The center of activity: the plasma membrane
For an organism to remain alive, chemicals must be able to move in and out of cells. To be specific, all this activity occurs across the cell membrane, which is a protective layer that separates the cell from the outside.
A very thin layer of protein and fat makes up the cell membrane. It is semipermeable. Some experts prefer to call it "selectively" permeable because it lets certain molecules through but not others.
So, what are some of the things that go in and out of a cell? Both plants and animals take in and release gaseous substances (oxygen, carbon dioxide, water vapor) during respiration, photosynthesis, and excretion. Intake of nutrients and excretion of waste products are also facilitated by these processes.
Movement of substances across the membrane happens mainly in two ways:
- Passive transport: There is no expenditure of energy on the organism's part to move things in and out. Diffusion and osmosis are types of passive transport.
- Active transport: Energy is needed to move substances in and out of the cell.
In the next chapters we'll find out more about these.
Quick check: Why are diffusion and osmosis called "passive"?
Quick check: Properties of the cell membrane
- Lets some substances through but not all
- Made up of a layer of protein and fats
- Protects the cell
The cell membrane is....
(3 correct options)