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Overview: The Cellular Internet
Cell-to-cell communication is essential for organisms Biologists have discovered some universal strategies and mechanisms of cellular regulation Response is determined by combined effects of multiple signals that are received
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Yeast Mating-a classic system
factor Receptor 1 Exchange of mating factors a a factor Yeast cell, mating type a Yeast cell, mating type 2 Mating a Figure 11.2 Communication between mating yeast cells An example of cell signalling New a/ cell a/ 3
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Yeast Mating System Exemplifies Cell Communication
Aka Signal Transduction Simple but all the key features are present Reception, Transduction, Response
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A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response
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Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes The concentration of signaling molecules allows bacteria to detect population density Quorum-sensing in Vibrio fischerii
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When cells are touching signalling is easy Plasma membranes
Gap junctions between animal cells Plasmodesmata between plant cells (a) Cell junctions Figure 11.4 Communication by direct contact between cells (b) Cell-cell recognition
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In many other cases, cells must share information with cells they are not touching
They use (1) local regulators, messenger molecules that travel only short distances And (2) chemicals called hormones for long-distance signaling
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Local signaling in animals is either paracrine or synaptic
Target cell Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse Secreting cell Secretory vesicle Figure 11.5 Local and long-distance cell communication in animals Local regulator diffuses through extracellular fluid Target cell is stimulated (a) Paracrine signaling (b) Synaptic signaling Both are short-range but synaptic signaling occurs only in the nervous system
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The messenger molecules are called hormones
Long-distance signaling usually involves different organs communicating Endocrine cell Blood vessel Hormone travels in bloodstream to target cells Figure 11.5 Local and long-distance cell communication in animals Target cell (2) Hormonal signaling The messenger molecules are called hormones
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There are three stages in cell signaling
Reception Transduction Response EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Figure 11.6 Overview of cell signaling Signaling molecule
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RECEPTION TRANSDUCTION RESPONSE
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The binding between a signal molecule (ligand) and receptor is highly specific
A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins 3 examples from many possible
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Reception- Example I A G protein-coupled receptor is a plasma membrane receptor that works with the help of a G protein The G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive GTP/GDP are chemically very similar to ATP/ADP but contain Guanine not Adenine
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Mechanism of G protein coupled reception
Plasma membrane G protein-coupled receptor Inactive enzyme Activated receptor Signaling molecule GDP G protein (inactive) Enzyme GDP GTP CYTOPLASM 1 2 Activated enzyme Figure 11.7 Membrane receptors—G protein-coupled receptors, part 2 GTP GDP P i Cellular response 3 4 The relay protein is called a “G Protein”
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Reception- Example II Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosines Has intra- and extracellular domains, also membrane domain A receptor tyrosine kinase can trigger multiple signal transduction pathways at once Kinase is an enzyme that attaches a phosphate to a substrate
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Fully activated receptor tyrosine kinase
Signaling molecule (ligand) Ligand-binding site Signaling molecule Helix Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosine kinase proteins Dimer CYTOPLASM 1 2 Activated relay proteins Figure 11.7 Membrane receptors—receptor tyrosine kinases Cellular response 1 Tyr Tyr P Tyr Tyr P Tyr Tyr P P Tyr Tyr P Tyr Tyr P Tyr Tyr P P Cellular response 2 Tyr Tyr P Tyr Tyr P Tyr P Tyr P 6 ATP 6 ADP Activated tyrosine kinase regions Fully activated receptor tyrosine kinase Inactive relay proteins 3 4
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Reception- Example III
A ligand-gated ion channel receptor acts as a gate when the receptor changes shape When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor
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Ligand- gated ion channel 1 Signaling molecule (ligand) Gate closed
Ions Ligand- gated ion channel Plasma membrane Ligand-gated ion channel receptor 2 Gate open Cellular response Figure 11.7 Membrane receptors—ion channel receptors 3 Gate closed
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An example of a response
Reception Signal Transduction: an example of a key type of transduction pathway and response An example of a response This type of signal transduction pathway is called a cascade Binding of epinephrine to G protein-coupled receptor (1 molecule) Transduction Inactive G protein Active G protein (102 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (102) ATP Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104) Cyclic AMP is the key inside the cell. Figure Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine Inactive phosphorylase kinase Active phosphorylase kinase (105) Inactive glycogen phosphorylase (Do not memorize this cascade) Active glycogen phosphorylase (106) Response Glycogen Glucose-1-phosphate (108 molecules)
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Cyclic AMP is the “second messenger”
Adenylyl cyclase Phosphodiesterase Pyrophosphate P P i ATP cAMP AMP Figure Cyclic AMP Cyclic AMP is the “second messenger”
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Ethylene Response in plants
Identify: Reception, Transduction, Response
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