Water-soluble ligands are polar and therefore cannot pass through the plasma membrane unaided sometimes, they are too large to pass through the membrane at all. Because these molecules are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors. Steroid hormones have similar chemical structures to their precursor, cholesterol. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while they are being transported through the bloodstream. Other hydrophobic hormones include thyroid hormones and vitamin D. Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen the male sex hormone, testosterone and cholesterol, which is an important structural component of biological membranes and a precursor of steriod hormones ( Figure). Steroids are lipids that have a hydrocarbon skeleton with four fused rings different steroids have different functional groups attached to the carbon skeleton. Important members of this class of ligands are the steroid hormones. Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. The types of molecules that serve as ligands are incredibly varied and range from small proteins to small ions like calcium (Ca 2+). Produced by signaling cells and the subsequent binding to receptors in target cells, ligands act as chemical signals that travel to the target cells to coordinate responses.
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