Cell Transport and Osmosis
Track what moves, which direction it moves, and whether a protein or energy is required.
Cell Transport and Osmosis
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Direction, permeability, and energy determine how substances cross membranes
Passive transport moves substances down a concentration or electrochemical gradient without direct ATP use. Simple diffusion crosses the bilayer; facilitated diffusion uses a channel or carrier. Osmosis is water movement across a selectively permeable membrane. Active transport moves substances against a gradient using energy directly or indirectly.
Tonicity
Hypertonic solutions draw water out; hypotonic solutions drive water in; isotonic solutions produce no sustained net water change.
Ask about permeability
A gradient matters only if the membrane or an available transport protein permits movement.
Substance → gradient → protein → energy
Problem: Glucose enters a cell from high to low concentration through a carrier, without ATP.
Recognize: It moves down the gradient, so transport is passive. It needs a carrier, so it is not simple diffusion.
Do it fast: Facilitated diffusion.
Why it works
Naming these four features prevents common mix-ups among diffusion, osmosis, facilitated diffusion, and active transport.
Five forms you should recognize
Small nonpolar O₂ moves directly through the bilayer from high to low concentration.
Water moves toward the side with more nonpenetrating solute.
A carrier moves glucose down its gradient without ATP.
A pump uses energy to maintain an ion gradient.
Mass gain indicates net water entry; mass loss indicates net water exit.
Check before you commit
- Calling every protein-assisted movement active
- Describing osmosis as solute movement
- Assuming equilibrium means motion stops
- Ignoring whether a solute crosses the membrane
- Confusing hypertonic with high water concentration
- Forgetting that carriers can saturate
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Cell Transport and Osmosis FAQ
Does facilitated diffusion use ATP?
No. It uses a membrane protein but moves down the relevant gradient.
Why do plant cells not usually burst in pure water?
The cell wall resists expansion and supports turgor pressure.
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