Final answer:
The polar phosphate heads of the phospholipid bilayer are hydrophilic and thus oriented toward the water-containing extracellular and intracellular fluids. The hydrophobic lipid tails face inward, away from water, which results in the formation of a stable bilayer crucial for cell function.
Step-by-step explanation:
The polar phosphate heads of the phospholipid bilayer are oriented toward the extracellular fluid (ECF) and intracellular fluid (ICF) because they are hydrophilic, which means they are attracted to water. This characteristic of the phosphate heads is crucial because both the inside and the outside of cells are aqueous environments. The hydrophobic lipid tails, in contrast, are repelled by water and so they face each other in the middle of the cell membrane, creating a barrier that allows the cell to stay intact while immersed in a water-based environment.
Phospholipids are amphipathic molecules, containing both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region. The amphipathic nature of phospholipids enables the formation of the bilayer, with the hydrophilic heads facing the aqueous surroundings and the hydrophobic tails sequestered away from the water, thus forming a stable boundary that is essential to the functionality of cells.