RQ = CO₂ evolved / O₂ consumed
From the equation: CO₂ = 102, O₂ = 145
RQ = 102/145 = 0.703
0.703 falls between 0.5 and 0.95 → this is a fat (lipid substrate)
C₅₁H₉₈O₆ = tripalmitin (a triglyceride/fat molecule)
Fats have low oxygen-to-carbon ratio → need more O₂ → RQ < 1
Respiratory Quotient (RQ) = ratio of volume of CO₂ released to volume of O₂ consumed during respiration. RQ = CO₂ / O₂. It is a dimensionless ratio measured using a Ganong's respirometer. RQ value tells us which substrate is being respired. Key values: Carbohydrates: RQ = 1.0 (CO₂ produced = O₂ consumed). Fats (lipids): RQ ≈ 0.7 (more O₂ needed, less CO₂ produced). Proteins: RQ ≈ 0.8-0.9 (intermediate). Organic acids (malic acid in succulents): RQ > 1. Anaerobic fermentation: RQ = ∞ (CO₂ released but no O₂ consumed).
Fats (triglycerides) have the general composition rich in C and H, with very little O compared to carbohydrates. Carbohydrate empirical formula: CH₂O (one O for every C). Fats: approximately CH₂ (very little O, mostly C-H bonds). To oxidise the many C-H bonds in fat: much more O₂ is required. The O₂ consumed >> CO₂ produced → RQ < 1. Tripalmitin: 2(C₅₁H₉₈O₆) + 145 O₂ → 102 CO₂ + 98 H₂O. RQ = 102/145 = 0.703. Typical fat RQ range: 0.6-0.8. Fat respiration releases more energy per gram than carbohydrate (9 kcal/g vs 4 kcal/g) because of greater H content.
Glycolysis occurs in cytoplasm, under both aerobic and anaerobic conditions. Steps: Glucose (C₆H₁₂O₆, 6C) → 2 Pyruvate (C₃H₄O₃, 3C). Yield: 2 ATP (net, after spending 2 ATP in initial steps). 2 NADH. 2 Pyruvate. Enzymes: hexokinase (phosphorylates glucose), phosphofructokinase (PFK, rate-limiting, key regulatory enzyme), pyruvate kinase. Regulation: PFK inhibited by ATP (high energy) and citrate. Activated by AMP and ADP. Aerobic: pyruvate → acetyl-CoA (pyruvate decarboxylation, mitochondrial matrix). Anaerobic: pyruvate → lactate (animals, by lactate dehydrogenase) or ethanol + CO₂ (yeast, by pyruvate decarboxylase + alcohol dehydrogenase).
Krebs cycle (citric acid cycle / TCA cycle) occurs in mitochondrial matrix. Per acetyl-CoA (2C entering cycle): Products: 3 NADH, 1 FADH₂, 1 GTP (= 1 ATP), 2 CO₂. Per glucose (2 pyruvates → 2 acetyl-CoA): 6 NADH, 2 FADH₂, 2 GTP, 4 CO₂. Key steps: Oxaloacetate (4C) + Acetyl-CoA (2C) → Citrate (6C) [by citrate synthase]. Citrate → Isocitrate → α-Ketoglutarate (5C) + CO₂ [isocitrate dehydrogenase]. α-Ketoglutarate → Succinyl-CoA (4C) + CO₂ [α-ketoglutarate dehydrogenase]. Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate. Oxaloacetate regenerated → cycle continues. Location of CO₂ release: during Krebs cycle (not glycolysis in aerobic respiration).
ETS on inner mitochondrial membrane. Complexes: Complex I (NADH dehydrogenase): NADH → NAD⁺, pumps 4 H⁺ into intermembrane space. Complex II (succinate dehydrogenase): FADH₂ → FAD, does NOT pump H⁺. Complex III (cytochrome bc1 / ubiquinol-cytochrome c oxidoreductase): pumps 4 H⁺. Complex IV (cytochrome oxidase): reduces O₂ to H₂O, pumps 2 H⁺. ATP Synthase (Complex V / F₀F₁-ATPase): H⁺ flows from intermembrane space → matrix through ATP synthase → ADP + Pi → ATP. ATP yield: NADH → 2.5 ATP. FADH₂ → 1.5 ATP. Per glucose (modern estimates): Glycolysis: 2 ATP + 2 NADH. Pyruvate decarboxylation: 2 NADH. Krebs: 2 GTP + 6 NADH + 2 FADH₂. ETS total: ~30-32 ATP. Total ≈ 30-38 ATP per glucose.
Fermentation = anaerobic breakdown of organic molecules for energy. Two main types in living organisms: Lactic acid fermentation: glucose → 2 pyruvate → 2 lactate (by lactate dehydrogenase). NAD⁺ regenerated. No CO₂ produced. Net: 2 ATP. Occurs in: animal muscles during intense exercise, RBCs (no mitochondria), Lactobacillus bacteria. Alcoholic fermentation: glucose → 2 pyruvate → 2 acetaldehyde (pyruvate decarboxylase, releases CO₂) → 2 ethanol (alcohol dehydrogenase, regenerates NAD⁺). Net: 2 ATP. Occurs in: yeast (Saccharomyces cerevisiae), some plant seeds in waterlogged conditions. RQ of fermentation: alcoholic = infinity (CO₂ released, no O₂). Note: yeast can do both aerobic and anaerobic — switches based on O₂ availability (Pasteur effect).
Carbohydrates: enter as glucose (directly) or after glycogenolysis (glycogen → glucose). RQ = 1.0. Most preferred substrate. 4 kcal/g. Fats: fatty acids activated to acyl-CoA → β-oxidation (in mitochondrial matrix) → acetyl-CoA → Krebs cycle. One palmitic acid (16C): β-oxidation produces 8 acetyl-CoA + 7 NADH + 7 FADH₂ → then Krebs. Total: ~129 ATP per palmitate. 9 kcal/g. RQ ≈ 0.7. Proteins: amino acids → deamination (remove NH₂ group → urea) → carbon skeleton enters at various points of Krebs cycle or glycolysis. Glutamate → α-ketoglutarate. Aspartate → oxaloacetate. Alanine → pyruvate. RQ ≈ 0.8-0.9 depending on amino acid composition. 4 kcal/g. Used as fuel only when carbohydrates and fats depleted (starvation).
Pentose phosphate pathway (PPP) / hexose monophosphate shunt: alternative to glycolysis. Occurs in cytoplasm. Two phases: Oxidative phase: glucose-6-phosphate → ribulose-5-phosphate + 2 NADPH + CO₂. Non-oxidative phase: interconversion of sugar phosphates. Products: NADPH (reducing power for: fatty acid synthesis, glutathione reduction, nitric oxide synthesis, cytochrome P450 reactions). Ribose-5-phosphate (for nucleotide and nucleic acid synthesis). CO₂ from oxidative decarboxylation. Importance: provides NADPH for: biosynthesis of fatty acids, cholesterol, steroids. Keeping glutathione in reduced form → antioxidant defence. Ribose-5-phosphate for nucleotide synthesis. Active in: liver (fat synthesis), red blood cells (glutathione reduction for antioxidant defence), adrenal cortex (steroid synthesis), lactating mammary gland.