01Passive vs. active
Passive transport moves substances down their concentration gradient (high → low) and needs no energy from the cell. Diffusion, facilitated diffusion and osmosis are all passive.
Active transport moves substances against their gradient (low → high) and requires energy, usually ATP, plus a carrier protein.
02The sodium-potassium pump
The classic example: each cycle uses 1 ATP to pump 3 Na⁺ out and 2 K⁺ in. Because more positive charge leaves than enters, the inside of the cell becomes negative — this is the membrane potential that neurons depend on.
Gradients are stored energy. Cells later let ions flow back down their gradient to power other work (see 2.9 co-transport).
03Notebook box
Passive: down gradient, no ATP — diffusion, facilitated diffusion, osmosis.
Active: against gradient, ATP — pumps, endocytosis, exocytosis.
Worked example: A cell is treated with a drug that blocks ATP production. Which stops: O₂ uptake or Na⁺/K⁺ pumping? → The pump stops; O₂ diffusion continues.