Some pass, some are blocked
The membrane chooses what goes in and out.
A doorkeeper
Imagine a doorkeeper who lets in friends but stops strangers. A cell membrane acts a bit like that.
What would a cell membrane that let everything through be like?
What this lesson covers
The idea
The cell membrane is selectively permeable: it allows some substances to pass through it while blocking others.
A doorkeeper
Imagine a doorkeeper who lets in friends but stops strangers. A cell membrane acts a bit like that.
What would a cell membrane that let everything through be like?
- Useless: the cell could not control what is inside
- Perfect: more is always better
- A wall that blocks everything
Send substances
Send each substance at the membrane. See what happens.
Selectively permeable
The cell membrane is selectively permeable: it allows some substances to pass through it while blocking others.
Oxygen and carbon dioxide cross the thin membranes of the alveoli (air sacs) in the lungs.
- Substance | Crosses the membrane?
- Oxygen | yes
- Carbon dioxide | yes
- Water | yes (osmosis)
- Large protein, starch grain | no, blocked
Notes
The cell membrane is selectively permeable: it lets some substances pass and blocks others.
Check yourself
“Selectively permeable” means the membrane…
Which crosses the membranes of the alveoli in the lungs?
Why is it useful that the membrane is selective?
- lets some substances through and blocks others — correct. Yes.
- lets everything through. That would be fully permeable.
- blocks everything. Nothing could then enter or leave.
- Oxygen and carbon dioxide — correct. Yes.
- Large proteins and starch grains. Those are blocked.
- Whole cells. A membrane does not let whole cells through.
- The cell can take in what it needs and keep unwanted things out — correct. Yes. It controls what moves in and out.
- So that the cell can never take in anything. Then it could not live.
- So that the cell can grow without limit. Selectivity is about control, not size.