Chapter 7 – Signal Transduction and Cell Communication vbnet Copy Edit

Signal transduction pathway

A series of molecular events by which a signal on a cell's surface is converted into a specific cellular response.

Paracrine

A signaling mechanism where the target cells are near the signal-releasing cells.

Juxtacrine

Signaling between adjacent cells through direct contact or shared cytoplasm.

Autocrine

A type of signaling where a cell targets itself with signals it produces.

Hormonal

Signaling over long distances via hormones that travel through the bloodstream.

Gap junctions

Specialized intercellular connections between animal cells that allow direct communication.

Plasmodesmata

Cytoplasmic channels between plant cells that pass through cell walls and allow molecule exchange.

Signaling networks

Complex interactions between multiple signaling pathways within a cell.

Receptor

A protein that detects a signal molecule (ligand) and performs an action in response.

Connexon

Protein subunits that form gap junctions between cells.

Connexin

The protein that makes up connexons in gap junctions.

Desmotubule

A tube-like structure in plasmodesmata that connects the ER of adjacent cells.

Ligand

A molecule that binds specifically to a receptor site of another molecule.

G protein-coupled receptor

A membrane receptor that works with the help of a G protein to relay signals.

Protein kinase receptor

A receptor that activates its own kinase activity upon ligand binding.

Ion channel receptor

A receptor that acts as a gate for ions when it changes shape.

Intracellular receptors

Receptors located inside the cell that bind to hydrophobic signal molecules.

Agonists

A molecule that mimics a signal and activates a receptor.

Antagonists

A molecule that blocks or dampens a signal by binding to a receptor.

Phosphorylation

The addition of a phosphate group to a molecule, often used to activate proteins.

Kinase

An enzyme that catalyzes phosphorylation by transferring phosphate from ATP.

Tyrosine

An amino acid often targeted in phosphorylation for signal transduction.

Phosphatase

An enzyme that removes phosphate groups from proteins.

G protein

A protein that binds GTP and activates downstream signaling pathways.

Effector protein

A protein activated by a receptor that produces a cellular response.

Reception, Transduction, Response

The three basic stages of cell signaling: detecting a signal (reception), relaying it internally (transduction), and generating a cellular effect (response).

Gap Junctions

Direct cytoplasmic connections between adjacent animal cells that allow molecules and ions to pass freely.

Crosstalk

Interaction between different signaling pathways that can enhance or inhibit each other.

Receptor Locations

Receptors may be membrane-bound (e.g., GPCRs, kinase receptors) or intracellular for hydrophobic signals like steroids.

GPCR Mechanism

Ligand binds to GPCR, which activates a G protein by replacing GDP with GTP, triggering downstream effects.

Protein Kinase Receptor Mechanism

Ligand binding activates the receptor's kinase domain, leading to autophosphorylation and signal relay.

Ion Channel Receptor Mechanism

Binding of ligand opens or closes the ion channel, allowing ions to flow across membranes.

Intracellular Receptor Mechanism

Receptors inside the cell bind hydrophobic ligands and often act as transcription factors.

Caffeine Action

Caffeine blocks adenosine receptors, preventing drowsiness and increasing alertness.

Acetylcholine Receptor Action

Ligand binding opens a channel allowing Na+ to flow into neurons, initiating muscle contraction.

Insulin Receptor Action

Binding of insulin activates kinase activity, leading to glucose uptake through GLUT transporters.

Amplification in Signaling

One ligand-receptor interaction can activate multiple downstream proteins, amplifying the signal.

Ras and Cancer

Mutated Ras in cancer cells stays active, continuously sending growth signals.

Tissue-Specific Responses

Different cells respond differently to the same signal based on receptor type and intracellular machinery.

cAMP Production

Adenylyl cyclase converts ATP to cAMP, a key second messenger in many pathways.

Protein Kinase Cascade

A series of kinases activate each other in sequence, amplifying the original signal.

Nitric Oxide Production

NO is synthesized from arginine by nitric oxide synthase and diffuses to cause smooth muscle relaxation.

Calcium Mobilization

Ca²⁺ stored in ER is released in response to IP₃, triggering downstream effects.

IP3/DAG Pathway

Phospholipase C cleaves PIP2 into IP3 and DAG; IP3 releases Ca²⁺, DAG activates PKC.

Bipolar Disorder and IP3

IP3 signaling is implicated in bipolar disorder; lithium alters this pathway.

Smell and Nervous System

Odorant receptors trigger GPCR pathways leading to neuron firing in olfactory system.

Fight or Flight (Epinephrine)

Epinephrine activates cAMP, which phosphorylates enzymes to break down glycogen for energy.

Signal → Gene Transcription

Some pathways activate transcription factors that regulate gene expression.

Signal Regulation

Phosphatases and phosphodiesterases deactivate signals by removing phosphate or degrading cAMP.

Viagra Mechanism

Viagra inhibits phosphodiesterase that degrades cGMP, maintaining blood vessel dilation.

Plasmodesmata

Plant cell junctions that allow signaling molecules to pass between cells.

Volvocine Line & Multicellularity

Green algae show a range from unicellular to colonial to multicellular forms, illustrating evolution of complexity.

Adenylyl cyclase

An enzyme that converts ATP to cAMP in response to a signal.

Phosphodiesterase

An enzyme that breaks down cAMP into AMP, terminating the signal.

GDP

A nucleotide that binds to inactive G proteins.

GTP

A nucleotide that activates G proteins when bound.

Phosphorylation cascade

A series of protein activations where each step amplifies the signal.

First messenger

The initial signaling molecule that binds to the receptor.

Secondary messengers

Small molecules like cAMP, Ca²⁺, IP₃ that relay signals from receptors to target molecules.

cAMP

A common secondary messenger involved in activating protein kinases.

Ca²⁺

A calcium ion that acts as a versatile second messenger in many pathways.

Nitric oxide

A gaseous signaling molecule involved in smooth muscle relaxation.

Diacylglycerol (DAG)

A lipid-derived second messenger involved in PKC activation.

Inositol triphosphate (IP3)

A second messenger released from membrane lipids; helps release Ca²⁺ from ER.

Glycogen phosphorylase

An enzyme that breaks down glycogen into glucose-1-phosphate.

Phospholipase

An enzyme that cleaves phospholipids to produce DAG and IP₃.

Phospholipase C

A specific enzyme that cleaves PIP₂ into IP₃ and DAG.

PIP2

A membrane phospholipid cleaved into IP₃ and DAG during signaling.

Nitric oxide

An enzyme that synthesizes nitric oxide from arginine.