The boundary of the cell

The idea

The cell membrane, or plasma membrane, is the thin boundary around every cell that protects its contents and defines its individuality; through it the cell exchanges materials and communicates with its surroundings and neighbouring cells.

Contents with no container

Imagine the soup in a bowl with the bowl taken away. The soup would spread everywhere.

What would happen to a cell’s contents with no boundary?

• They would mix with the surroundings
• Nothing; they would stay together anyway
• They would become bigger

Remove the membrane

Switch the membrane off, then let time pass.

The membrane defines the cell

The cell membrane (also called the plasma membrane) is the thin boundary around every cell. It protects the contents and defines the individuality of the cell. Through it the cell exchanges materials and communicates with its surroundings and its neighbouring cells.

Even single-celled organisms exchange materials and respond to their surroundings through the cell membrane.

• Job of the membrane | Example
• Protects the contents | keeps the cell’s parts together
• Defines the cell | this cell is separate from its neighbour
• Exchanges materials | takes in what it needs, lets waste out
• Communicates | messages to neighbouring cells

Notes

The cell membrane (plasma membrane) is the thin boundary of every cell. It protects the contents, gives the cell its individuality, and lets the cell exchange materials and communicate.

Check yourself

Another name for the cell membrane is…

Which cells have a cell membrane?

Which is not a job of the cell membrane?

Even a single-celled organism needs a membrane because…

• the plasma membrane — correct. Yes.
• the cell wall. The wall is a separate outer layer in plants, fungi and bacteria.
• the nucleus. The nucleus is inside the cell.
• Every living cell — correct. Yes. It is the universal feature of a cell.
• Only animal cells. Plant cells and bacteria have one too.
• Only large cells. Every cell has it, large or small.
• Making the cell’s DNA — correct. DNA is made inside the cell, not by the membrane.
• Protecting the contents. That is a job of the membrane.
• Letting the cell exchange materials. That is a job of the membrane.
• Helping the cell communicate with neighbours. That is a job of the membrane.
• it must exchange materials and respond to its surroundings through it — correct. Yes.
• single cells have no surroundings. They live in surroundings and must respond to them.
• membranes are only for big cells. Every cell has one.