The plasma membrane is a bilayer of phospholipids and proteins that provides a barrier between the interior and exterior of a cell. It maintains homeostasis by regulating what passes in and out. Phospholipids are the primary components, with a hydrophilic phosphate head and hydrophobic fatty acid tails that form a bilayer. This structure, along with embedded proteins, allows the membrane to be selectively permeable and help control what enters and exits the cell.