Charge a tiny membrane, disconnect its supply, and choose where the stored driving force goes. Discover how cells couple fuel chemistry to ATP—and why a bypass changes the result.
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Materials can cycle while free energy needs a continuing input. The route back across a membrane determines whether its stored driving force can help make ATP.
If the electron supply stops, must an already charged ATP maker stop immediately?
A finite stored driving force can remain, but further synthesis still needs ingredients and enough energy for the chosen reaction.
Digestion breaks food into materials the body can absorb. Cells use chemical reactions involving those materials to help make ATP, a molecule used in many coupled processes. Moving muscles is one application; cells also use ATP for transport and building. This lesson does not calculate how far a snack lets anyone run.
Electrons are transferred from fuel-related reactions to carriers such as NADH. A chain of membrane proteins passes them onward and helps transfer protons across the inner mitochondrial membrane. This establishes a proton-motive force: an electrical and chemical difference that can drive proton return.
Proton return through ATP synthase can drive its rotor and the changes at its catalytic sites that make ATP from ADP and inorganic phosphate, Pi. The ingredients and sufficient driving force are both essential. This coupling is a major part of oxidative phosphorylation.
The already charged teaching membrane can support some further synthesis after new electron supply stops. It cannot do so forever. Try the bypass from the same starting state: it dissipates the stored driving force without making ATP.
At the terminal electron-transfer step, oxygen accepts electrons and water is formed. Carbon dioxide is released in upstream fuel-processing reactions. Oxygen does not deliver carbon, and ATP synthase is not where oxygen directly burns glucose.
When ATP is used in coupled cellular processes, ADP and phosphate can become available again. Synthesis can reuse them with a new free-energy input. Our ATP-use event is a bookkeeping step, not one muscle contraction, running stride or second of life.
Glycolysis in the cytosol converts one glucose to two pyruvates and gives a net substrate-level ATP contribution; it does not directly consume oxygen. Pyruvate conversion releases CO2 and makes acetyl-CoA. The citric acid cycle regenerates its oxaloacetate pool while transferring reducing equivalents. Not all ATP is made by mitochondrial ATP synthase. Reactome’s human pathway entries are curated synthesis, with some molecular detail inferred from other species.
The net oxidation account C6H12O6 + 6 O2 → 6 CO2 + 6 H2O balances matter overall. It does not identify the path of each oxygen atom or show every intermediate. Two CO2 are released in conversion of two pyruvates; complete oxidation through the cycle gives four more net CO2. Newly arriving acetyl carbons need not leave in the first turn. ATP turnover is a separate coupled account, not an extra carbon product in this equation.
NADH transfers reducing equivalents and returns to NAD+; it does not transform into ATP. In the scoped mammalian chain, complexes I, III and IV contribute to outward proton transfer. Complex II feeds electrons into the quinone pool without pumping protons. Its FAD cofactor is enzyme-bound, not a freely delivered FADH2 parcel.
At canonical complex IV, four electrons reduce one O2 to two waters. Four chemical protons are consumed in that reaction and four additional protons are pumped. Our lumped two-NADH supply event consumes one O2, forms two redox waters and transfers 20 outward H+ equivalents across the idealized I/III/IV chain. Buffered chemical protons are separate from that pumped account.
Watt et al. resolved an eight-member c ring in bovine F1–c8, PDB 2XND. Its ideal complete turn translocates eight protons and supports three ATP catalytic events: intrinsic ratio 8/3. The full-turn packet is not three simultaneous reactions or a one-to-one match between 45° rotor increments and 120° catalytic steps. Ring sizes vary between organisms; transport and leaks add costs beyond the intrinsic ratio.
PDB 6ZPO is a bovine ATP synthase model fitted to cryo-EM data, with an IF1 inhibitor in the experimental preparation. Its catalytic head faces the matrix. Source inspection retains the inhibitor; an uninhibited working animation is a schematic adaptation. The paper’s Results places proton entry on the intermembrane-space side; a conflicting “matrix” statement in its abstract must not reverse the teaching direction.
The workbench isolates voltage by externally buffering both sides at equal pH. Capacitance C=10⁻¹⁴ F, supply budget B=440000/NA joules per two-NADH event and ATP demand G=50000/NA joules per ATP are authored conditions, not measurements from the pictured specimen. Finite inventories gate events while chemical activities and G are held fixed by assumption.
Let n count net outward monovalent charge equivalents, e=1.602176634×10⁻¹⁹ C and NA=6.02214076×10²³ mol⁻¹. Q=ne, V=Q/C and U=(ne)²/(2C). V is positive for the intermembrane-space side relative to matrix. The number n counts net stored charge, not every dissolved proton or pH. Exact physical constants do not turn the chosen capacitance into measured biology.
A supply event needs two NADH, one O2 and enough budget for U(n+20)−U(n). An ATP full-turn event needs at least eight stored charge equivalents, three ADP and three Pi, with U(n)−U(n−8)≥3G. Failed guards leave state unchanged. The full-turn abstraction can reject a packet even when a real enzyme could undergo smaller substeps.
The bypass returns up to 20 stored charge equivalents per event, sending the lost U to dissipation without changing ATP. ATP use returns one ADP and one Pi and transfers G to cellular use/dissipation. The balance includes stored electrical free energy, ATP synthesis free energy, dissipation and cellular use. A nonzero U is not necessarily enough for another forward synthesis event.
If an external apparatus holds the proton-motive force fixed, one ideal c8 turn can receive 8FΔp per mole of turns, with F=eNA≈96485.332 C/mol. At 0.15 V this is about 115.78 kJ/mol of turns; at 0.20 V it is about 154.38. Our chosen three-ATP demand is 150 kJ/mol of turns, so only the latter passes. This externally clamped comparison is not the discharging finite workbench.
Define favorable inward Δp=(ψIMS−ψmatrix)+(2.303RT/F)(pHmatrix−pHIMS). At 310 K, a 0.5 pH-unit difference contributes about 30.76 mV and represents a proton concentration ratio of about 3.16. The voltage-only workbench omits this term rather than adding it twice. Real enzymes may reverse under suitable conditions; reverse ATP hydrolysis and IF1 regulation are not simulated.
Jones et al. studied inverted bovine-heart membrane vesicles. Figure 2 reports fitted NADH-oxidation/ATP-synthesis slopes of 0.484/0.337 in control, 0.921/−0.003 with gradient dissipation, and −0.011/−0.015 with complex-I inhibition, in μmol min⁻¹ mg⁻¹. Preserve the small negative near-background fits; they are not negative molecular inventories. These are separate measured outputs, not fitted traces of our workbench or human exercise data.
EMD-43064 is a tomogram from wild-type mouse embryonic fibroblasts in Fry et al.’s study. Moving between slices changes spatial depth, not time. The grayscale is reconstructed electron-scattering density, not voltage or visible moving protons. A fitted bovine protein structure, a mouse-cell tomogram and a hypothetical assay are different evidence layers.
A snack’s energy cannot be converted into a promised running distance by counting ATP cards. Digestion, delivery, metabolic choices and many losses intervene. Our useful question is which connections allow coupling.
Ask what preparation was measured and which output changed. Faster NADH oxidation and lower ATP synthesis can occur together when coupling is disrupted. The treatment is evidence to interpret, not an at-home instruction.
Track materials and energy separately. A missing ingredient should not erase charge; a bypass should not invent ATP; resetting must declare the new starting condition. These checks transfer to many scientific models.
Draw a membrane line with an ATP-synthase route and a bypass. Label the intermembrane-space side and matrix side. Make eight large H+ markers, three ADP cards, three Pi cards and three ATP cards. Add a condition card: sufficient driving force for this one illustrated full cycle.
Predict how many ATP cards appear. With the sufficient-force card present, move all eight H+ markers through ATP synthase and exchange three ADP/Pi pairs for three ATP cards. Write where the materials came from and what supplied the driving force. This is one net full-cycle account, not simultaneous molecular substeps.
Reset exactly to step1, including the externally supplied condition card. Move the same eight markers through the bypass. Leave ADP and Pi unchanged; create no ATP. Record one similarity and one difference between the routes. Returning markers dissipates driving force here without ATP coupling.
Reset again, then set aside one ADP card. Circle what is missing. Our complete-cycle rule needs three ADP and three Pi, so the full event cannot finish. Leave all other cards unchanged. This coarse rule does not say real ATP synthase lacks partial substeps.
Take one ATP card saved from step2. Exchange it for one ADP card and one Pi card to represent use. Keep those materials for later. Write “new free-energy input needed” beside them: returning the cards does not restore spent driving force or correspond to a running stride.
Gather three ADP and three Pi, but replace the condition card with “driving force too small.” Predict the result: no forward synthesis under this condition. Explain why ingredients alone are insufficient. Record a prediction, result and explanation for each case.
What changes when the same paper markers return by a different route?
Original indoor reasoning activity, not a laboratory protocol or physical measurement. Use paper only; no biological material, food, chemicals, appliances or exercise. The paper model assumes one full-cycle energy condition; it does not compute changing voltage, pH, force or timing. Resetting is an external intervention, not free biological recharging.
It accepts electrons and helps form water. Oxygen is the terminal electron acceptor. Water formation there is distinct from carbon dioxide released in upstream fuel chemistry.
Not necessarily; stored driving force can support some further synthesis. Disconnecting the supply preserves the current store. Further synthesis still needs ingredients, a connected route and enough driving force.
The bypass dissipates driving force without coupling it to ATP formation. A return route and a coupled chemical reaction are different. This bypass discharges the store but has no ATP-forming machinery.
Stop while stored charge may remain. The event cannot proceed without its reactants. A missing ingredient and a depleted store are different states.
They can be reused for ATP synthesis with new free-energy input. The molecular pool turns over. Reusing materials is not the same as recycling all the free energy without a new input.
Ten ideal turns transfer 80 H+ equivalents and make 30 ATP. The intrinsic 8:3 account applies to this rotor idealization. It does not include transport costs, leaks, other pathways or a whole-cell glucose yield.
Electron-transfer activity and ATP coupling can be separated. Jones and colleagues studied prepared membrane vesicles. Their separate oxidation and synthesis measurements test coupling, not running performance.
The slice supports architecture; the motion represents a separately defined mechanism. The tomogram comes from a frozen mouse-cell specimen. Color and motion are explanatory overlays, not recorded molecular trajectories or electrical measurements.
Original structural study. Intrinsic ideal 8/3 ratio excludes transport and leaks; 2XND is a subcomplex.
Watt et al. (2010) · bovine c8 rotorX-ray 3.5 Å; Watt et al. ANP is an ATP analog. PDB data CC0; source coordinates, not motion.
PDB 2XND · original coordinatesResults locates entry on the intermembrane-space side; abstract contains a conflicting matrix wording. Inhibited experimental structure is distinct from working adaptation.
Spikes, Montgomery & Walker (2020)Cryo-EM-derived 4.0 Å model from IF1-containing preparations. Cite Spikes et al.; PDB data CC0. Preserve inhibitor in source mode.
PDB 6ZPO · original coordinatesBovine and bacterial inverted vesicle assays support four pumped H+ per two electrons at complex I in these systems. Figure2 fitted slopes are not our event model. CC BY4.0.
Jones et al. (2017) · original experimentPrinted page4989. Preserve fitted negative near-background slopes, units and experimental preparation. Any crop requires attribution and adaptation notice.
Jones et al. · institutional PDFSource research group: canonical oxidases use four electrons per O2; four protons enter water and four more are pumped. Do not generalize to all bacterial oxidases.
Max Planck · terminal oxidasesSource-owned human pathway curation, not a new experiment. Glycolysis does not directly consume O2.
Reactome · glycolysisCurated human reaction; carbon bookkeeping is separate from electron-carrier and ATP accounts.
Reactome · pyruvate dehydrogenaseCurated human pathway with many molecular details inferred from model species. Its simplified downstream ATP yield is not used here.
Reactome · citric acid cycleCurated pathway; used to distinguish carrier transfer, oxygen reduction and proton coupling, not a fixed whole-cell yield.
Reactome · respiratory electron transportDefined Opa1 states in embryonic fibroblasts. Frozen specimen density is not moving protons or voltage. Paper figures CC BY4.0; retain specific panel/specimen attribution.
Fry et al. (2024) · original tomography studyCredit Navarro, Fry, Ananda, Ge, McDonald, Ali, Hakim, Lugo, Luce and Chao; Fry et al. 2024. Spatial slices are not time frames.
EMD-43064 · original depositionLarge compressed source volume, approximately221 MB. Model dimensions511×720×173 and spacing2.206nm. A finite specimen volume, not an entire cell.
EMD-43064 · original mapPDB archive data are CC0; scientific attribution to the depositions and original papers remains provided. Does not assign a license to unrelated images.
RCSB PDB · reuse policyUnrestricted public distribution under the archive policy; do not relabel it as an invented CC-BY license. Paper figures have their separate CC BY4.0 terms.
EMDB · reuse policyExact SI constants used in the authored energy calculation. They do not validate chosen assay parameters as biological measurements.
NIST · SI defining constantsIndependent subject review is pending.
Read the sources and model assumptions
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