Ions and polar molecules cannot slip through the oily middle of the membrane. They cross through proteins. The question that sorts every case is simple: are they going down their gradient or up it?
Key idea
Facilitated diffusion moves substances down their gradient through channel or carrier proteins, with no energy from the cell. Active transport uses energy, usually ATP, to move substances against their gradient. The sodium-potassium pump is the classic example.
| Kind | Direction, protein, energy | Examples |
|---|---|---|
| Simple diffusion | Down the gradient; no protein; no energy | O₂, CO₂ |
| Facilitated diffusion | Down the gradient; a channel or carrier; no energy | Glucose carriers; ion channels; aquaporins |
| Active transport | Against the gradient; a pump; uses ATP | The sodium-potassium pump; roots taking up minerals |
Why it works
Going down a gradient happens on its own, like rolling downhill; a channel or carrier just gives the particle a path. Going up a gradient is like pushing a cart uphill: it needs a power source. The sodium-potassium pump uses one ATP to move 3 Na⁺ out of the cell and 2 K⁺ in, both against their gradients. The gradients it builds are stored energy: nerve cells use them to send signals, and many cells use the sodium gradient to pull glucose in.
Worked example: Passive or active?
Root cells hold many times more potassium than the soil water around them, yet they keep taking more in. Passive or active?
- Potassium moves from the soil (low) into the cell (high).
- That is against the gradient.
- Moving against a gradient needs energy: active transport.
Answer: Active transport
Worked example: Pump arithmetic
A cell's sodium-potassium pumps use 400 ATP. How many Na⁺ and K⁺ ions do they move?
- Each ATP moves 3 Na⁺ out and 2 K⁺ in.
- Na⁺: $400 \times 3 = 1200$ out. K⁺: $400 \times 2 = 800$ in.
Answer: $1200$ Na⁺ out and $800$ K⁺ in
Common mistake
Thinking any transport that uses a protein must use energy. Channels and carriers in facilitated diffusion use none: the gradient drives the movement. Only moving against the gradient costs ATP. Sort by direction first, not by whether a protein is involved.