Final answer:
Estradiol and testosterone are considered lipids because they are steroid hormones derived from cholesterol with a structure allowing them to diffuse through lipid membranes and influence gene expression.
Step-by-step explanation:
The vertebrate sex hormones estradiol and testosterone are considered to be lipids because they are classified as steroid hormones, which are a type of lipid-derived hormone. These hormones are synthesized from cholesterol, and their structure is characterized by a hydrocarbon skeleton with four fused rings, typical of the steroid classification. Like other lipids, steroid hormones are fat-soluble, allowing them to easily diffuse across the plasma membranes of target cells, which are also lipid-rich environments. Once inside the cell, these hormones bind to specific receptor proteins and form a hormone-receptor complex that can move into the nucleus and directly influence gene expression.
Another characteristic of these steroid hormones is their insolubility in water. To travel through the bloodstream to their target cells, they must bind to transport proteins, which greatly extends their circulating half-life compared to peptide hormones, which are water-soluble. For instance, the steroid hormone cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, which is a peptide hormone, has a half-life of approximately one minute.
Testosterone and estradiol are significant in regulating a vast array of physiological processes, primarily reproductive functions. They are produced by the gonads (testes in males and ovaries in females) and other locations such as the adrenal glands, which also release other types of steroid hormones like aldosterone and cortisol. The fact that these hormones are lipid-soluble is crucial for their mode of action, as it allows them to penetrate cell membranes and affect cellular processes by modulating gene expression.