If two fatty acids, glycerol, and a phosphate group undergo a dehydration reaction, they form a phospholipid molecule. Phospholipids are the main components of cell membranes and are responsible for their structure and function. They consist of a hydrophilic (water-loving) head group and two hydrophobic (water-hating) fatty acid tails. The hydrophilic head group interacts with water molecules, while the hydrophobic fatty acid tails interact with each other and form a barrier to water and other polar molecules. This arrangement gives cell membranes their characteristic semipermeable properties, allowing certain substances to enter and exit the cell while blocking others.