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Cell CommunicationCell Communication

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Communication Communication MethodsMethods Cell-to-cell contact Local signaling Long distance signaling

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Cell-to-Cell Cell-to-Cell CommunicationsCommunications Cell junctions directly connect the

cytoplasm of adjacent cells Ex: cardiac cells for rhythmicity

Surface receptors can give/send information Ex: specific immune response

Plasma membranes

Plasmodesmatabetween plant cells

Gap junctionsbetween animal cells

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Local SignalingLocal Signaling Adjacent cells are signaled.

Chemical messengers released Ex: Neurotransmitters via neurons

(a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid.

(b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell.

Local regulator diffuses through extracellular fluid

Target cell

Secretoryvesicle

Electrical signalalong nerve celltriggers release ofneurotransmitter

Neurotransmitter diffuses across

synapse

Target cellis stimulated

Local signaling

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Long Distance SignalingLong Distance Signaling Use of hormones

Both plants and animals use hormones (e.g. Insulin)

Can affect many cells in Other parts of the body

Protein or Steroid types Hormone travelsin bloodstreamto target cells

(c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells.

Long-distance signaling

Bloodvessel

Targetcell

Endocrine cell

Figure 11.4 C

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How Do Cells How Do Cells Communicate?Communicate? Signal Transduction Signal Transduction

PathwaysPathways Convert signals on a cell’s surface

into cellular responses Are similar in microbes and

mammals, suggesting an early origin

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EXTRACELLULARFLUID

Receptor

Signal molecule

Relay molecules in a signal transduction pathway

Plasma membraneCYTOPLASM

Activationof cellularresponse

Figure 11.5

3 Phases of Signal Transduction3 Phases of Signal Transduction

Reception1 Transduction2 Response3

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Step One - ReceptionStep One - Reception Reception occurs when a signal molecule

(ligand) binds to a receptor protein.Receptor protein is on the cell surface Ligand and receptor have a unique bonding

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Step Two - TransductionStep Two - Transduction Signal initiated by conformational change

of receptor protein Signal is turned into a cellular response. Signaling cascades relay signals to target Multistep pathways can amplify a signal

Second messengers involved

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Signal molecule

Activeproteinkinase

1

Activeproteinkinase

2

Activeproteinkinase

3

Inactiveprotein kinase

1

Inactiveprotein kinase

2

Inactiveprotein kinase

3

Inactiveprotein

Activeprotein

Cellularresponse

Receptor

P

P

P

ATPADP

ADP

ADP

ATP

ATP

PP

PP

PP

Activated relaymolecule

i

Phosphorylation cascade

P 

P

i

i

P

A phosphorylation cascade

Figure 11.8

A relay moleculeactivates protein kinase 1.

1

2 Active protein kinase 1transfers a phosphate from ATPto an inactive molecule ofprotein kinase 2, thus activatingthis second kinase.

Active protein kinase 2then catalyzes the phos-phorylation (and activation) ofprotein kinase 3.

3

Finally, active proteinkinase 3 phosphorylates aprotein (pink) that brings about the cell’s response tothe signal.

4 Enzymes called proteinphosphatases (PP)catalyze the removal ofthe phosphate groupsfrom the proteins, making them inactiveand available for reuse.

5

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ATP

GTP

cAMP

Proteinkinase A

Cellular responses

G-protein-linkedreceptor

AdenylylcyclaseG protein

First messenger(signal moleculesuch as epinephrine)

Cyclic AMP example…

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321

IP3 quickly diffuses throughthe cytosol and binds to an IP3–gated calcium channel in the ERmembrane, causing it to open.

4 The calcium ionsactivate the nextprotein in one or moresignaling pathways.

6 Calcium ions flow out ofthe ER (down their con-centration gradient), raisingthe Ca2+ level in the cytosol.

5

DAG functions asa second messengerin other pathways.

Phospholipase C cleaves aplasma membrane phospholipidcalled PIP2 into DAG and IP3.

A signal molecule bindsto a receptor, leading toactivation of phospholipase C.

EXTRA-CELLULARFLUID

Signal molecule(first messenger)

G protein

G-protein-linkedreceptor

Variousproteinsactivated

Endoplasmicreticulum (ER)

Phospholipase C

PIP2

IP3

(second messenger)

DAG

Cellularresponse

GTP

Ca2+

(second messenger)

Ca2+

IP3-gatedcalcium channel

Ex: Inositol P3

and calcium

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Step Three - ResponseStep Three - Response Cell signaling leads to regulation of

cytoplasmic activities or transcription

Signaling pathways regulate a variety of cellular activities

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Pathways can also regulate genes by activating transcription factors that turn genes on or off

Reception

Transduction

Response

mRNANUCLEUS

Gene

P

Activetranscriptionfactor

Inactivetranscriptionfactor

DNA

Phosphorylationcascade

CYTOPLASM

Receptor

Growth factor

Figure 11.14

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There are three main types of plasma membrane receptors:G-protein-linkedTyrosine kinasesIon channel

Types of ReceptorsTypes of Receptors

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G-protein-linked receptorsG-protein-linked receptors Very common, diverse functions Only results in single pathway

response

G-protein-linkedReceptor

Plasma Membrane

EnzymeG-protein(inactive)CYTOPLASM

Cellular response

Activatedenzyme

ActivatedReceptor

Signal molecule Inactivateenzyme

GDP

GDP

GTP

GTP

P i

GDP

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Receptor tyrosine kinasesReceptor tyrosine kinases Multiple pathway response Regulates/coordinates many cell

functionsSignalmolecule

Signal-binding site

CYTOPLASM

Tyrosines

Signal moleculeHelix in the

Membrane

TyrTyrTyr

TyrTyr

Tyr TyrTyrTyr

TyrTyr

Tyr

TyrTyrTyr

TyrTyr

Tyr TyrTyrTyr

TyrTyr

Tyr

TyrTyrTyr

TyrTyr

Tyr

DimerReceptor tyrosinekinase proteins(inactive monomers)

PPP

PP

P TyrTyrTyr

TyrTyr

TyrPPP

PP

PCellularresponse 1

Inactiverelay proteins

Activatedrelay proteins

Cellularresponse 2

Activated tyrosine-kinase regions(unphosphorylateddimer)

Fully activated receptortyrosine-kinase(phosphorylateddimer)

6 ATP 6 ADP

Figure 11.7

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Ion channel receptorsIon channel receptors

Cellularresponse

Gate open

Gate close

Ligand-gatedion channel receptor

Plasma Membrane

Signalmolecule(ligand)

Figure 11.7

Gate closed Ions

When ligand binds, channel can open or close.

Ex: neurotransmitters bind as ligands for ion channels

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Hormone(testosterone)

EXTRACELLULARFLUID

Receptorprotein

DNA

mRNA

NUCLEUS

CYTOPLASM

Plasmamembrane

Hormone-receptorcomplex

New protein

Figure 11.6

*Intracellular Receptors*Intracellular Receptors Target protein is INSIDE the cell Must be hydrophobic molecule

1 The steroid hormone testosterone passes through the plasma membrane.

The bound proteinstimulates thetranscription ofthe gene into mRNA.

4

The mRNA istranslated into aspecific protein.

5

Testosterone bindsto a receptor proteinin the cytoplasm,activating it.

2

The hormone-receptor complexenters the nucleusand binds to specific genes.

3

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Evolutionary Significance Unicellular and multicellular cell

communication have similarities Yeast cells signal for sexual

reproduction through signal transduction process.

Bacteria secrete molecules to sense density of own population. Quorum Sensing (survival purpose)

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Yeast Sexual ReproductionYeast Sexual Reproduction

factorReceptor

Exchange of mating factors. Each cell type secretes a mating factor that binds to receptors on the other cell type.

1

Mating. Binding of the factors to     receptors induces changes      in the cells that     lead to their     fusion.

New a/ cell. The nucleus of the fused cell includes all the genes from the a and a cells.

2

3

factorYeast cell,mating type a

Yeast cell,mating type

a/

a

a

Yeast cells identify their mates by cell

signaling.

Suggests early evidence of cell

signaling.