chapter 9 cell communication

Intercellular signaling

Communication between cells.

Intracellular signaling

Communication within a cell.

Ligands

Chemical signals released by signaling cells in the form of small, usually volatile or soluble molecules that bind to another specific molecule to deliver a signal.

Target cells

Cells that are affected by chemical signals because they contain specific receptor proteins that bind those signals.

Paracrine signaling

A form of chemical signaling where signals act locally between cells that are close together by diffusing through the extracellular matrix, eliciting quick and short-lasting responses.

Synapse

The junction between nerve cells where signal transmission occurs.

Neurotransmitters

Chemical ligands released from a presynaptic nerve cell that travel across a chemical synapse of $20\text{--}40\text{ nm}$ to bind receptors on a postsynaptic cell.

Endocrine signaling

A form of signaling involving signals that originate from distant endocrine cells, travel via the bloodstream, and produce slower but longer-lasting responses.

Hormones

Signaling molecules produced in endocrine glands that travel large distances through the bloodstream at low concentrations to act on target cells in other body regions.

Autocrine signaling

A form of signaling in which the signaling cell secretes a ligand that binds to receptors on its own surface or to neighboring cells of the same type.

Intracellular mediators

Small signaling molecules or ions, such as calcium ions ($\text{Ca}^{2+}$), that diffuse directly between adjacent cells through gap junctions or plasmodesmata.

Plasmodesmata

Fluid-filled channels connecting the plasma membranes of neighboring plant cells, creating a plant-wide communication network.

Internal receptors

Receptors found in the cytoplasm of a cell (also called intracellular or cytoplasmic receptors) that bind hydrophobic ligands that cross the plasma membrane and directly regulate gene transcription.

Gene expression

The cellular process of transforming the information in a cell's DNA into a sequence of amino acids that forms a protein.

Cell-surface receptors

Membrane-anchored integral proteins spanning the plasma membrane that bind external ligands and transduce extracellular signals into intracellular signals.

Signal transduction

The process through which an extracellular signal bound to a cell-surface receptor is converted into an intracellular signal.

Ion channel-linked receptors

Cell-surface receptors that bind a ligand and open a channel across the plasma membrane to allow specific ions, such as sodium, calcium, magnesium, and hydrogen, to pass through.

G-protein-linked receptors

Cell-surface receptors containing seven transmembrane domains that bind a ligand and activate an associated heterotrimeric G-protein by facilitating the exchange of GDP for GTP.

Choleragen

A toxin produced by the bacterium Vibrio cholerae that modifies a G-protein controlling an intestinal chloride channel, locking it active and leading to severe fluid loss and dehydration.

Enzyme-linked receptors

Cell-surface receptors with a single alpha-helical transmembrane region whose intracellular domain is either an enzyme or directly interacts with an enzyme.

Kinase

An enzyme that transfers phosphate groups from ATP to another molecule or protein.

Tyrosine kinase receptor

An enzyme-linked receptor that dimerizes upon ligand binding and transfers phosphate groups from ATP to tyrosine residues on its intracellular domain.

Dimerization

The binding of two neighboring receptor molecules to each other to form a stable complex, enabling their intracellular domains to interact and activate.

Steroids

Hydrophobic lipid ligands characterized by a hydrocarbon skeleton with four fused rings that can directly diffuse across the plasma membrane to bind internal receptors.

Nitric oxide (NO)

A gaseous ligand with a very short half-life that diffuses directly across the plasma membrane to interact with smooth muscle receptors and induce tissue relaxation.

Signaling pathway

A cascade of biochemical events inside a cell triggered by receptor activation, where enzymes, second messengers, and activated proteins sequentially interact.

Signal integration

The process in which signals from two or more different cell-surface receptors merge to activate the same response inside the cell.

Phosphorylation

The addition of a phosphate group ($\text{PO}_4^{3-}$) to a molecule or amino acid residue (such as serine, threonine, or tyrosine), catalyzed by a kinase.

Phosphatase

An enzyme that removes phosphate groups attached to proteins, reversing the effects of kinase-mediated phosphorylation.

Second messengers

Small molecules (such as $\text{Ca}^{2+}$, $\text{cAMP}$, DAG, and $\text{IP}_3$) that propagate and spread a signal through the cytoplasm after the receptor has been activated.

Cyclic AMP (cAMP)

A second messenger synthesized from ATP by adenylyl cyclase that functions primarily to bind and activate cAMP-dependent kinase (A-kinase).

Adenylyl cyclase

An enzyme that converts ATP into cyclic AMP ($\text{cAMP}$) during intracellular signaling.

Phospholipase C

An enzyme that cleaves phosphatidylinositol bisphosphate ($\text{PIP}_2$) into diacylglycerol (DAG) and inositol triphosphate ($\text{IP}_3$).

Diacylglycerol (DAG)

A lipid second messenger produced by the cleavage of $\text{PIP}_2$ that remains in the plasma membrane and activates protein kinase C (PKC).

Inositol triphosphate (IP3)

A second messenger produced by the cleavage of $\text{PIP}_2$ that diffuses into the cytoplasm and binds to ligand-gated calcium channels in the endoplasmic reticulum to release $\text{Ca}^{2+}$.

MAPK/ERK pathway

A phosphorylation cascade (Ras-Raf-MEK-ERK) that communicates signals from a cell-surface receptor to the nucleus to regulate gene transcription and protein translation.

Growth factors

Ligands that promote cell growth and division, typically binding to cell-surface receptor tyrosine kinases (RTKs).

Apoptosis

A mechanism of programmed cell death that enables a damaged, infected, or unneeded cell to die in a controlled manner without releasing damaging internal molecules.

Phosphodiesterase

An enzyme that terminates signal cascades by degrading cyclic AMP ($\text{cAMP}$) into AMP.

Haploid cell

A cell with one-half the normal number of chromosomes.

Diploid cell

A cell with two sets of each chromosome, which is what normal body cells contain.

Mating factor

Signaling molecule secreted by yeast cells to communicate to nearby yeast cells that they are available to mate.

Differential interference contrast microscopy

A light microscopy technique that enhances the contrast of the sample.

Quorum sensing

Method of cellular communication used by bacteria that informs them of the abundance of similar (or different) bacteria in the environment.

Autoinducer

Signaling molecule secreted by bacteria to communicate with other bacteria of its kind.

Acyl-homoserine lactone (AHL)

A small, hydrophobic autoinducer molecule that enters target bacteria and binds to transcription factors, switching gene expression on or off.

Biofilms

Complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host.

Apoptosis

Programmed cell death.

Autocrine signal

Signal that is sent and received by the same or similar nearby cells.

Cell-surface receptor

Cell-surface protein that transmits a signal from the exterior of the cell to the interior, even though the ligand does not enter the cell.

Chemical synapse

Small space between axon terminals and dendrites of nerve cells where neurotransmitters function.

Cyclic AMP (cAMP)

Second messenger that is derived from ATP.

Cyclic AMP-dependent kinase

(Also, protein kinase A, or PKA) kinase that is activated by binding to cAMP.

Diacylglycerol (DAG)

Cleavage product of PIP2 that is used for signaling within the plasma membrane.

Dimer

Chemical compound formed when two molecules join together.

Dimerization

(Of receptor proteins) interaction of two receptor proteins to form a functional complex called a dimer.

Endocrine cell

Cell that releases ligands involved in endocrine signaling (hormones).

Endocrine signal

Long-distance signal that is delivered by ligands (hormones) traveling through an organism's circulatory system from the signaling cell to the target cell.

Enzyme-linked receptor

Cell-surface receptor with intracellular domains that are associated with membrane-bound enzymes.

Extracellular domain

Region of a cell-surface receptor that is located on the cell surface.

G-protein-linked receptor

Cell-surface receptor that activates membrane-bound G-proteins to transmit a signal from the receptor to nearby membrane components.

Growth factor

Ligand that binds to cell-surface receptors and stimulates cell growth.

Inhibitor

Molecule that binds to a protein (usually an enzyme) and keeps it from functioning.

Inositol phospholipid

Lipid present at small concentrations in the plasma membrane that is converted into a second messenger; it has inositol (a carbohydrate) as its hydrophilic head group.

Inositol triphosphate (IP3)

Cleavage product of PIP2 that is used for signaling within the cell.

Intercellular signaling

Communication between a cell.

Internal receptor

(Also, intracellular receptor) receptor protein that is located in the cytosol of a cell and binds to ligands that pass through the plasma membrane.

Intracellular mediator

(Also, second messenger) small molecule that transmits signals within a cell.

Intracellular signaling

Communication within cells.

Ion channel-linked receptor

Cell-surface receptor that forms a plasma membrane channel, which opens when a ligand binds to the extracellular domain (ligand-gated channels).

Kinase

Enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule.

Ligand

Molecule produced by a signaling cell that binds with a specific receptor, delivering a signal in the process.

Neurotransmitter

Chemical ligand that carries a signal from one nerve cell to the next.

Paracrine signal

Signal between nearby cells that is delivered by ligands traveling in the liquid medium in the space between the cells.

Phosphatase

Enzyme that removes the phosphate group from a molecule that has been previously phosphorylated.

Phosphodiesterase

Enzyme that degrades cAMP, producing AMP, to terminate signaling.

Receptor

Protein in or on a target cell that bind to ligands.

Second messenger

Small, non-protein molecule that propagates a signal within the cell after activation of a receptor causes its release.

Signal integration

Interaction of signals from two or more different cell-surface receptors that merge to activate the same response in the cell.

Signal transduction

Propagation of the signal through the cytoplasm (and sometimes also the nucleus) of the cell.

Signaling cell

Cell that releases signal molecules that allow communication with another cell.

Signaling pathway

(Also signaling cascade) chain of events that occurs in the cytoplasm of the cell to propagate the signal from the plasma membrane to produce a response.

Synaptic signal

Chemical signal (neurotransmitter) that travels between nerve cells.

Target cell

Cell that has a receptor for a signal or ligand from a signaling cell.