Problem 19 - Entrance Test

This question asks for a key difference in the mechanisms of action of steroid and peptide hormones. * Steroid Hormones (e.g., cortisol, estrogen, testosterone): * Lipid-soluble: They are derived from cholesterol and can easily pass through the lipid bilayer of the cell membrane. * Intracellular receptors: Their receptors are located in the cytoplasm or nucleus of target cells. * Direct gene influence: Upon binding to their receptors, the hormone-receptor complex typically translocates to the nucleus and acts as a transcription factor, directly binding to DNA (hormone response elements) to alter gene expression, leading to the synthesis of new proteins. This process is generally slower but has long-lasting effects. * Transport: They are transported in the blood largely bound to carrier proteins (e.g., albumin, specific binding globulins) because they are lipid-soluble and thus poorly soluble in water (blood plasma). * Peptide Hormones (e.g., insulin, ADH, oxytocin, TSH): * Water-soluble: They are made of amino acids and cannot easily pass through the cell membrane. * Cell surface receptors: Their receptors are located on the outer surface of the target cell membrane. * Second messenger systems: Upon binding to their cell surface receptors, they activate intracellular signaling pathways involving second messengers (e.g., cAMP, cGMP, IP3, DAG, Ca2+). These second messengers then initiate a cascade of enzymatic reactions that alter cellular activity. This action is generally rapid. * Transport: They are transported freely in the blood (dissolved in plasma) because they are water-soluble. Let's evaluate the options: A. Incorrect. This is reversed. Steroid hormones bind to intracellular receptors, and peptide hormones bind to cell surface receptors. B. Incorrect. This is reversed. Peptide hormones typically act rapidly via second messenger systems, while steroid hormones alter gene expression over a longer period. C. Correct. Peptide hormones, being unable to cross the cell membrane, rely on secondary messenger systems to relay their signal inside the cell. Steroid hormones, being lipid-soluble, cross the cell membrane and directly bind to intracellular receptors, influencing gene transcription. D. Incorrect. While steroid hormones require carrier proteins, peptide hormones are typically transported freely dissolved in the blood due to their water-soluble nature. So, 'both' is incorrect.