⚡ 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 ($3C$):

🧪 Glycolysis (EMP Pathway): 10-Step Cytoplasmic Splitting of Glucose

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

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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 (Krebs TCA Cycle) Pathway Map

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

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4. ⚡ Electron Transport System (ETS) & Chemiosmosis

Located on the inner mitochondrial membrane (cristae):

⚡ Mitochondrial Electron Transport Chain & Complex V ATP Synthase

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

• $1\text{ NADH}⟶\mathbf{3}\text{ ATP}$ (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)