What primarily composes the plasma membrane?

The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins. This structure is fundamental to the membrane's function and integrity. The phospholipid bilayer is made up of two layers of phospholipids, which have hydrophilic (water-attracting) heads that face outward toward the aqueous environment and hydrophobic (water-repelling) tails that face inward, creating a barrier that separates the inside of the cell from the outside.

Embedded within this bilayer are proteins that can serve various functions, including acting as channels or transporters for molecules, serving as receptors for signaling, or providing structural support. This asymmetrical arrangement allows the plasma membrane to be selectively permeable, enabling the cell to maintain homeostasis by controlling the entry and exit of substances.

In contrast, the other choices do not accurately depict the composition of the plasma membrane. A single layer of proteins would not provide the necessary barrier function and would lack the structural integrity needed for a membrane. A cellulose wall is typical in plant cells and is not a feature of the plasma membrane in any cell type. DNA and RNA are nucleic acids found within the cell's nucleus and cytoplasm, but they do not form the membrane structure itself.