Final answer:
Peptide hormones are water-soluble and circulate freely in the bloodstream as they do not require transport proteins, unlike lipid-soluble hormones.
Step-by-step explanation:
Peptide hormones are water-soluble, allowing them to travel freely in the bloodstream without the need for a transport protein. Unlike steroid and thyroid hormones, which require specific binding proteins due to their lipid-soluble nature, peptide hormones including insulin and catecholamines circulate in free form. The lipid-insoluble property of these hormones means they do not pass through plasma membranes and instead bind to cell surface receptors on their target cells. This is crucial for their function as signaling molecules, initiating a cascade of events within the cell to exert their biological effects.