A selective boundary
The cell membrane separates the cell from its surroundings without sealing it off. It is selectively permeable: some substances cross readily, while others need a protein pathway or cannot cross under the same conditions.
Small nonpolar molecules such as oxygen have no separated positive and negative ends, so they can move through the membrane by simple diffusion. Charged particles and many polar molecules, whose charge is unevenly distributed, are blocked by the membrane’s oily interior, so they often cross through specific channel or carrier proteins.
Follow the gradient—and the energy
A concentration gradient is a difference in the amount of a substance between two places. Moving down the gradient means moving from higher concentration to lower concentration. In passive transport, the cell does not directly spend energy on the movement.
For ions, electrical charge also affects which direction they tend to move. Concentration and charge together form an electrochemical gradient. Facilitated diffusion is still passive even though a membrane protein helps.
Moving a substance against its relevant gradient requires active transport and cellular energy. Osmosis is the passive movement of water across a selectively permeable membrane.
Remember these ideas
Key ideas
- Selective permeability means the membrane allows some substances to cross more easily than others.
- Passive transport moves a substance down its relevant concentration or electrochemical gradient.
- Facilitated diffusion uses a membrane protein but does not directly require cellular energy.
- Active transport uses cellular energy to move a substance against its relevant concentration or electrochemical gradient.
Make the reasoning visible
Choose a transport route
- 1
Oxygen is small and nonpolar, so it can cross the membrane by simple diffusion when moving down its gradient.
- 2
A charged ion moving down its electrochemical gradient cannot pass through the membrane’s oily interior easily, so it can use a channel protein by facilitated diffusion.
- 3
If the ion must move against its electrochemical gradient, it will not move that way on its own even when a protein pathway exists.
- 4
Against-gradient movement requires active transport and an energy source, commonly through a membrane pump.
Try the next step
Name the evidence
Glucose moves from an area of higher concentration to lower concentration through a carrier protein. Is this simple diffusion, facilitated diffusion, or active transport?
Why does using a protein pathway not automatically make a transport process active?