Final answer:
Insulin is the hormone made of amino acids that does not easily cross cell membranes, as it is a non-steroid hormone that is water-soluble, unlike lipid-soluble hormones like testosterone and estrogen.
Step-by-step explanation:
The hormones that are made of amino acids and do not cross cell membranes easily are non-steroid hormones such as insulin. These hormones are water-soluble and because of their solubility, they cannot diffuse through the lipid bilayer of cell membranes. Instead, they bind to specific receptor proteins on the outside surface of cell membranes. Upon binding, these hormones trigger an internal second messenger system that affects cell functions.
Testosterone, estrogen, and other steroids are lipid-soluble hormones and therefore can easily cross cell membranes to bind with internal receptors, influencing gene expression from within the cell.