Overview
This section explains how cells use chemical signals and receptors to communicate, focusing on the types of signaling mechanisms and cellular receptors in multicellular organisms.
Types of Cellular Communication
- Intercellular signaling is communication between cells; intracellular signaling is communication within a cell.
- Signaling molecules, called ligands, are released by signaling cells and bind to specific receptors on target cells.
Forms of Signaling in Multicellular Organisms
- Paracrine signaling acts locally between nearby cells via diffusion through the extracellular matrix.
- Endocrine signaling uses hormones, which travel through the bloodstream to affect distant cells.
- Autocrine signaling occurs when a cell targets itself with signals it produces.
- Direct signaling across gap junctions allows small molecules to pass directly between adjacent cells.
Paracrine and Synaptic Signaling Examples
- Paracrine signals are rapid and short-lived due to local degradation or reabsorption.
- Synaptic signaling between nerve cells uses neurotransmitters to transmit signals rapidly across synapses.
Endocrine, Autocrine, and Direct Signaling Details
- Endocrine signals are slower but longer-lasting due to dilution in the bloodstream.
- Autocrine signaling is important during development, immune responses, and programmed cell death.
- Gap junctions (animals) and plasmodesmata (plants) allow direct cytoplasmic sharing of small molecules.
Types of Cellular Receptors
- Internal receptors (cytoplasmic) respond to hydrophobic ligands and often directly affect gene expression.
- Cell-surface (transmembrane) receptors bind external ligands and transmit signals into the cell.
Categories of Cell-Surface Receptors
- Ion channel-linked receptors open/closes ion channels in response to ligand binding.
- G-protein-linked receptors activate G-proteins, triggering intracellular signaling cascades.
- Enzyme-linked receptors, including tyrosine kinase receptors, activate intracellular enzymes when ligands bind.
Signaling Molecules (Ligands)
- Small hydrophobic ligands (e.g., steroid hormones) diffuse across membranes and require carrier proteins in blood.
- Water-soluble ligands (e.g., peptides, proteins) bind to cell-surface receptors and can be large or polar.
- Other ligands, like nitric oxide, are gases that can diffuse across membranes and act locally due to their short lifespan.
Key Terms & Definitions
- Ligand — A molecule that binds to a specific receptor to transmit a signal.
- Receptor — A protein on or in a cell that binds a ligand and initiates a response.
- Paracrine signaling — Local signaling between nearby cells.
- Endocrine signaling — Long-distance signaling using hormones transported by blood.
- Autocrine signaling — A cell targets itself with its own signaling molecules.
- Gap junction — A channel allowing direct signal transfer between adjacent cells.
- Internal receptor — A cytoplasmic receptor for hydrophobic ligands.
- Cell-surface receptor — A membrane receptor for hydrophilic ligands.
- Ion channel-linked receptor — Opens/closes ion channels upon ligand binding.
- G-protein-linked receptor — Activates G-proteins to mediate signaling.
- Enzyme-linked receptor — Activates or is associated with intracellular enzymes when bound by a ligand.
Action Items / Next Steps
- Review the four signaling mechanisms and their real-life examples.
- Compare internal and cell-surface receptors, focusing on their ligand types and actions.
- Understand ligand structure (hydrophobic vs. hydrophilic) and how it determines signaling pathways.