Biological Systems MCAT Practice Exam 2026 – Complete Study Resource

Peptide hormones primarily utilize second messenger systems, such as cAMP, to exert their effects on target cells. Unlike steroid hormones, which can pass through the cell membrane and directly influence gene transcription because of their lipophilic nature, peptide hormones are hydrophilic and cannot cross the membrane. Instead, they bind to specific receptors located on the surface of target cells.

When a peptide hormone binds to its receptor, it typically activates a G-protein coupled receptor (GPCR). This activation leads to the production of second messengers like cyclic adenosine monophosphate (cAMP) or inositol trisphosphate (IP3), which then propagate the signal within the cell. These second messengers initiate various intracellular responses, such as enzyme activation, changes in ion permeability, or a variety of cellular processes.

This method of signaling allows peptide hormones to amplify their effects rapidly and efficiently across the cell membrane without needing to penetrate it. Because of this mechanism, cAMP and other second messengers play a crucial role in the regulatory functions of peptide hormones in many physiological processes.