01ATP: the cell's currency
ATP (adenosine triphosphate) stores energy in the bonds between its phosphate groups. Removing the last phosphate (hydrolysis) releases energy: ATP → ADP + Pᵢ, ΔG ≈ −7.3 kcal/mol.
Cells couple this exergonic reaction to endergonic ones, often by transferring the phosphate to another molecule (phosphorylation), making it more reactive.
02Energy laws in cells
First law: energy is not created or destroyed, only converted (light → chemical → kinetic).
Second law: every conversion loses some energy as heat, increasing entropy. Organisms need a constant energy input to stay ordered. That's why energy flows through ecosystems instead of cycling.
Electron carriers — NAD⁺/NADH, FAD/FADH₂, NADP⁺/NADPH — carry high-energy electrons between reactions.
03Notebook box
Oxidation = losing electrons (OIL); reduction = gaining electrons (RIG). NAD⁺ + 2e⁻ + H⁺ → NADH (reduced, energy-rich).
Worked example: Why must cells constantly regenerate ATP? → Each molecule is used and rebuilt thousands of times a day; storage is minimal.