Bioenergetics

ATP — the universal energy currency

Adenosine triphosphate carries two high-energy phosphoanhydride bonds; hydrolysis of the terminal one releases ΔG ≈ −30.5 kJ/mol under standard conditions, more under cellular conditions. Punjab Textbook Chapter 14 and Campbell Chapter 9 emphasise that ATP is constantly recycled — a resting human regenerates roughly its own body mass in ATP per day. The cell makes ATP three ways: substrate-level phosphorylation (in glycolysis and Krebs), oxidative phosphorylation (mitochondrial ETC), and photophosphorylation (chloroplast thylakoid).

Glycolysis — cytoplasmic, anaerobic, ten steps

Glucose (6C) is invested with 2 ATP, split into two 3-carbon trioses, and oxidised to two pyruvates with net production of 2 ATP and 2 NADH. Key regulator: phosphofructokinase (step 3), inhibited by ATP and citrate. Under anaerobic conditions, pyruvate is reduced to lactate (vertebrate muscle, via lactate dehydrogenase) or to ethanol + CO₂ (yeast, via pyruvate decarboxylase and alcohol dehydrogenase) — both regenerating NAD⁺ so glycolysis can continue.

Link reaction and Krebs cycle

Pyruvate is transported into the mitochondrial matrix and oxidatively decarboxylated by the pyruvate dehydrogenase complex to acetyl-CoA, releasing 1 CO₂ and 1 NADH per pyruvate. The Krebs cycle (Hans Krebs, Nobel 1953) per turn produces 3 NADH, 1 FADH₂, 1 GTP/ATP (substrate-level), and 2 CO₂. Per glucose (two acetyl-CoAs), the link reaction plus Krebs gives 8 NADH, 2 FADH₂, 2 ATP, and 6 CO₂.

Oxidative phosphorylation

The electron transport chain in the inner mitochondrial membrane consists of complexes I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome b-c₁), IV (cytochrome c oxidase), with mobile carriers ubiquinone and cytochrome c. Electrons from NADH enter at I and from FADH₂ at II; oxygen is the terminal electron acceptor at IV, reduced to H₂O. Proton pumping at I, III, and IV creates a gradient (~ pH 1.4 difference, ~ 200 mV) across the inner membrane that drives ATP synthase (Mitchell's chemiosmotic theory, Nobel 1978). Yields per glucose: ~ 2.5 ATP/NADH and ~ 1.5 ATP/FADH₂, totalling ~ 30-32 ATP, depending on the shuttle used to bring cytoplasmic NADH into the mitochondrion.

Photosynthesis as the mirror reaction

Light reactions on thylakoid membranes use PSII (P680) to split water and PSI (P700) to reduce NADP⁺ to NADPH; the Z-scheme creates a proton gradient that drives ATP synthase, just as in mitochondria. The Calvin cycle in the stroma fixes 3 CO₂ via Rubisco onto 3 RuBP (5C) to form 6 PGA (3C), reduced to 6 G3P; one G3P leaves as product, five are recycled to regenerate 3 RuBP. Net stoichiometry per G3P produced: 9 ATP and 6 NADPH consumed, 3 CO₂ fixed. Common traps: oxygen comes from H₂O, not CO₂; the Calvin cycle is light-independent in mechanism but light-dependent in supply because it needs ATP and NADPH; C₄ plants concentrate CO₂ to suppress Rubisco's wasteful oxygenase activity (photorespiration).

Limiting factors and respiratory quotient

Blackman's law of limiting factors states that photosynthesis rate is set by whichever of light intensity, CO₂ concentration, or temperature is in shortest supply. Greenhouse growers exploit this by enriching air to ~ 1000 ppm CO₂. The respiratory quotient RQ = CO₂ produced ÷ O₂ consumed is 1.0 for carbohydrate (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O), ~ 0.7 for fat (more reduced, needs more O₂), and ~ 0.8 for protein. A starving person's RQ falls below 1.0 as they switch from carbohydrate to fat catabolism — a recurring MDCAT calculation. Anaerobic respiration in muscle produces lactate but no CO₂, giving an effectively undefined RQ; in yeast fermentation only CO₂ is released without O₂ consumption, so the RQ is infinite. These ratios let physiologists infer substrate use without having to biopsy tissue.