Final answer:
True, only transmembrane proteins can span the plasma membrane and are capable of functioning on both sides of the bilayer or transporting molecules across it.
Step-by-step explanation:
The statement "Only transmembrane proteins can function on both sides of the bilayer or transport molecules across it" is True. Transmembrane proteins span the entire plasma membrane and are involved in a variety of cellular processes including transporting molecules. These proteins can act as channels or pumps to facilitate the movement of substances that cannot permeate the lipid bilayer on their own, such as ions and polar molecules. Examples include channel proteins that allow passive transport of ions and the sodium/potassium pump which actively transports sodium and potassium ions across the membrane. On the other hand, peripheral proteins associate more loosely with the membrane and typically do not span the bilayer or transport molecules.