cell membrane
The cell membrane also decides what may enter or leave a cell. The membrane is said to be semipermeable or selectively permeable, allowing only certain ions and organic molecules to cross the membrane. The plasma membrane contain many proteins, as well as other lipids called sterols. The proteins have various functions, such as channels channel proteins that allow certain molecules into the cell, and receptors receptor proteins that bind to signal molecules. In Figure above , the smaller green molecules shown between the phospholipids are cholesterol molecules. Cholesterol helps keep the plasma membrane firm and stable over a wide range of temperatures. At least ten different types of lipids are commonly found in plasma membranes. Each type of cell or organelle will have a different percentage of each lipid, protein and carbohydrate.