Final answer:
The cell membrane is primarily composed of phospholipids arranged in a phospholipid bilayer, featuring hydrophilic heads facing the aqueous environments and hydrophobic tails facing inwards, creating a barrier for cell regulation.
Step-by-step explanation:
The cell membrane is mostly made up of a type of lipid called phospholipids. These phospholipids are arranged in a double layer known as a phospholipid bilayer. This arrangement is critical for the membrane's function because phospholipids have a hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails. The hydrophilic heads are in contact with the aqueous environments inside and outside the cell, while the hydrophobic tails face each other, forming a non-aqueous layer in the middle of the membrane. This structure allows the cell membrane to act as a barrier, regulating the entry and exit of substances, thereby maintaining the proper environment for the cell's operations.