USA Biology Olympiad (USABO) Practice Exam

How do steroid hormones differ from most polypeptide hormones regarding cellular permeability?

1. Cannot cross cell membranes

2. Are stored in vesicles

3. Diffuse across cell membranes

4. Bind to surface receptors

The correct answer is: Diffuse across cell membranes

Steroid hormones differ from most polypeptide hormones in their structure and, consequently, their ability to interact with cell membranes. Steroid hormones are derived from cholesterol and are lipophilic (fat-soluble) in nature. This characteristic allows them to easily diffuse across the lipid bilayer of cell membranes without the need for transport mechanisms. Once inside the target cell, steroid hormones can bind to specific intracellular receptors, and the hormone-receptor complex can then influence gene expression by interacting with the DNA in the nucleus. This mechanism stands in contrast to most polypeptide hormones, which are generally hydrophilic (water-soluble) and cannot pass through the lipid bilayer of cell membranes. Instead, polypeptide hormones typically bind to receptors on the surface of the cell, triggering a series of intracellular responses without entering the cell. Thus, the ability of steroid hormones to diffuse across cell membranes is a fundamental aspect of their function, enabling them to initiate physiological responses directly within the cell.