Cell Communication: Direct and Indirect Signaling and Chemical Messengers

A set of Question-and-Answer flashcards covering direct vs. indirect cell communication, ligand/receptor basics, messenger classifications (paracrine, autocrine, neurotransmitters, hormones, cytokines), lipophobic vs lipophilic messengers, storage/release mechanisms, receptor types, GPCR signaling and amplification, second messengers, and key examples.

What are the two general mechanisms of cell-to-cell communication discussed in the lecture?

Direct communication via gap junctions and indirect communication via chemical messengers.

What structures form the channels that enable direct cell-to-cell communication between adjacent cells?

Gap junctions formed by connexons (connexin subunits).

What can pass through gap junction channels to enable direct communication between neighboring cells?

Cytosolic contents, ions, and other small signaling molecules.

How does indirect (chemical) communication differ from direct communication in terms of messenger delivery?

Indirect communication uses secreted chemical messengers that diffuse to target cells and bind to receptors to elicit a response.

How do chemical messengers reach their target cells in indirect signaling?

They diffuse down their concentration gradient until they bind to receptors on the target cell.

What is a ligand in the context of cell signaling?

Any molecule that binds reversibly to a protein (typically a receptor) to elicit a response.

What determines receptor specificity for a chemical messenger?

Receptors have specific shapes that fit the ligand; binding is highly specific and reversible.

Define paracrine signaling and give an example mentioned in the notes.

A local signaling mechanism where the messenger diffuses through interstitial fluid to nearby cells; example: histamine.

Define autocrine signaling.

A chemical messenger is released by a cell and binds to receptors on the same cell that released it.

What is a neurotransmitter and where is it typically released?

A chemical messenger produced by neurons and released into a synaptic cleft for local action.

What is a neurohormone?

A hormone released by a neuron into the bloodstream to act on distant targets.

What are cytokines and where do they typically act?

Immune cell–derived chemical messengers that usually act locally via paracrine signaling.

What distinguishes lipophobic (hydrophilic) ligands from lipophilic (lipophilic) ligands regarding receptor location?

Lipophobic ligands bind to membrane-bound receptors; lipophilic ligands bind to intracellular receptors (inside the cell).

Why can’t lipophobic ligands cross the cell membrane on their own?

Because they are water-soluble and the membrane is a lipid bilayer; they need membrane receptors to transmit signals.

How are lipophobic ligands stored and released?

Stored in secretory vesicles and released by exocytosis; receptors are on the cell membrane.

How are lipophilic ligands produced and released, and where are their receptors located?

Produced on demand (not stored in vesicles), released by diffusion; receptors are intracellular (cytosol or nucleus) and often bound to carrier proteins in blood.

Name the four lipophobic ligand groups and the two lipophilic ligand groups discussed.

Lipophobic: amino acids, amines, peptides, and proteins. Lipophilic: steroids and eicosanoids.

What are catecholamines and how are they formed?

Dopamine, norepinephrine, and epinephrine; derived from the amino acid tyrosine and can act as neurotransmitters or hormones depending on release context.

What are peptide and protein ligands, and how are they produced and released?

Lipophobic messengers produced in cells, stored in vesicles, and released by exocytosis; examples include enkephalins and endorphins.

What are steroids and eicosanoids, and how do they differ in storage and receptor location?

Lipophilic messengers derived from cholesterol (steroids) or arachidonic acid (eicosanoids); produced on demand and act via intracellular receptors.

What is the mechanism by which aspirin affects signaling related to prostaglandins?

Aspirin inhibits cyclooxygenase, reducing prostaglandin synthesis and pain signaling.

What is the pathway of second messengers in a G protein–coupled receptor (GPCR) signal via phospholipase C?

Ligand binds receptor -> G protein activates phospholipase C -> PIP2 is converted to DAG and IP3; DAG activates PKC, IP3 triggers Ca2+ release from the endoplasmic reticulum.

What is signal amplification in GPCR signaling?

A single ligand-bound GPCR can activate multiple downstream enzymes and channels, greatly amplifying the cellular response.

What three factors determine the magnitude of the target cell response after ligand binding?

Concentration of the messenger, number of receptors, and the affinity of the receptor for the messenger.

What are upregulation and downregulation in receptor dynamics?

Upregulation: increase in receptor number to heighten sensitivity; Downregulation: decrease in receptor number leading to tolerance.

Give an example of downregulation/tolerance discussed in the notes.

Exposure to MDMA reduces serotonin receptor numbers; effects can persist for months after exposure.

What are the two long-distance communication options discussed for chemical messengers?

Nervous signaling via long axons (neurotransmitters at synapses) and endocrine signaling via bloodstream (hormones).