Cellular Respiration, Part 2 — Simple Notes (BIO 1107)

Big picture

  • Goal: pull energy out of food and package it as ATP.

  • Where we are: after glycolysis (→ 2 pyruvate, 2 ATP, 2 NADH).

  • Next: Pyruvate oxidation → Citric Acid Cycle → Oxidative phosphorylation.

  • If O₂ is scarce: Fermentation keeps glycolysis running.

Citric Acid Cycle (a.k.a. Krebs / TCA)

Where: mitochondrial matrix
Why: finish oxidizing the carbon fuel; load up electron carriers (NADH, FADH₂)

Per turn (1 acetyl-CoA):

  • In: acetyl-CoA (2C), 3 NAD⁺, 1 FAD, 1 ADP + Pi, 2 H₂O

  • Out: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP (as GTP), CoA recycled

Per glucose (2 turns):

  • 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂

Ideas to hold:

  • It’s a cycle: starts/ends with oxaloacetate.

  • “Complete oxidation” of the acetyl group → the carbon leaves as CO₂.

  • ATP here is made by substrate-level phosphorylation (enzyme moves a phosphate to ADP).

Oxidative Phosphorylation (ETC + ATP synthase)

Where: inner mitochondrial membrane
Inputs: NADH, FADH₂, O₂, ADP + Pi
Outputs: ~28 ATP, H₂O, NAD⁺, FAD

1) Electron Transport Chain (ETC)

  • Complex I takes e⁻ from NADH; Complex II from FADH₂.

  • e⁻ flow: I/II → Q (CoQ) → III → cytochrome c → IV → O₂ (final e⁻ acceptor) → H₂O.

  • As e⁻ drop in free energy, Complexes I, III, IV pump H⁺ into the intermembrane space.

2) Proton Motive Force (PMF)

  • Pumped H⁺ build an electrochemical gradient (high H⁺ outside, low inside).

  • This gradient = stored potential energy.

3) ATP Synthase

  • H⁺ flow back through ATP synthase (F₀ channel spins; F₁ makes ATP) → chemiosmosis.

  • Yield (typical): ~2.5 ATP per NADH, ~1.5 ATP per FADH₂.

  • Total ATP per glucose ≈ 32 (textbook tally: 4 by SLP + ~28 by OP).

Quick Tally per Glucose (aerobic)

  • Glycolysis: 2 ATP (SLP) + 2 NADH

  • Pyruvate Oxidation: 2 NADH

  • Citric Acid Cycle: 2 ATP (SLP) + 6 NADH + 2 FADH₂

  • OxPhos from carriers: ~ (10 NADH × 2.5) + (2 FADH₂ × 1.5) ≈ 25 + 3 = 28 ATP

  • Grand total: ~32 ATP

(Real yields vary with shuttle systems/leaks, but 30–32 is the classic range.)

Fermentation (Anaerobic “Plan B”)

When: no/low O₂ → ETC stops → NADH can’t dump e⁻ → need NAD⁺ to keep glycolysis going.
What it does: oxidizes NADH → NAD⁺ by reducing an organic molecule (no ETC, no O₂).

Lactic Acid Fermentation (animals, some bacteria)

  • Pyruvate + NADH → Lactate + NAD⁺

  • Net from glucose (glycolysis + fermentation): 2 ATP total

  • Happens in muscle at high exertion.

Ethanol (Alcohol) Fermentation (yeast, plants)

  • Pyruvate → acetaldehyde + CO₂; then
    Acetaldehyde + NADH → Ethanol + NAD⁺

  • Net from glucose: 2 ATP total

  • Used in bread/wine/beer.

Key idea: Fermentation = NAD⁺ recycling, not big ATP production.

Electron & Energy Flow (super short)

  • Glucose is oxidized → e⁻ to NADH/FADH₂ → e⁻ to ETC → O₂ reduced to H₂O.

  • Energy “steps down”: C–H bonds → NADH/FADH₂ → PMF → ATP.

Must-know vocab (flashcard-ready)

  • Oxidation: loss of e⁻ (OIL)

  • Reduction: gain of e⁻ (RIG)

  • Substrate-level phosphorylation: enzyme moves a phosphate from a substrate → ADP

  • Oxidative phosphorylation: ETC + chemiosmosis make ATP using O₂

  • Proton motive force (PMF): H⁺ gradient across inner mito membrane

  • Final e⁻ acceptor (aerobic): O₂ → H₂O

ATP - protien molecule

whats its doing - makign atp