⚡ Cellular Respiration & Fermentation

Respiration is an enzyme-governed catabolic, exergonic process in which chemical bond energy within organic substrates is stepwise released and trapped as universal metabolic currency: Adenosine Triphosphate (ATP).

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1. 🧪 Glycolysis (EMP Pathway: Embden, Meyerhof, Parnas)

Occurs in the cytoplasm of all living organisms (aerobic & anaerobic). Breaks down 1 glucose ($6C$) into 2 pyruvates ($3C$):

  Glucose (6C) ──[1 ATP]──► G6P ──► F6P ──[1 ATP]──► F-1,6-BP (6C)  <-- Preparatory Phase
                                                              │
                                       ┌──────────────────────┴──────────────────────┐
                                       ▼                                             ▼
                                   DHAP (3C) ◄──────[Triose Phosphate Isomerase]────► PGAL (3C)
                                                                                       │ [x2]
                               2 Pyruvate (3C) ◄──[2 ATP]── 2 PEP ◄──[2 NADH+2ATP]─────┘ <-- Payoff Phase

$$\text{Net Yield of Glycolysis:}\mathbf{1}\text{ Glucose}⟶\mathbf{2}\text{ Pyruvate}\mathbf{+}\mathbf{2}\text{ ATP (Net Substrate-Level)}\mathbf{+}\mathbf{2}\text{ NADH}\mathbf{+}\mathbf{2}{\mathbf{H}}^{\mathbf{+}}$$

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2. 🌉 The Link Reaction (Oxidative Decarboxylation of Pyruvate)

Occurs in the mitochondrial matrix catalyzed by the multi-enzyme Pyruvate Dehydrogenase Complex:

$$\mathbf{2}\text{ Pyruvate}\mathbf{+}\mathbf{2}{\text{ NAD}}^{\mathbf{+}}\mathbf{+}\mathbf{2}\text{ CoA-SH}\underset{\text{TPP, Lipoic Acid, }{\mathbf{Mg}}^{\mathbf{2}\mathbf{+}}}{\overset{\text{Pyruvate Dehydrogenase}}{\to }}\mathbf{2}\text{ Acetyl-CoA}\mathbf{+}\mathbf{2}{\text{ CO}}_{\mathbf{2}}\uparrow \mathbf{+}\mathbf{2}\text{ NADH}\mathbf{+}\mathbf{2}{\mathbf{H}}^{\mathbf{+}}$$

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3. 🔄 The Krebs Cycle (Tricarboxylic Acid / Citric Acid Cycle)

Discovered by Hans Krebs (1937) in the mitochondrial matrix:

                          THE CITRIC ACID CYCLE METABOLIC ROADMAP
                                  Oxaloacetate (4C) + Acetyl-CoA (2C)
                                                  │
                                                  ▼ [Citrate Synthase]
                                             Citrate (6C)
                                                  │
                                                  ▼ [Isocitrate Dehydrogenase]
                                     α-Ketoglutarate (5C) + CO2 + NADH
                                                  │
                                                  ▼ [α-KG Dehydrogenase Complex]
                                      Succinyl-CoA (4C) + CO2 + NADH
                                                  │
                                                  ▼ [Succinate Thiokinase: Substrate-Level GTP/ATP]
                                             Succinate (4C)
                                                  │
                                                  ▼ [Succinate Dehydrogenase: Inner Membrane Complex II]
                                              Fumarate (4C) + FADH2
                                                  │
                                                  ▼
                                              Malate (4C) ──► Oxaloacetate (4C) + NADH

$$\text{Per Turn of Citric Acid Cycle:}\mathbf{3}\text{ NADH}\mathbf{+}\mathbf{1}{\text{ FADH}}_{\mathbf{2}}\mathbf{+}\mathbf{1}\text{ GTP/ATP}\mathbf{+}\mathbf{2}{\text{ CO}}_{\mathbf{2}}$$$$\text{Per Glucose Molecule (2 Turns):}\mathbf{6}\text{ NADH}\mathbf{+}\mathbf{2}{\text{ FADH}}_{\mathbf{2}}\mathbf{+}\mathbf{2}\text{ GTP/ATP}\mathbf{+}\mathbf{4}{\text{ CO}}_{\mathbf{2}}$$

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4. ⚡ Electron Transport System (ETS) & Chemiosmosis

Located on the inner mitochondrial membrane (cristae):

  [MATRIX]                 Complex I (NADH DH) ──► UQ ◄── Complex II (Succinate DH)
                              │                     │
                              ▼                     ▼
  [INTERMEMBRANE SPACE]    4 H+ pumped           Complex III (Cyt bc1) ──► Cytochrome c
                                                    │
                                                    ▼
                                                 Complex IV (Cyt c Oxidase: a, a3, 2 Cu centers)
                                                    │
                                                    ▼ [Final Electron Acceptor: 1/2 O2 + 2H+ -> H2O]
                                                 Complex V (ATP Synthase: F0 proton channel + F1 catalytic head)

$$\text{Total Net Aerobic ATP Yield per Glucose}\mathbf{=}\mathbf{36}\mathbf{-}\mathbf{38}\text{ ATP}$$

• $1\text{ NADH}⟶\mathbf{3}\text{ ATP}$ (or $2.5\text{ ATP}$ modern consensus).
• $1{\text{ FADH}}_2⟶\mathbf{2}\text{ ATP}$ (or $1.5\text{ ATP}$ modern consensus).

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5. 💨 Respiratory Quotient ($RQ$)

$$RQ=\frac{\text{Volume of }C{O}_2\text{ evolved}}{\text{Volume of }{O}_2\text{ consumed}}$$

Substrate	Balanced Catabolic Equation	Theoretical $RQ$ Value
Carbohydrates (Glucose)	$C_6{H}_{12}{O}_6+6O_2⟶6C{O}_2+6H_{2}O$	$1.0$
Fats / Lipids (Tripalmitin)	$2(C_{51}{H}_{98}{O}_6)+145O_2⟶102C{O}_2+98H_{2}O$	$0.7$ ($\frac{102}{145}$)
Proteins	Catabolism via deamination and oxidation	$\text{a}\approx 0.9$
Organic Acids (Malic Acid)	$C_4{H}_6{O}_5+3O_2⟶4C{O}_2+3H_{2}O$	$1.33$ ($>1.0$)
Anaerobic Respiration	Glucose fermentation (No $O_2$ consumed)	$\mathrm{\infty }$ (Infinity)