The membrane and its two species
A membrane with channels on one face and pumps on the other. Channels are free and downhill. Pumps cost ATP and go uphill.
Drag a molecule wherever you like. Then stop, and watch what happens on its own.
Try dragging a sodium ion from inside out through the channel. It will go — one molecule can go either way.
| Na+ | in 0 | out 0 | no gradient |
| glucose | in 0 | out 0 | no gradient |
| water | 0 | 0 | nothing holds it — it follows its own gradient |
ATP + H₂O → ADP + Pᵢ
Breaking bonds costs. Forming bonds pays. This releases energy because the products' bonds pay back more than the reactants' cost.
pump cycles 0
There is no control anywhere that makes something go a particular way. You can push one molecule. You cannot push the crowd.
| solute | inside | outside | net flux | held by |
|---|---|---|---|---|
| water | 0 osm | 0 osm | — | nothing holds it — it follows its own gradient, which total solute sets |
direction is computed, never chosen
3 Na⁺ out, 2 K⁺ in, per ATP
cell volume 1.00 of 1.00
inside 0 osm · outside 0 osm
holding — and it costs continuously
ATP + H\u2082O \u2192 ADP + P\u1D62 \u0394G\u00B0\u2032 = \u221230.5 kJ/mol
Breaking bonds costs. Forming bonds pays. Nothing was stored inside the bond.
cycles run 0 \u00B7 spent 0 kJ/mol-equivalent
No moves yet — predict before you run.