Substances that are large, polar, or charged are most likely to require a protein to facilitate their diffusion across the cell membrane.

Here's why:

* Cell membranes are selectively permeable: They allow small, non-polar molecules (like oxygen, carbon dioxide, and water) to pass through easily.

* Polar and charged molecules face challenges: These molecules are repelled by the hydrophobic interior of the cell membrane.

* Proteins provide pathways: Membrane proteins act as channels, carriers, or pumps to help these molecules cross the membrane.

Here are some examples of substances that typically require protein assistance:

* Sugars (glucose, fructose): Large and polar.

* Amino acids: Large and charged.

* Ions (sodium, potassium, calcium): Charged.

* Large macromolecules (proteins, nucleic acids): Too large to pass through the membrane.

Key takeaway: The cell membrane acts like a gatekeeper, letting small, non-polar molecules pass freely, while requiring assistance for larger, polar, or charged substances. Membrane proteins play a crucial role in this selective transport.