Animal and plant cells: a cell membrane

The idea

The cell membrane is the thin boundary that surrounds every cell, whether prokaryotic or eukaryotic. It is a partially permeable barrier built from a phospholipid bilayer with embedded proteins. It holds the cytoplasm and cell contents together, separates the cell from its surroundings, and controls which substances enter and leave. Small molecules such as oxygen and carbon dioxide diffuse across it, water moves by osmosis, and dissolved substances may be moved by active transport. In animal cells it is the outermost layer; in plant cells it lies just inside the cell wall. A useful method is to compare it with a selectively permeable gate: glucose and ions can be admitted or expelled, while large molecules cannot pass freely.

What earns the marks

• States that the cell membrane is a thin partially permeable barrier surrounding the cell.
• Explains that it controls the movement of substances into and out of the cell.
• Identifies it as the outer boundary of animal cells but lying inside the cell wall in plant cells.
• Links the membrane to processes such as diffusion, osmosis and active transport.
• Recognises that it also encloses the cytoplasm and keeps cell contents together.

Common mistakes and how to put them right

• Calling the membrane fully permeable.

  Put it right by stating it is partially permeable, so only some substances pass through.

• Confusing the membrane with the cell wall.

  Put it right by noting the wall is rigid cellulose and fully permeable, while the membrane is flexible and partially permeable.

• Saying the membrane only lets water through.

  Put it right by explaining that gases, water and dissolved substances can cross, often with the help of transport proteins.

Exam tips

• Use the phrase partially permeable rather than semi-permeable when describing the membrane.
• When asked to compare plant and animal cells, state clearly that only plant cells have a cell wall outside the membrane.
• Link the membrane to a named transport process to show understanding rather than just labelling it.