All cells are surrounded by membranes, but not all organelles are surrounded by a membrane.
Cell surface membranes and other membranes
Cell surface membranes surround cells - they control the substances that enter and leave the cell.
- This called being partially permeable as some substances can move across the membrane, but others cannot.
- Cell surface membranes are also sometimes known as plasma membranes.
Membranes are also sometimes found surrounding organelles, the separate the contents of the organelle from the cytoplasm, splitting the cell into different compartments.
- These membranes also also partially permeable.
- Their function is to control what substances enter and leave the organelle.