Final answer:
Cell membranes are primarily composed of a phospholipid bilayer, with additional components such as proteins, cholesterol, and carbohydrates providing structure and function.
Step-by-step explanation:
Cell membranes are made up of phospholipid bilayers. These layers consist of molecules that have a glycerol backbone, two fatty acid chains, and a phosphate-linked head group. The phospholipid molecules are arranged in two layers with their hydrophilic (water-attracting) heads facing the outside and inside of the cell, and their hydrophobic (water-repelling) tails facing each other.
This structure allows cell membranes to regulate the entry and exit of substances. In addition to phospholipids, cell membranes also contain proteins and cholesterol, which provide flexibility, and carbohydrates that are attached to lipids and proteins on the cell's exterior, important for cell recognition and communication.