Final answer:
Steroid hormones require transport proteins to remain soluble in the bloodstream for transport. They diffuse across the plasma membranes of target cells and bind to intracellular receptors to influence gene expression.
Step-by-step explanation:
Steroids, such as estrogen and testosterone, are lipid-soluble hormones that can diffuse across the plasma membrane of cells due to their hydrophobic nature. For steroids to be transported over long distances in the bloodstream, they must remain soluble; hence, they bind to transport proteins.
These proteins, often called binding, carrier, or transport proteins, such as Thyroxine-binding globulin (TBG), transport the hormones to their target cells.
Once reaching their destination, the steroids are released from the carrier protein and can easily diffuse across the lipid bilayer of target cells' plasma membranes. Inside the cell, they bind to specific intracellular receptors and form a complex that enters the nucleus, influencing gene expression by acting as transcription factors.