Receptors and cell signaling

What is a ligand?

A molecule that binds a specific protein, such as a receptor.

What makes a cell a target for a signal?

It has the appropriate receptor and cellular machinery to respond.

Where are cell-surface receptors located?

In the plasma membrane, with regions facing outside and inside the cell.

Which ligands usually bind cell-surface receptors?

Hydrophilic or large ligands that cannot readily cross the bilayer.

Where are intracellular receptors located?

In the cytoplasm or nucleus.

Which ligands usually bind intracellular receptors?

Small hydrophobic ligands, such as steroid hormones.

What is direct signaling?

Communication through cell contact or connections between neighboring cells.

What is juxtacrine signaling?

A membrane-bound ligand interacts with a receptor on a touching cell.

What direct connections occur in animals and plants?

Gap junctions in animals; plasmodesmata in plants.

What is paracrine signaling?

A released signal acts on nearby cells.

What is autocrine signaling?

A released signal acts on the same cell that released it.

What is neuronal signaling?

A neuron releases neurotransmitters across a synapse to its target.

What is endocrine signaling?

Hormones travel through the bloodstream to distant target cells.

What are the five steps of cell signaling?

Signal → reception → transduction → response → termination.

What is signal transduction?

The process of relaying a received signal inside the cell.

What is a second messenger?

A small intracellular molecule or ion, such as cAMP or Ca²⁺, that relays a signal.

What responses can signaling cause?

Gene expression, altered metabolism, secretion, cell growth, or apoptosis.