What type of molecule is the primary target of steroid hormones within the cell?

Steroid hormones primarily target nuclear receptors within the cell. These hormones, being lipophilic, can easily pass through the cell membrane and bind to specific receptors located in the cytoplasm or nucleus. Once bound to the nuclear receptor, the steroid-hormone complex undergoes a conformational change that allows it to interact with DNA in the nucleus. This interaction leads to the regulation of gene expression, ultimately influencing various physiological processes such as metabolism, growth, and reproduction.

Nuclear receptors function as transcription factors that, upon hormone binding, can either activate or repress the transcription of target genes. This mechanism is critical because it allows steroid hormones to exert their effects over longer time periods, in contrast to hormones that act through cell membrane receptors, which often produce more immediate effects.

The other types of molecules mentioned do not serve as primary targets for steroid hormones. For instance, cell membrane receptors are typically associated with peptide hormones, while plasma proteins may transport steroid hormones in the bloodstream but do not act as sites for hormone action. Nuclear DNA, while essential for the cellular processes that the steroid hormones regulate, is not the immediate target of the hormone itself; it is the nuclear receptors that mediate the interaction between the steroid hormones and DNA, leading to the desired cellular response