Definition · Plain-language
Cell membrane
The cell membrane is the thin layer around a cell that controls which substances enter and leave it.
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The boundary of the cell
The cell membrane, sometimes called the plasma membrane, is the thin layer that forms the outer edge of a cell, separating its contents from the outside world. It holds the cell together and gives it a defined boundary. Every cell has one, whether plant, animal or bacterial. In an animal cell the membrane is the outermost layer; in a plant cell it lies just inside the rigid cell wall. Though extremely thin, the membrane is vital, because it controls the cell’s contact with everything around it.
Controlling what enters and leaves
The membrane’s most important job is to control which substances pass in and out of the cell. It is partially permeable (also called selectively permeable), meaning it lets some substances through, such as water and oxygen, while keeping others out. This control allows the cell to take in the nutrients it needs, let out waste products, and keep useful substances inside. Materials cross the membrane in several ways, including diffusion, osmosis and active transport, depending on the substance and the direction it needs to go.
Membrane versus cell wall
The cell membrane is easy to confuse with the cell wall, but they are different. The membrane is a thin, flexible layer found in all cells, and it controls what passes in and out. The cell wall is a much thicker, rigid layer found only in plant cells (and fungi and bacteria), made of cellulose in plants, that gives support and shape. A plant cell has both: the wall on the outside for strength, and the membrane just inside it for control. An animal cell has only the membrane.
Key facts
At a glance
- Definition: the thin layer that surrounds and bounds a cell
- Also called: the plasma membrane
- Main job: controls what enters and leaves the cell
- Property: partially permeable (lets some substances through)
- Found in: all cells, plant and animal
- Not the same as: the cell wall, which is rigid and only in plant cells
Common misconceptions
What people often get wrong
Often heard: The cell membrane and the cell wall are the same thing.
Actually: They are different layers. The membrane is a thin, flexible layer in all cells that controls what passes in and out. The cell wall is a thick, rigid layer found only in plant cells (and fungi and bacteria) that gives support. Plant cells have both.
Often heard: The cell membrane lets everything pass through freely.
Actually: The membrane is partially permeable, not fully open. It lets some substances through, such as water and oxygen, while blocking others. This selective control is exactly what lets the cell manage its internal conditions.
Often heard: Only animal cells have a cell membrane.
Actually: All cells have a cell membrane, including plant and bacterial cells. In a plant cell the membrane sits just inside the cell wall. The membrane is a universal feature of cells; the cell wall is not.
Going deeper
