What is the primary function of phospholipids in cell membranes?

The primary function of phospholipids in cell membranes is to allow for the formation of a bilayer that serves as a barrier to the external environment while facilitating selective transport of materials. Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. When they are arranged in a bilayer, the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward, toward the aqueous environment both inside and outside the cell.

This unique structure creates a semi-permeable membrane that allows certain substances to pass through while blocking others, thus regulating the internal environment of the cell. Transport proteins and channels embedded in the phospholipid bilayer further assist in the selective transport of ions and molecules, making the membrane critical for maintaining homeostasis and enabling communication and absorption of nutrients.

While phospholipids contribute to the structural integrity of the cell membrane, their main role is not to store energy or act as a filter, but rather to facilitate the controlled transport of materials across the membrane.