What structure consists of two layers of lipids stacked with hydrophobic tails facing inward?

The correct answer is the lipid bilayer, which is a fundamental structure in biological membranes. It is composed of two layers of phospholipids, with the hydrophilic (water-attracting) heads oriented towards the aqueous environment outside and inside the cell, while the hydrophobic (water-repelling) tails face inward, shielded from water. This arrangement creates a stable barrier that separates the interior of the cell from its external environment, allowing for selective permeability. This selective permeability is crucial for maintaining homeostasis within the cell, as it regulates the movement of substances in and out of the cell.

In contrast, a phospholipid monolayer consists of only one layer of phospholipids and does not provide the same structural integrity or functional capabilities as a bilayer. A protein membrane does not refer to a specific structural unit like the lipid bilayer but rather to membranes that include protein components, which play roles in transport, signaling, and structural support. The cell wall, found in plants, fungi, and bacteria, is a rigid structure external to the cell membrane, providing additional support and protection, but is not composed of lipid layers.