Bioenergetics and ATP yield
Cells capture energy from food in ATP, the energy currency. Complete oxidation of one glucose gives 32 ATP (newer) or 38 ATP (older books).
Free energy and ATP in simple words
Free energy (G) is the part of a molecule's energy that can do useful work. ΔG is its change during a reaction; ΔG°′ is the standard value at pH 7 used in tables. Negative ΔG: energy is released and the reaction can go on its own (exergonic, e.g. oxidation of glucose). Positive ΔG: energy must be put in (endergonic, e.g. making proteins). ΔG = 0: the reaction is at equilibrium. ATP is the energy currency: like money, it is earned in exergonic reactions (breaking down food) and spent in endergonic ones (building molecules, muscle contraction, active transport). Its two outer phosphate bonds are high-energy bonds.
High-energy compounds (energy released on hydrolysis)
| Compound | ΔG°′ (kcal/mol) | Role |
|---|---|---|
| Phosphoenolpyruvate | −14.8 | Gives ATP in glycolysis (pyruvate kinase) |
| 1,3-Bisphosphoglycerate | −11.8 | Gives ATP in glycolysis (phosphoglycerate kinase) |
| Creatine phosphate | −10.3 | Quick ATP store in muscle and brain |
| ATP → ADP + Pi | −7.3 | Energy currency of the cell |
| Glucose-6-phosphate (low-energy, for contrast) | −3.3 | Cannot make ATP from ADP |
ATP from one glucose: the breakdown table
| Stage | Product | Older (NADH = 3, FADH2 = 2) | Newer (NADH = 2.5, FADH2 = 1.5) |
|---|---|---|---|
| Glycolysis | 2 ATP (net) | 2 | 2 |
| Glycolysis | 2 NADH (in cytoplasm) | 6 (or 4 by glycerol-P shuttle) | 5 (or 3 by glycerol-P shuttle) |
| Pyruvate → acetyl-CoA (×2) | 2 NADH | 6 | 5 |
| TCA cycle (×2) | 6 NADH | 18 | 15 |
| TCA cycle (×2) | 2 FADH2 | 4 | 3 |
| TCA cycle (×2) | 2 GTP | 2 | 2 |
| Total per glucose | — | 38 (or 36) | 32 (or 30) |
Why two totals?
P/O ratio = ATP made per oxygen atom used in the electron transport chain. In the exam, write the value your teacher uses and mention the other.
Turning food into ATP needs niacin (NAD+), riboflavin (FAD), thiamine and pantothenic acid (link reaction and TCA cycle), plus iron and copper in the electron transport chain. ATP works mostly bound to magnesium (Mg-ATP). Iron-deficiency anaemia, very common among Indian women, lowers oxygen delivery and ATP production and shows up as tiredness. Carbohydrate gives about 4 kcal per gram.
2 marks: “What are high-energy compounds? Give two examples.” Compounds that release a large amount of free energy on hydrolysis, e.g. ATP, creatine phosphate, phosphoenolpyruvate.
5 marks: “Calculate the ATP yield from complete oxidation of one glucose molecule.” Tabulate glycolysis, link reaction and TCA yields; total 38 (36) by older values or 32 (30) by newer P/O ratios.
10 marks: “Write an essay on bioenergetics with reference to glucose oxidation.” Define free energy and ΔG, list high-energy compounds, explain ATP as energy currency, then give the full ATP table with both totals.
Old: 3 and 2 → 38. New: 2.5 and 1.5 → 32. (ATP per NADH and per FADH2 → ATP per glucose.)