Overview
This lecture covers the mechanisms of cell communication, focusing on signal transduction, types of signaling (local and long-distance), and the molecular details of hormone signaling in plants and animals.
Cell Communication and Signal Transduction
- Cell-to-cell communication in multicellular organisms involves transmitting chemical messages called signals.
- Signal transduction is the process of receiving, transmitting, and responding to signals, either generally (initiation to response) or specifically (reception to response).
- A signal response can affect cytoplasmic activities or gene expression (turning genes on or off).
Types of Cell Signaling
- Autocrine (Intracellular) Signaling: Signal initiation, reception, and response occur within the same cell.
- Local Signaling: Signals act on neighboring cells, often via gap junctions (animals) or plasmodesmata (plants).
- Cell Surface Macromolecule Signaling: Signal transmission occurs between surface proteins of adjacent cells (mainly in animals).
- Paracrine Signaling: Locally produced regulators (not true hormones) diffuse to influence cells in the vicinity.
- Synaptic Signaling: Unique to neurons, involving neurotransmitters crossing the synapse between nerve cells.
Long-Distance (Hormonal) Signaling
- Plants: Hormones called growth substances (e.g., auxin) are produced by tissues and diffuse to target cells, leading to varied effects (growth, response to light/gravity).
- Animals: Hormones are produced by endocrine glands; peptide hormones bind to cell-surface receptors, while steroid hormones enter cells directly to influence target tissues specifically.
Signal Reception and Transduction Mechanisms
- Steroid Hormones: Non-polar molecules cross the membrane, bind intracellular receptors, and regulate gene transcription in the nucleus.
- Peptide Hormones: Polar, large molecules bind to cell-surface receptors, activating relay molecules (e.g., G proteins) to transmit the signal inside the cell.
G Protein and Signal Relay Pathways
- G proteins act as intermediaries, transferring the signal from the receptor to other intracellular molecules.
- Phosphorylation Cascade: G protein activates a series of protein kinases, each activating the next via phosphorylation, culminating in a cellular response.
- Second Messenger Mechanism: G protein activates adenylyl cyclase, producing cyclic AMP (cAMP) as the second messenger, which then activates a protein kinase to elicit a response.
Key Terms & Definitions
- Signal Transduction — The cellular process of receiving, transmitting, and responding to a signal.
- Autocrine Signaling — Communication within the same cell.
- Paracrine Signaling — Local signaling to nearby cells.
- Synaptic Signaling — Nerve cell communication via neurotransmitters across a synapse.
- Hormone — Chemical messenger influencing cells at a distance.
- Steroid Hormone — Lipid-soluble hormone that enters cells and regulates gene transcription.
- Peptide Hormone — Water-soluble hormone that binds cell-surface receptors.
- G Protein — Membrane protein relaying signals from receptors to effectors inside the cell.
- Phosphorylation Cascade — A sequence of events where kinases activate each other via phosphorylation.
- Second Messenger (cAMP) — Intracellular signaling molecule activated by adenylyl cyclase in response to an external signal.
Action Items / Next Steps
- Review the distinction between phosphorylation cascade and second messenger mechanisms.
- Understand the different modes of signaling and be able to classify examples.
- Be familiar with the roles of steroid vs. peptide hormones in signal reception and transduction.