A membrane is a thin soft layer of material. A membrane separates two things.
Membrane as tissue [change]
A membrane can mean a thin layer of cells or tissue. This layer covers, separates, or lines a tissue or organ. An example is the mucous membrane that is the skin that lines the inside of your nose and mouth.
Plasma membranes [change]
The plasma membrane covers cells. Plasma membranes also divide the cell into different spaces called organelles. Organelles are special areas of the cell that do different work. For example, the nucleus holds the DNA in a cell. The mitochondria make energy for the cell.
Plasma membranes are made of lipids (fats) and protein. The lipids keep the inside of the cell separate from the outside. The proteins do many things. They give the cell messages from outside. They let some things (like glucose, calcium, and potassium) go into and out of the cell.