passive versus active transport
/ PASS-iv VUR-sus AK-tiv TRANS-port /
Think of a ball on a hillside. Rolling it downhill is free — you just let go and gravity does the work. Pushing it uphill costs effort — you have to spend energy to fight gravity. Moving substances across a membrane is just like this, and the two cases have names: passive transport is the free downhill kind, and active transport is the costly uphill kind.
Passive transport moves a substance down its gradient, from where it is crowded to where it is sparse, with no energy spent by the cell. Simple diffusion, facilitated diffusion through channels and carriers, and osmosis are all passive — the cell merely allows the natural flow, perhaps providing a protein doorway. Active transport does the opposite: it moves a substance up its gradient, from sparse to crowded, against its natural direction. Going uphill is not free, so the cell must pay with energy, usually by burning the fuel molecule ATP.
The two together give a cell complete control over its contents. Passive transport lets it ride the easy flows — taking in oxygen, releasing waste. Active transport lets it defy them — hoarding nutrients far above outside levels, or pumping ions out to build the gradients that nerves and muscles run on. A simple test tells them apart: if blocking the cell's energy supply stops the movement, it was active; if the movement continues regardless, it was passive.
Oxygen drifts into a cell passively, no effort needed. But the same cell pumps sodium back out actively, burning ATP, to keep its inside ready for the next signal — stop the ATP and that pumping stops.
Downhill is free (passive); uphill costs ATP (active).
Facilitated diffusion still uses a protein, yet it is passive, not active — the giveaway is direction and energy, not whether a protein is involved.