Final answer:
The lipids in cell membranes are arranged in a bilayer structure, with hydrophilic heads facing outward and hydrophobic tails facing inward. They are not evenly distributed between the inner and outer membrane leaflets.
Step-by-step explanation:
The lipids in cell membranes are arranged in a bilayer structure. The hydrophilic heads of the phospholipids face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior. The hydrophobic tails of the phospholipids face inward, meeting at the interface of the two layers.
The arrangement of lipids in the cell membrane allows for the selective permeability of the membrane, as hydrophobic substances can pass through the hydrophobic interior of the bilayer, while hydrophilic substances interact with the hydrophilic heads.
Therefore, lipids are not evenly distributed between the inner and outer membrane leaflets. The phospholipid bilayer has distinct polar and nonpolar regions that provide structural integrity and allow for cellular processes such as membrane transport and signaling.