Final answer:
The two main categories of extracellular signaling molecules that interact with cell surface receptors are hormones and ligands, where hormones are a type of ligand that travels through the bloodstream to target cells, while ligands include any molecule that binds specifically to a receptor.
Step-by-step explanation:
The two major categories of extracellular signaling molecules that bind to cell surface receptors are hormones and ligands. Hormones are specialized signaling molecules that travel long distances through the organism's body, typically via the bloodstream, to reach their target cells. Ligands are a broader category that includes any molecule that binds specifically to a receptor, which can include hormones but also other molecules, such as neurotransmitters and others, depending on the context of the interaction.
Ligands can be categorized as either small hydrophobic ligands, such as steroid hormones, which can cross plasma membranes and interact with internal receptors, or water-soluble ligands, which cannot penetrate the plasma membrane and thus bind to cell-surface receptors initiating a signaling cascade. These interactions coordinate responses and ensure communication between cells, playing a critical role in maintaining homeostasis and coordinating biological processes.