IB Biology Topic C2.1.6: Transmembrane Receptors VS Intracellular Receptors Notes

Receptor Location Follows Signal Solubility

1. A receptor is a protein that recognises a specific signalling molecule and starts a cellular response.
2. Receptors sit in one of two places: transmembrane receptors span the plasma membrane, and intracellular receptors sit in the cytoplasm or nucleus.
3. Which one a signal uses depends on a single property: whether the signal can cross the hydrophobic lipid bilayer.

Water-soluble signals stay outside and use surface receptors; lipid-soluble signals pass through and use internal receptors.

Transmembrane Receptors Catch Signals That Cannot Get In

1. Their ligands are hydrophilic molecules like peptides and neurotransmitters that cannot cross the lipid bilayer.
2. A hydrophobic stretch of amino acids anchors the receptor inside the membrane.
   1. Its hydrophilic ends stay exposed to the watery fluid on each side, with the ligand-binding site facing outward.
3. The signal never enters the cell: binding changes the receptor's shape and triggers signal transduction across the membrane instead.

Intracellular Receptors Catch Signals That Slip Through

1. Their ligands are lipophilic and small, such as steroid hormones, so they diffuse straight through the lipid bilayer.
2. The receptor has a hydrophilic surface so it stays soluble in the watery cytoplasm or nucleus.
3. The signal enters the cell and binds its receptor inside, and the complex typically acts as a transcription factor that changes gene expression.

The Two Types Side by Side

Solubility Decides the Route, Not Chance

1. A signal cannot choose its receptor: its solubility forces where the receptor must be.
2. Hydrophilic signals are blocked by the membrane, so their receptors must sit on the surface where the signal can reach them.
3. Lipophilic signals pass through freely, so their receptors can wait inside where they act directly on genes.