BioMasterTransport across the cell membrane
When does the cell spend ATP to move something, and when not? Here you separate passive from active transport, with osmosis and the facilitated-diffusion trap.
Direct answer: Passive transport moves substances down the concentration gradient and does NOT use ATP (simple diffusion, facilitated diffusion and osmosis). Active transport moves substances against the gradient and DOES use ATP (for example, the sodium-potassium pump).
Comparison table
| Passive transport | Active transport | |
|---|---|---|
| Direction | Down the gradient | Against the gradient |
| Energy use | No (spontaneous) | Yes (ATP) |
| Examples | Simple diffusion, facilitated diffusion, osmosis | Na⁺/K⁺ pump, endocytosis, exocytosis |
Types of passive transport
- Simple diffusion: small or nonpolar molecules (O₂, CO₂) cross the bilayer directly.
- Facilitated diffusion: polar or large molecules (glucose, ions) pass through channel or carrier proteins — without using ATP.
- Osmosis: diffusion of water across the membrane, from the less concentrated to the more concentrated side.
Osmosis: hypotonic, isotonic and hypertonic
In a hypotonic medium the cell gains water and swells (it may burst). In an isotonic medium it is at equilibrium. In a hypertonic medium it loses water and shrivels (crenation in animals; plasmolysis in plants).
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Study Biology free on BioMasterFrequently asked questions
What is the difference between passive and active transport?
Passive transport goes down the gradient without using ATP; active transport goes against the gradient and uses ATP.
Does facilitated diffusion use energy?
No. Although it uses membrane proteins, it moves down the gradient and does not consume ATP: it is passive transport.
What is osmosis?
It is the diffusion of water across the membrane, from the less concentrated solution to the more concentrated one.