Final answer:
A protein that spans both layers of the bilayer is known as an integral protein, and more specifically, when it extends across the membrane, it is a transmembrane protein. These are crucial for substance transport across the cell membrane.
Step-by-step explanation:
The protein that spans both layers of the bilayer is known as an integral protein. Specifically, when these proteins extend across the membrane and are exposed on both sides, they are referred to as transmembrane proteins. Integral proteins play critical roles in various cellular functions, including acting as channels or transporters to allow the movement of substances across the otherwise impermeable lipid bilayer. These proteins interact with the hydrophobic core of the phospholipid bilayer with hydrophobic regions, while their hydrophilic regions protrude from the bilayer to interact with the internal or external cellular environment. Transmembrane proteins can be composed of multiple membrane-spanning segments, either alpha-helices or beta-sheets, that provide the structural foundation for their functional roles within the plasma membrane.