Receptor sites on the outside of the plasma membrane are

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The question asks about the nature of receptor sites located on the exterior surface of the plasma membrane.
Option 1: carbohydrates. While carbohydrates can be present on membrane proteins and lipids as glycoproteins or glycolipids, the receptor sites themselves are typically built from proteins that recognize and bind specific molecules. Carbohydrates by themselves are not generally the functional receptors that mediate signal transduction. This makes this choice misleading.
Option 2: proteins. This is the correct understanding: many membrane receptors are proteins—either integral membrane proteins or G-protein-coupled receptors—that bind ligands outside the cell and trigger a cascade inside the cell. The binding domain is arranged to recognize specific shapes, charges, or chemical groups on signaling molecules, transmitting information across the membrane.
Option 3: cyclic AMP. Cyclic AMP is a second messenger produced inside the cell in response to receptor activation, not a receptor itself. It participates in intracellular signaling once generated, so it is not the extracellular receptor site.
Option 4: ATP. While ATP can act as a signaling molecule in certain contexts (e.g., purinergic signaling), it is not the general term for extracellular receptor sites on the plasma membrane. Receptors that bind extracellular ligands are proteins; ATP can serve as a ligand for specific receptor proteins, but ATP itself is not the receptor site.
In summary, among the options, proteins best describe the structural and functional nature of receptor sites on the outside of the plasma membrane, whereas the other choices either describe ligands or intracellular messengers rather than the receptor structures themselves.

ANSC20001_2025_SM1 Week 2 - The plasma membrane and membrane potential (Example MCQs)

Receptor sites on the outside of the plasma membrane are

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