Final answer:
Steroid hormones can directly enter cells and bind to intracellular hormone receptors to affect gene expression. Non-steroid hormones cannot directly enter cells and bind to cell surface receptors, activating second messenger systems within the target cell.
Step-by-step explanation:
Hormones can affect cells through two general modes of action: steroid hormones and non-steroid hormones. Steroid hormones, such as estradiol and testosterone, are lipid-derived and can directly enter cells. They bind to intracellular hormone receptors, forming hormone-receptor complexes that affect gene expression in the nucleus of the target cell. On the other hand, non-steroid hormones, such as adrenaline and oxytocin, are peptide or amino acid-derived hormones that cannot directly enter cells. They bind to cell surface receptors, activating second messenger systems that affect various processes within the target cell.