They protect the cell. The membrane is seen as a double layer since electron dense material has been used for contrasting. It stains both the inner and the outer membrane's surface. The membrane is the area between the dark layers. Some surround vacuoles , others mitochondria , chloroplasts and the nucleus.
Compartments are reaction areas clearly separated spatially.
They consist mainly of lipids and proteins. They act as a permeability barriers and are permeable only for selected substances. When fragmented do they always reform as vesicles. Phospholipids are the most common molecules in membranes. This structure lends a tendency to form organized structures and monomolecular layers whose degree of organization is dependent on the length of the fatty acid residues and the number of double bonds to phospholipids.