Who am I?

I am single-layered but maintain cellular osmotic pressure.

Cell membrane or Plasma membrane

The cell membrane is a selectively permeable membrane i. e. it allows the entry of only certain molecules inside the Cell. Because of this property only certain molecules like water, gases, salts etc. can enter and exit the cell. Plasma membrane thus has the property to resist any change in the cellular environment that may occur due to changes in the outside environment.