According to the Encyclopedia Brittanica, the cell membrane has two main functions: regulating which molecules go in and out of the cell and separating incompatible processes occurring within organelles. Without the cell membrane, toxins would inevitably enter the cell and cause damage.
The cell membrane is the thin layer on the outer boundary of a cell or internal cell compartment, as defined by the Encyclopedia Britannica. Membranes are composed mainly of a lipid bilayer. These lipids give the membrane a fluid consistency comparable to a light oil. The fatty chains inside the membrane allow small, fat-soluble molecules to enter the cell while repelling large, water-soluble molecules. This allows molecules like oxygen to enter the cell while barring harmful molecules like sugar and calcium. Also, membrane channels allow ions to pass freely in and out of the cell. This enables proteins manufactured in the cell to travel to where they are needed.
Certain components of the cell have their own membranes, the Encyclopedia Brittanica explains. The nucleus has a double membrane with sizable pores. These pores allow materials to be transferred between the nucleus and cytoplasm. The mitochondria, ribosomes and endoplasmic reticulum also have their own membranes. The cell membrane may be thin, but it protects the cell by dictating everything that goes in and out of the cell and its components.