The plasma membrane, also known as the cell membrane, is a bilipid membrane with protein molecules embedded in between. The membrane encloses the cytoplasm and all the cellular organelles and is the barrier between inside the cell and its surrounding environment. It is selectively permeable in that certain molecules can pass through the plasma membrane if there is a concentration gradient on either side of the membrane, and if the molecules are small enough to be transported across. The plasma membrane is found in all cells. However, a cell wall is found in plant cells and is composed of cellulose material. Cellulose is a polysaccharide. It is nonliving. Its function is to protect and act as a structural support for the plant cell.