Final answer:
Polar ligands bind to cell-surface receptors, while nonpolar ligands can pass through the plasma membrane and bind to internal receptors, influencing gene expression and protein synthesis within the cell.
Step-by-step explanation:
Polar ligands create a complex with a cell-surface receptor. Nonpolar signaling molecules can traverse the plasma membrane to reach their target of an internal receptor.
Ligands are crucial in cell signaling pathways and act as keys to their corresponding receptors. Polar ligands, which include water-soluble molecules such as peptides and proteins, are unable to cross the plasma membrane due to their polarity. Thus, they bind to receptors that are located on the cell surface.
Once bound, these receptors can undergo a change in shape or initiate a series of cellular responses through mechanisms like ion channel change or G-protein activation. On the other hand, nonpolar ligands, such as steroid hormones, can easily pass through the hydrophobic core of the plasma membrane due to their lipophilic nature. Upon entering the cell, they bind to internal receptors located within the cytoplasm. Following this binding, the ligand-receptor complex migrates to the nucleus, where it can directly interact with DNA to modify gene expression and ultimately protein synthesis.