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Cell Transport
Active vs passive transport
Moving things across a membrane comes in two kinds — one pays ATP to push uphill, the other coasts downhill for free.
Active transport
Energy-dependent movement AGAINST the concentration gradient, powered by ATP (e.g., the sodium–potassium pump). It can build up a substance on one side.
Passive transport
Movement DOWN the gradient with no energy spent — simple diffusion, facilitated diffusion through carriers/channels, and osmosis for water.
The trap
The deciding line is energy plus direction. Active = ATP, against the gradient. Passive = no ATP, along the gradient. Facilitated diffusion uses a protein but is still passive — a carrier is not the same as spending energy.
Quick check
Facilitated diffusion is called passive because it:
- AUses ATP
- BMoves solutes against the gradient
- CSpends no energy, just moves down the gradient
- DOnly moves water