Final answer:
When synthesized at the rough ER, peptide hormones are in the form of polypeptide chains. These chains go through post-translational modifications, are packaged into vesicles, and when released, activate specific receptors on the surface of target cells using a second messenger system.
Step-by-step explanation:
Peptide hormones, when synthesized at the rough endoplasmic reticulum (RER), are in the polypeptide chain stage. These chains are inserted directly into the ER and begin their journey towards becoming functional hormones. As they are synthesized, the growing polypeptide moves through the cisternae of the RER where it undergoes post-translational modifications before being packaged into vesicles. Secreted peptides, such as insulin, are typical examples of peptides that are synthesized on the RER. They begin as precursor proteins that are processed within the RER to become mature, functional hormones, ready for storage in vesicles. Once appropriately triggered, these hormones are released into the bloodstream, where they cannot pass through plasma membranes due to their water-soluble nature, hence their receptors are found on the cell surface. Furthermore, these hormones use second messenger systems, such as cAMP, IP3, or DAG, to relay their signals inside the target cells since they cannot penetrate the cell membrane. This step is crucial for initiating the hormone's specific effects without directly interacting with the cell's genome. The hormone's receptor interactions and the subsequent signalling pathway activation illustrate the complex nature of the cellular response to hormonal stimuli. Through this, peptide hormones regulate numerous physiological processes, including metabolism, growth, and the release of other hormones.