

3. Krebs Cycle:

• Location: Mitochondrial matrix
• Aerobic (requires O₂ indirectly)

4. Electron Transport Chain:

• Location: Inner mitochondrial membrane (cristae)
• Requires O₂ as final electron acceptor

(c) Products of each stage:

1. Glycolysis (glucose → 2 pyruvate):

• ATP: 2 net (4 produced - 2 invested)
• NADH: 2
• Pyruvate: 2

2. Pyruvate Oxidation (2 pyruvate → 2 acetyl-CoA):

• NADH: 2
• CO₂: 2
• Acetyl-CoA: 2

3. Krebs Cycle (2 turns, one per acetyl-CoA):

• ATP (GTP): 2
• NADH: 6
• FADH₂: 2
• CO₂: 4

4. Electron Transport Chain:

• ATP: ~34 (from NADH and FADH₂)
• H₂O: 6 (O₂ reduced)

(d) Total ATP yield:

From NADH:

• Glycolysis: 2 NADH × 2.5 ATP = 5 ATP
• Pyruvate oxidation: 2 NADH × 2.5 ATP = 5 ATP
• Krebs: 6 NADH × 2.5 ATP = 15 ATP
• Subtotal: 25 ATP

(Note: Glycolysis NADH may yield only 1.5 ATP each if using glycerol phosphate shuttle = 3 ATP total)

From FADH₂:

• Krebs: 2 FADH₂ × 1.5 ATP = 3 ATP

From substrate-level phosphorylation:

• Glycolysis: 2 ATP
• Krebs: 2 ATP (or GTP)
• Subtotal: 4 ATP

Total (using malate-aspartate shuttle): $\text{Total} = 25 + 3 + 4 = 32 \text{ ATP}$

Total (using glycerol phosphate shuttle): $\text{Total} = 23 + 3 + 4 = 30 \text{ ATP}$

$\boxed{\text{Total ATP: 30-32 per glucose (most commonly cited: 30-38)}}$

Note: Older textbooks cite 36-38 ATP using 3 ATP/NADH and 2 ATP/FADH₂. Modern estimates (accounting for proton leak and ATP/ADP transport) are lower: ~30-32 ATP.

Summary Table:

Efficiency:

• Glucose: 686 kcal/mol
• ATP: 7.3 kcal/mol
• Efficiency: (30 × 7.3) / 686 = ~32%
• Rest lost as heat (maintains body temperature)

Chemiosmotic Theory (Peter Mitchell, 1961):

(a) Electron Transport Chain - Creating the gradient:

Overview: ETC pumps H⁺ from matrix to intermembrane space, creating electrochemical gradient

Four protein complexes + 2 mobile carriers:

Complex I (NADH dehydrogenase):

• Accepts: 2e⁻ from NADH
• Passes to: Ubiquinone (CoQ)
• Pumps: 4 H⁺ out
• NADH → NAD⁺ + H⁺ + 2e⁻

Complex II (Succinate dehydrogenase):

• Accepts: 2e⁻ from FADH₂ (from Krebs cycle)
• Passes to: CoQ
• Pumps: 0 H⁺ (no pumping!)
• Lower entry point → less ATP

Ubiquinone (CoQ):

• Mobile carrier in membrane
• Carries electrons from Complex I/II to Complex III
• Also picks up H⁺ from matrix

Complex III (Cytochrome bc₁ complex):

• Accepts: 2e⁻ from CoQ
• Passes to: Cytochrome c
• Pumps: 4 H⁺ out
• Q-cycle mechanism

Cytochrome c:

• Mobile carrier (peripheral protein)
• Carries electrons from Complex III to Complex IV

Complex IV (Cytochrome oxidase):

• Accepts: 2e⁻ from cytochrome c
• Passes to: O₂ (final electron acceptor)
• Pumps: 2 H⁺ out
• Reaction: ½O₂ + 2H⁺ + 2e⁻ → H₂O

Total H⁺ pumped:

• Per NADH: 4 + 4 + 2 = 10 H⁺
• Per FADH₂: 0 + 4 + 2 = 6 H⁺

Electrochemical gradient created:

• Chemical gradient (ΔpH): ~0.5-1 pH units
  • Matrix pH ~8, intermembrane space pH ~7
• Electrical gradient (ΔΨ): ~180 mV (matrix negative)
• Proton-motive force (PMF):

$\Delta p = \Delta\Psi - \frac{2.3RT}{F}\Delta pH$

At 37°C: $\Delta p \approx 180 - 60(1) = 220 \text{ mV}$

(b) ATP Synthase - Using the gradient:

Structure:

F₀ portion (membrane-embedded):

• c-ring: 8-15 subunits forming rotor
• Proton channel through a-subunit
• Anchored in membrane

F₁ portion (matrix-facing):

• 3α and 3β subunits (catalytic)
• γ-subunit (central stalk/rotor)
• Extends into matrix

Mechanism (rotary catalysis):

Step 1: H⁺ enters channel in F₀

• H⁺ binds to c-ring subunit
• Neutralizes negative charge (Asp/Glu residue)

Step 2: Rotation

• Binding of H⁺ causes c-ring to rotate
• Each H⁺ binding causes ~30° rotation (for 12-subunit c-ring)

Step 3: H⁺ release

• c-ring rotation brings H⁺ to exit channel
• H⁺ released into matrix
• c-ring subunit returns to start

Step 4: Mechanical energy → chemical energy

• γ-subunit (connected to c-ring) rotates

• Rotation changes conformation of β-subunits

• 3 β-subunits cycle through 3 states:

  1. Open (O): ADP + Pi bind
  2. Loose (L): Binding stimulated
  3. Tight (T): ATP formed and released

Binding change mechanism:

• All three β-subunits in different states simultaneously
• 120° rotation changes: O → L → T → O
• ATP formed spontaneously when in T state
• Energy used to RELEASE ATP, not form it!

(c) H⁺ per ATP calculation:

Depends on c-ring size:

Most organisms: c-ring with 8-15 subunits

Complete rotation (360°):

• c-ring: 10 subunits (common in mitochondria)
• 1 full rotation = 10 H⁺ through F₀
• 1 full rotation = 3 ATP made (3 β-subunits × 1 ATP each)

$\frac{\text{H}^+}{\text{ATP}} = \frac{10}{3} \approx 3.3 \text{ H}^+\text{/ATP}$

For different c-ring sizes:

• 8 subunits: 8/3 = 2.7 H⁺/ATP
• 10 subunits: 10/3 = 3.3 H⁺/ATP
• 12 subunits: 12/3 = 4.0 H⁺/ATP
• 15 subunits: 15/3 = 5.0 H⁺/ATP

$\boxed{\text{Approximately 3-4 H}^+ \text{ per ATP (depends on organism)}}$

ATP yield from NADH:

NADH → 10 H⁺ pumped

At 3.3 H⁺/ATP: $\text{ATP} = \frac{10}{3.3} \approx 3 \text{ ATP/NADH}$

Modern estimates (accounting for ATP/ADP translocase): $\text{ATP} \approx 2.5 \text{ ATP/NADH}$

FADH₂: $\text{ATP} = \frac{6}{3.3} \approx 1.8 \approx 1.5 \text{ ATP/FADH}_2$

Energy coupling: $\Delta G_{pmf} = n\Delta p = (3.3)(220 \text{ mV}) = 726 \text{ mV}$

Enough to drive ATP synthesis: $\text{ADP} + \text{P}_i \rightarrow \text{ATP} + \text{H}_2\text{O} \quad \Delta G = +30.5 \text{ kJ/mol}$

Uncouplers:

• DNP (2,4-dinitrophenol): carries H⁺ across membrane
• Bypasses ATP synthase
• Energy dissipated as heat (thermogenesis)
• Brown fat uses UCP1 (uncoupling protein) naturally