Cell CommunicationChapter 11

11.1External signals are converted to responses within a cell

Why cell communication?

Cells must “talk” to coordinate activities

Evolved in single and multicellular organisms◦Ex: quorum sensing

in bacteria◦Ex: hormones in

plants and animals

Signaling by Distance

Cell to cell contact

Local◦Paracrine◦Synaptic

Long distance◦Endocrine

Signal Transduction Pathways

Receiving end of cell “conversation”

11.2Reception: A signaling molecule binds to a receptor protein, causing it to change shape

Reception

Chemical signal is detected by the target cell

Surface proteins or intracellular receptor

Ligand◦Signaling molecule that

binds specifically to another molecule

Reception: G Protein-Coupled Receptor

Ligand binds to G protein-coupled receptor on membrane

G protein becomes activated

Activated G protein binds to enzyme, activating it

G protein receptor is COUPLED with G protein

Reception: Receptor Tyrosine Kinases

Ligand binds to receptor tyrosine kinase protein monomers◦Kinase: enzyme that

transfers phosphate groups

Activated monomers form dimer

Phosphates from ATP added to activated dimer

Reception: Ion Channel Receptors

Ligand gated ion channel changes shape when ligand binds

Opens “gate” so ions can cross membrane

Reception: Intracellular Receptors

Receptor in cytoplasm or nucleus (NOT cell membrane)

Signal is hydrophobic or small enough to cross membrane◦Ex: steroid hormones, nitric

oxide

11.3Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell

Transduction

Converts the signal to a form that the cell can respond to

Often involves relay molecules

Transduction: Phosphorylation Cascades

Series of proteins activated by addition of phosphate group pass signal along

Like falling dominoes

Transduction: Secondary Messengers

Non-protein messengers pass signal along◦Ex: cyclic AMP (cAMP), Ca+, or IP3

11.4Response: Cell signaling leads to regulation of transcription or cytoplasmic activities

Response

Specific response by the cell

Almost any imaginable cellular activity turned on or off, depending on signal

Usually regulate enzyme activity

Response: Nuclear

Genes turned on to make proteins◦Activates

transcription

Genes turned off to stop making proteins◦Stops

transcription

Response: Cytoplasmic

Proteins made are modified, amplified, or terminated◦Translation of genes

modified, resulting proteins modified

Example: stimulation of glycogen breakdown by epinephrine

Cell Signaling Specificity

Which receptors and secondary messengers a cell has determines which signals it will respond to and how◦Ex: liver and heart cells respond differently to

epinephrine

11.5Apoptosis integrates multiple cell-signaling pathways

Apoptosis

Programmed cell deathPart of normal development and

differentiation◦Ex: formation of fingers and toes◦Ex: cancer cells

Complex cell signaling pathways