Final answer:
Steroid hormones act on target cells by binding to internal receptors in the cytoplasm or nucleus, forming a complex that influences gene expression.
Step-by-step explanation:
Steroid hormones in the human body act on target cells by binding to internal receptors. These receptors are located either in the cytoplasm or within the nucleus of the cell. Since steroid hormones are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell. After penetrating the cell membrane, they bind to internal receptors and form a hormone-receptor complex. This complex then moves to the chromatin within the cell nucleus and binds to specific segments of the cell's DNA.
Once bound to DNA, this complex functions as a transcription regulator, influencing gene expression by increasing or decreasing the synthesis of mRNA. The resulting mRNA is translated into proteins that alter the cell's structure or function by catalyzing chemical reactions, thus regulating specific cell processes.
Examples of steroid hormones include cortisol and sex hormones such as estrogen, which are made from lipids similar to cholesterol. Their ability to diffuse through the membrane and bind intracellularly distinguishes them from hormones that cannot cross membranes easily and instead bind to surface receptors.