Final answer:
Polar and large molecules cannot easily pass through the hydrophobic lipid layer of the cell membrane, and thus require transport proteins like transporters, channels, carriers, or processes like vesicle transport to facilitate their movement across the membrane.
Step-by-step explanation:
Polar and large molecules require transport proteins like transporters, channels, or carriers to move across the cell membrane because they cannot easily diffuse through the hydrophobic lipid bilayer. These molecules are either too large, too polar, or both, to pass through the cell membrane without assistance. Small nonpolar molecules can diffuse across the membrane easily, but polar molecules, such as water, ions like sodium and potassium, and large molecules like proteins, encounter the barrier presented by the membrane's lipids. Specialized structures, such as channel proteins, carrier proteins, and vesicle transport mechanisms, facilitate the movement of these substances.
For example, glucose enters cells using facilitated transport by glucose transporters. Different types of transport proteins, such as channel and carrier proteins, allow for specific molecules to pass. Vesicle transport is used for very large molecules like proteins, and involves the encasement of substances within a vesicle that is then transported into or out of the cell. This process requires energy, characterizing it as active transport, while facilitated diffusion does not require energy and is a form of passive transport.