In school I learned 2 as the correct answer. The cell membrane chooses which substances to allow in and out of cell - is what my teacher's definition was.
The correct answer is option 2, that is, the cell membrane lets certain substances enter the cell and keeps certain substances out of the cell.
All the cells are surrounded by a cell membrane. The cells membrane is selectively permeable, that is, it allows only some of the ions or molecules to pass through them by assistance of active or passive transport. For example, the molecules of water can passes passively through small pores on the membrane.
