Differentiate between: (a) Respiration and Combustion (b) Glycolysis and Kreb’s cycle (c) Aerobic respiration and fermentation

(a) Respiration and Combustion

• Respiration is a controlled, enzyme-catalyzed process in living cells that oxidizes organic substrates in small steps to produce ATP, with O₂ as the usual final electron acceptor and CO₂ and H₂O as end-products
• Combustion is an uncontrolled, non-biological oxidation of fuel outside cells, occurring in a single rapid step that releases energy as heat and light, with CO₂ and H₂O as main products
• Respiration conserves chemical energy in ATP; combustion dissipates energy primarily as heat

(b) Glycolysis and Kreb’s cycle

• Glycolysis is a linear ten-step pathway in the cytoplasm converting one glucose into two pyruvate molecules, yielding a net 2 ATP and 2 NADH by substrate-level phosphorylation, and functioning with or without oxygen
• Kreb’s cycle (citric acid cycle) is a cyclic eight-step pathway in the mitochondrial matrix that oxidizes acetyl-CoA to 2 CO₂ per turn, generating 3 NADH, 1 FADH₂ and 1 GTP (ATP) and feeding electrons into oxidative phosphorylation; it requires O₂ indirectly
• Glycolysis provides intermediates for fermentation and biosynthesis (e.g., 3-phosphoglycerate for amino acids); Kreb’s cycle supplies precursors for fatty acid, amino acid and nucleotide synthesis (e.g., citrate, α-ketoglutarate)

(c) Aerobic respiration and Fermentation

• Aerobic respiration uses O₂ as the final electron acceptor in the electron transport chain across the inner mitochondrial membrane, yielding CO₂, H₂O and approximately 30–34 ATP per glucose
• Fermentation operates in the absence of O₂, relying solely on glycolysis in the cytoplasm to produce 2 ATP per glucose and regenerating NAD⁺ by converting pyruvate into lactate or ethanol (plus CO₂), without further ATP synthesis
• Aerobic respiration sustains high-energy demands in multicellular organisms; fermentation enables ATP production under anaerobic or low-oxygen conditions in microbes and muscle cells