Which type of hormone is typically associated with membrane-bound receptors?

Peptide hormones are typically associated with membrane-bound receptors because of their chemical structure and characteristics. Peptide hormones are composed of chains of amino acids, which makes them hydrophilic (water-soluble). Due to their water solubility, they cannot easily cross the lipid bilayer of cell membranes. Instead, they bind to specific receptors located on the cell surface, which can initiate a signaling cascade within the cell upon hormone binding. This leads to various physiological effects, often through the activation of secondary messenger systems.

In contrast, steroid hormones and thyroid hormones are lipophilic (fat-soluble) and can easily diffuse through cell membranes to bind to intracellular receptors. These intracellular receptors then interact directly with DNA to influence gene expression. Fat-soluble hormones, like steroids, also exhibit similar behavior, as they work through intracellular receptors rather than binding to receptors on the cell membrane. Knowing these distinctions highlights why peptide hormones are primarily associated with membrane-bound receptors.