Overview
This lecture covers the types of signaling mechanisms in multicellular organisms, differences between internal and cell-surface receptors, and the relationship between ligand structure and signaling action.
Basics of Cell Communication
- Intercellular signaling occurs between cells; intracellular signaling happens within a cell.
- Ligands are chemical signals that bind receptors on target cells to convey messages.
- Each ligand usually binds a specific receptor, initiating a cellular response.
Forms of Signaling
- Paracrine signaling acts locally between nearby cells via ligand diffusion and typically has quick, short-lived effects.
- Endocrine signaling involves hormones traveling long distances via the bloodstream, producing slower, longer-lasting effects.
- Autocrine signaling occurs when a cell targets itself or similar neighboring cells, important in development and immune responses.
- Direct signaling via gap junctions (animals) or plasmodesmata (plants) allows small molecules/ions to pass directly between adjacent cells.
Types of Receptors
- Internal (intracellular) receptors are located in the cytoplasm and bind hydrophobic ligands, directly influencing gene expression.
- Cell-surface receptors (transmembrane) bind external ligands and transmit signals into the cell without ligand entry.
Categories of Cell-Surface Receptors
- Ion channel-linked receptors open or close ion channels in response to ligand binding.
- G-protein-linked receptors activate G-proteins, which then trigger intracellular signaling cascades.
- Enzyme-linked receptors have intracellular domains associated with enzymes; ligand binding activates enzymatic functions, e.g., tyrosine kinase activity.
Ligand Types and Specificity
- Small hydrophobic ligands (e.g., steroid hormones) diffuse across membranes and bind internal receptors; often require carrier proteins in blood.
- Water-soluble ligands (e.g., peptides, proteins) cannot cross membranes and bind to cell-surface receptors.
- Some gases, like nitric oxide (NO), directly diffuse across membranes and act as local signaling molecules.
Viruses and Cellular Receptors
- Viruses lack cell membranes and rely on binding to specific cell-surface receptors to infect hosts.
- Changes in viral surface proteins or host receptors can alter host specificity, sometimes enabling viruses to infect new species.
Key Terms & Definitions
- Ligand — a molecule that binds specifically to another molecule, usually a receptor, to signal a response.
- Receptor — a protein on or in a cell that binds a signaling molecule (ligand) to trigger a response.
- Paracrine signaling — cell signaling over short distances to nearby cells.
- Endocrine signaling — long-distance signaling via hormones traveling through the bloodstream.
- Autocrine signaling — cell signaling to itself or similar neighboring cells.
- Gap junctions/plasmodesmata — direct cell-to-cell communication channels in animals/plants.
- Ion channel-linked receptor — receptor that opens or closes ion channels in response to ligand binding.
- G-protein-linked receptor — membrane receptor that activates a G-protein to relay signals.
- Enzyme-linked receptor — receptor whose intracellular domain acts as or interacts with an enzyme.
Action Items / Next Steps
- Review and compare the four forms of cell signaling and their physiological examples.
- Study major categories of cell-surface receptors and their mechanisms.
- Prepare for questions on ligand properties and how they determine receptor type and signaling mechanism.