Water can cross the membrane in both directions, but solute never can, it's simply too big to fit through the holes. That one difference decides everything: water always ends up moving toward whichever side has more solute. Make both sides equal, and there's no pull either way, water just crosses back and forth evenly with no net direction.
Terms
- Osmosis
- Water moving across a selectively permeable membrane, from the side with more water (less solute) to the side with less water (more solute), until both sides reach equilibrium. It's really just diffusion, applied specifically to water, and it's how plant roots pull in water and how your own cells stay properly hydrated.
- Water molecule
- The small blue circles. They're small enough to slip through the membrane's holes, so they can cross in either direction.
- Solute molecule
- The larger orange circles. They're too big to fit through the holes, so each one stays on whichever side it started on for good.
- Membrane
- The wall down the middle, with tiny holes that water can pass through but solute can't. Real cell membranes work the same way: a two-layer fatty membrane with built-in proteins called aquaporins that let water through quickly. Without that control, cells would take on or lose too much water and stop working properly.
- Concentration
- How much solute is dissolved in the water on a given side. The more solute packed into the same space, the higher the concentration.
- Net water flow
- Water is always crossing in both directions at once. "Net flow" is just whichever direction has more water crossing overall, and that's always toward the more concentrated side.
- Equilibrium
- The point where the same amount of water is crossing in each direction, so the totals on each side stop changing, even though the two sides may never become perfectly equal.