Cambridge O Level Biology · Syllabus 5090 · Cells
Cell membrane
What is Cell membrane?
The thin partially permeable boundary present in every cell, animal, plant and bacterial. It controls the movement of substances into and out of the cell. In a plant or bacterial cell it lies immediately inside the cell wall, which supports the cell but does not control what passes through it.
This definition is part of the Cells chapter in Cambridge O Level Biology.
Cell membrane in context
Animal, plant and bacterial cells all share a cell membrane, cytoplasm and ribosomes, but they differ in what else they contain. An animal cell also has a nucleus and mitochondria. A plant cell adds a cellulose cell wall, a permanent sap vacuole and, in its photosynthetic cells only, chloroplasts. A bacterial cell has a cell wall that is not made of cellulose, circular DNA lying free in the cytoplasm and sometimes plasmids, but no nucleus, no mitochondria and no chloroplasts. In every cell the membrane controls what enters and leaves; a cell wall only supports, because it is fully permeable.
Common mistakes with Cell membrane
- “The cell wall controls what enters the cell.” Repair The cell membrane controls exchange; the wall supports. The wall is fully permeable, so it cannot select anything.
Questions students ask about Cell membrane
What is the difference between the cell wall and the cell membrane?
The cell membrane is the thin, partially permeable boundary present in every cell, and it controls the movement of substances into and out of the cell. The cell wall is a rigid layer of cellulose outside the membrane of a plant cell; it supports the cell and stops it bursting when water enters by osmosis. The wall is fully permeable, so it cannot select what passes through — writing “the cell wall controls what enters” loses the mark.
Do bacterial cells have DNA if they have no nucleus?
Yes. A bacterial cell has a circular molecule of DNA lying free in the cytoplasm, and sometimes small extra circles of DNA called plasmids. What it lacks is a nucleus to enclose that DNA, together with mitochondria and chloroplasts. It still has a cell membrane, cytoplasm, ribosomes and a cell wall, although that wall is not made of cellulose.