What is a common characteristic of water-soluble hormones?

Water-soluble hormones, such as insulin and glucagon, are primarily composed of amino acids, either in the form of polypeptides or as derivatives of amino acids. Their structure allows these hormones to dissolve in the bloodstream, which is primarily composed of water, facilitating their transport throughout the body to target tissues. Upon reaching their target cells, they bind to receptors on the cell surface, which triggers a cascade of signaling events inside the cell. This binding is essential for initiating physiological responses without entering the cell, contrasting with lipid-soluble hormones, which can easily cross cellular membranes due to their hydrophobic nature.

In this context, the other options do not align with the defining characteristics of water-soluble hormones. While lipid-based hormones are typically hydrophobic and can pass through cellular membranes directly, this is not applicable to water-soluble hormones. The belief that water-soluble hormones can only function within the cell is incorrect; they actually exert their actions from outside the cell by interacting with surface receptors. Additionally, the assertion regarding high energy requirements does not apply specifically to the functionality of these hormones, as their signaling mechanisms often rely on more efficient pathways that do not require significant energy expenditure.