Cell Biology
Cell membrane transport: passive and active
The membrane is selectively permeable, and how a substance crosses it depends on its size, charge, and whether the cell is willing to spend ATP.
Passive transport — no ATP required
- Simple diffusion — small, non-polar molecules (O2, CO2, steroid hormones) pass straight through the phospholipid bilayer, down their concentration gradient.
- Facilitated diffusion — larger or charged particles (glucose, ions) move down their gradient through a protein. Still passive, still no ATP — the protein provides a route, not a push.
- Osmosis — the movement of water across a semi-permeable membrane, from higher to lower water potential.
Facilitated diffusion is not active transport
Involving a protein does not make transport active. The distinction is direction and energy: passive transport always moves down a gradient and costs no ATP, regardless of whether a protein is involved.
Active transport — ATP required
- Primary active transport — directly uses ATP to move substances against their gradient. The sodium-potassium pump is the standard example: 3 Na+ out, 2 K+ in, per ATP.
- Secondary active transport (co-transport) — uses the gradient built by primary active transport rather than ATP directly. Glucose absorption in the small intestine works this way, riding the sodium gradient inward via SGLT1.
Bulk transport
Endocytosis brings large material into the cell in a vesicle (phagocytosis for solids, pinocytosis for fluids); exocytosis releases material out. Both require ATP.
Tonicity, quickly
| Solution | Relative solute | Effect on an animal cell |
|---|---|---|
| Hypotonic | Lower outside | Water enters; cell swells and may lyse |
| Isotonic | Equal | No net movement |
| Hypertonic | Higher outside | Water leaves; cell shrinks (crenation) |
This is why intravenous fluids must be isotonic, and it underlies fluid balance during exercise — including why overdrinking dilutes blood sodium.
Frequently asked questions
Is facilitated diffusion active or passive?
Passive. It moves substances down their concentration gradient and requires no ATP. The involvement of a transport protein does not make it active; only movement against a gradient using energy does.
How does the sodium-potassium pump work?
It is primary active transport, using one ATP to move three sodium ions out of the cell and two potassium ions in, against their concentration gradients.
What happens to a cell in a hypotonic solution?
Water moves into the cell because solute concentration is lower outside. An animal cell swells and may burst, a process called lysis.
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