where does the energy come from that is used to attach a 3rd phosphate to adp

The energy used to attach a third phosphate to ADP, forming ATP, primarily comes from cellular metabolic processes like oxidative phosphorylation in mitochondria or photophosphorylation in chloroplasts.

Main Energy Sources

ATP synthesis requires overcoming the energetically unfavorable phosphoanhydride bond formation, which demands free energy input exceeding the ~30.5 kJ/mol needed under standard conditions.

• Oxidative phosphorylation : Electrons from food-derived NADH/FADH₂ flow through the electron transport chain (Complexes I-IV), pumping protons to create an electrochemical gradient. ATP synthase harnesses proton flow back into the matrix to drive ADP + Pi → ATP.

• Substrate-level phosphorylation : Direct energy transfer in glycolysis or Krebs cycle, e.g., phosphoenolpyruvate donating phosphate to ADP via pyruvate kinase.

• Photosynthesis equivalent : Light-driven electron transport in thylakoids builds proton gradient for ATP synthase.

Process Mechanism

ATP synthase rotates like a molecular turbine, powered by proton motive force (~220 mV gradient), catalyzing ~3 ATP per full rotation (c-ring typically has 10-14 subunits).

The reaction ADP + Pi + energy input → ATP + H₂O is endergonic alone (ΔG°' +30.5 kJ/mol) but coupled to exergonic redox reactions yielding ~50-60 kJ/mol total ΔG.

Key Contexts

In low-oxygen muscle cells, glycolysis provides limited ATP via substrate- level means, but mitochondria dominate aerobic ATP yield (~30-32 ATP/glucose).

Recent discussions highlight ATP synthase's evolutionary conservation across life forms.

TL;DR : Proton gradient from electron transport chains supplies the energy via ATP synthase.

Information gathered from public forums or data available on the internet and portrayed here.