Final answer:
Receptor sites are hydrophobic as they can bind to hydrophobic ligands, whereas cell membranes are amphipathic due to having both hydrophilic heads and hydrophobic tails.
Step-by-step explanation:
Receptor sites on proteins can either bind to hydrophobic or hydrophilic ligands. Internal receptors are usually integral membrane proteins that bind to hydrophobic ligands, such as steroid hormones that can cross the cell membrane, and they initiate a signaling cascade that influences gene transcription and protein synthesis. Cell surface receptors, on the other hand, are adapted to bind to hydrophilic ligands that cannot cross the membrane on their own. Upon ligand binding, these receptors set off a different type of signaling cascade within the cell.
Cell membranes have a complex structure primarily composed of phospholipid molecules that are amphipathic. This means that they have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. When phospholipids assemble into a bilayer to form a cell membrane, their hydrophilic heads face towards the aqueous environment while the hydrophobic tails face inward, away from water. This arrangement allows the cell membrane to effectively separate the internal cell environment from the external one.
Based on the information provided and understanding the characteristics of receptor sites and cell membranes, the answer to the student's question would be: Receptor sites are hydrophobic, and cell membranes are amphipathic.