Final answer:
Hydrophilic signaling molecules like peptides and proteins cannot pass through the cell membrane; instead, they bind to cell-surface receptors, initiating a signal transduction cascade within the cell.
Step-by-step explanation:
Hydrophilic signaling molecules such as peptides and proteins cannot pass through the lipid bilayer of a cell membrane due to their polar nature. Instead, these water-loving molecules rely on other means to communicate their message to the target cell. They bind to specialized structures on the cell surface called cell-surface receptors, particularly to the receptors' extracellular domain. The binding of the signaling molecule, often referred to as a ligand, activates the receptor and initiates a signal transduction cascade inside the cell, leading to a cellular response.
This signaling mechanism is crucial in the body's communication system and is significantly different from that of hydrophobic molecules, which can pass directly through the plasma membrane and typically act on intracellular receptors. Hydrophobic signaling molecules often serve as transcription factors, influencing gene expression by interacting with DNA inside the nucleus. However, hydrophilic molecules need the assistance of cell-surface receptors to affect cellular processes.