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The Cellular Internet
Trillions of cells in a multicellular organism must communicate together to enable growth, survival, and reproduction
The same cell-signaling mechanisms tend to show up again and again
Concept 11.1
Yeast cells use cell signaling to identify mates- two sexes, type a and type a- cells secrete a chemical signal that binds to receptors on the opposite cell- causes the two cells to grow toward each other so they can fuse and mate
Concept 11.1
Signal transduction pathway: process by which a signal on a cell’s surface is converted into a specific cellular response
Concept 11.1Receptor
factor
a factor
a
a
Exchangeof matingfactors
Yeast cell,mating type a
Yeast cell,mating type
Mating
New a/cell
a/
1
2
3
Concept 11.1
Some cells communicate through direct contact- use of cell junctions- animal cells use membrane-bound cell surface molecules
Concept 11.1Plasma membranes
Gap junctionsbetween animal cells
(a) Cell junctions
Plasmodesmatabetween plant cells
(b) Cell-cell recognition
Concept 11.1
Some molecules only travel a short distance- local regulators (ex. growth factors)
Synaptic signaling is found in the nervous system- electrical signal triggers the release of neurotransmitters across a synapse
Concept 11.1
Hormones are used for long-distance signaling- specialized cells release hormones into the circulatory system to travel to other parts of the body
Concept 11.1
Local signaling
Target cell
Secretingcell
Secretoryvesicle
Local regulatordiffuses throughextracellular fluid
(a) Paracrine signaling (b) Synaptic signaling
Target cellis stimulated
Neurotransmitter diffuses across synapse
Electrical signalalong nerve celltriggers release ofneurotransmitter
Long-distance signaling
Endocrine cell Bloodvessel
Hormone travelsin bloodstreamto target cells
Targetcell
(c) Hormonal signaling
Concept 11.1
The receiving end of a signal can be organized into three stages:
Reception- a chemical signal is bound to a receptor protein at the cell’s surface
Transduction- converts the signal to a form that can produce a cellular response
Concept 11.2
Reception - the receptor protein on a target
cell allows the cell to “hear” the message from a signal- the signal is complementary in shape and attaches to the protein- the receptor protein changes shape, and thus is activated
Concept 11.2
Receptors can be intracellular or plasma membrane bound
To reach the intracellular receptors, messengers must be able to pass through the plasma membrane (either small or hydrophobic)- ex. steroid hormones or NO (nitric oxide)
Concept 11.2
Most water soluble molecules are going to bind to plasma membrane receptors- receptor transmits information from the outside environment to the inside of the cell
Concept 11.2
G protein-coupledreceptor
Plasmamembrane
EnzymeG protein(inactive)
GDP
CYTOPLASM
Activatedenzyme
GTP
Cellular response
GDP
P i
Activatedreceptor
GDP GTP
Signaling moleculeInactiveenzyme
1 2
3 4
Concept 11.3
Transduction- usually a multi-step process- benefit in that it greatly amplifies a signal
Protein kinase: an enzyme that transfers phosphate groups from ATP to a protein
Concept 11.3Signaling molecule
Receptor Activated relay
molecule
Inactiveprotein kinase
1Activeproteinkinase
1Inactiveprotein kinase
2
ATPADP Active
proteinkinase
2
P
PPP
Inactiveprotein kinase
3
ATPADP Active
proteinkinase
3
P
P PPi
ATPADP P
ActiveproteinPPP i
Inactiveprotein
Cellularresponse
Phosphorylation
cascade
i
Concept 11.3
Many signaling pathways also involve small, nonprotein molecules called second messengers- ex. cyclic AMP (cAMP) carries a signal initiated by epinephrine from the plasma membrane of a liver cell into the cell’s interior
Concept 11.3
- the binding of epinephrine to a liver cell activates adenlyl cyclase, which in turn can synthesize many molecules of cAMP
Concept 11.3
First messenger
G proteinAdenylylcyclase
GTP
ATPcAMP Second
messenger
Proteinkinase A
G protein-coupledreceptor
Cellular responses
Concept 11.4
ResponseIn the liver cells, the final step
activates the enzyme that catalyzes the breakdown of glycogen into glucose
Many other signaling pathways regulate the synthesis of an enzyme by turning genes on and off
Concept 11.4
Signal Amplification- at each step in the cascade, the number of activated products is much greater than the preceding steps- b/c the proteins persist in an active form long enough to process numerous molecules
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