Final answer:
Phospholipids form a bilayer in cell membranes because their hydrophobic tails avoid water and face inward, while their hydrophilic heads face the aqueous environment inside and outside the cell, creating a semi-permeable barrier.
Step-by-step explanation:
The phospholipids are arranged in two layers in a cell surface membrane primarily due to the amphipathic nature of these molecules, meaning each has a hydrophilic (water-loving) head and hydrophobic (water-hating) tails. When introduced to a water-based environment, such as the cytosol inside the cell or the extracellular fluid, phospholipids orient themselves in a bilayer with the hydrophobic tails facing inward, away from the water, and the hydrophilic heads facing outwards towards the water. This arrangement creates a phospholipid bilayer that is an essential characteristic of the plasma membrane serving as a semi-permeable barrier, which separates the internal cellular environment from the external surroundings and regulates the transport of substances in and out of the cell.