Final answer:
Phospholipids form a bilayer in cell membranes with hydrophilic heads facing outward and hydrophobic tails facing inward, creating a hydrophobic barrier vital for cell function.
Step-by-step explanation:
Phospholipids are crucial for the structure and function of cell membranes. These molecules consist of a hydrophilic (water-attracting) 'head' and two hydrophobic (water-repelling) 'tails'. In the cell membrane, phospholipids are arranged in a bilayer, with the hydrophobic tails facing inward, away from water, and the hydrophilic heads facing outward towards the aqueous environment on either side of the membrane. This arrangement contributes to the membrane's structure by forming a hydrophobic barrier that regulates the passage of substances in and out of the cell, ensuring that the internal environment of the cell is maintained separate from the external environment.