Final answer:
The cell membrane lipid bilayer is composed of phospholipids arranged in two layers with hydrophobic tails facing inward and hydrophilic heads facing outward, creating a stable and selectively permeable barrier.
Step-by-step explanation:
The composition of lipids in the outer and inner monolayers of a cell membrane lipid bilayer is primarily made up of molecules called phospholipids. Phospholipids have a glycerol backbone attached to two fatty acid tails, which are hydrophobic, and a phosphate group that makes up the polar head, which is hydrophilic. The bilayer is arranged such that the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward, towards the water-based environment inside and outside of the cell. This structure creates a stable barrier that is selectively permeable, allowing for the controlled entry and exit of substances.