What is a phospholipid bilayer?

A phospholipid bilayer is composed of two layers of phospholipid molecules that are arranged with their hydrophobic (water-repelling) tails facing inward toward each other, while the hydrophilic (water-attracting) heads face outward towards the aqueous environments both inside and outside of the cell. This unique arrangement is critical in forming the structural foundation of cell membranes, allowing them to maintain integrity while also being selectively permeable to various substances.

The bilayer structure enables the formation of a barrier that separates the interior of the cell from the external environment, playing a crucial role in protecting the cell and facilitating communication and transport. The fluid nature of the bilayer allows for movement of proteins and lipids within the layer, contributing to the dynamic functionality of the cell membrane, including the ability to form vesicles for transport and communicate with other cells.

Other choices do not accurately describe the fundamental structure of the phospholipid bilayer; for example, a single layer of proteins would not provide the necessary separation or characteristics of a membrane, while cholesterol contributes to membrane fluidity but is not the primary structural component. A membrane with a protein core does not align with the accepted model of cell membrane architecture.