No pumps, no ATP, no intent — just molecules moving at random until the crowding evens out. The interesting part is what the cell does to make that useful.
Diffusion looks purposeful and is not. Molecules move at random; the crowded region simply loses more than it gains, and that imbalance is the whole phenomenon.
Thermal energy keeps every molecule moving. No ATP is spent, because the motion was already happening.
Movement never stops at equilibrium. Only the net movement reaches zero, once both directions match.
Diffusion of water across a partially permeable membrane. Same process, spotlight on the solvent.
Tap each part. Pay attention to the phrasing on the arrow — it is the version that earns marks.
Tap the membrane, the water, the solute, the net flow arrow, or either side of the vessel.
Four steps, no ATP anywhere in them. Watch what limits the process — it becomes a graph in the next section.
On paper, simple and facilitated diffusion look alike: both passive, both downhill. On a graph they are unmistakable.
Nothing binds, so transport is fast. Many are gated, which lets the cell open and close them on demand.
Binds, flips, releases. Specific to its solute and limited by how many copies exist.
Simple diffusion climbs forever. Facilitated diffusion hits a ceiling. That plateau is the exam's favourite clue.
Every particle starts on the left. Watch the counts even out — then keep watching the crossing tallies.
Three carriers, and a slider for how much glucose is waiting outside. Push the concentration up and watch what the rate does.
Tap a node to light up its connections. The pink dashed link is the one that turns a graph into a diagnosis.
Everything on this map is downhill. Nothing here costs the cell anything.
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
Diffusion is a process, because the molecules are already moving and no ATP is spent. Net movement runs down a concentration until the system reaches , at which point molecules still move but there is no further net change. The diffusion of water across a partially permeable membrane is , and its direction is predicted by . Ions cross through a hydrophilic , while glucose binds to a that changes shape. Because those proteins are finite in number, the rate plateaus once they are all .
Single best answer. You get the reasoning as soon as you commit.