Final answer:
Proteins exhibit diverse functions primarily due to their ability to specifically bind with molecules such as water-soluble ligands, which often interact with cell-surface receptors. While small molecules might penetrate the cell membrane, larger biologics typically require transport mechanisms. Understanding these interactions is crucial and can involve complex techniques like X-ray crystallography.
Step-by-step explanation:
The chief characteristic that allows proteins' diverse functions is their ability to bind specifically and tightly with other molecules. Molecules, such as water-soluble ligands that are polar, cannot pass through the plasma membrane unaided. These types of ligands, which include small molecules, peptides, and proteins, most often bind to the extracellular domain of cell-surface receptors. Some ligands are capable of crossing the membrane if they are small enough and comply with Lipinski's rule of five, which indicates a reasonable potential for cellular membrane penetration.
However, larger therapeutic biologics often require assistance to enter cells, such as transport through peptides for biomedicine. Macromolecules like proteins may require chemical modifications or prosthetic groups to be biologically active. To study these complex interactions, techniques like X-ray crystallography are utilized, although its application is limited for certain types of macromolecular structures, like membrane-bound G-protein coupled receptors (GPCRs).