Chapter 11 – Cell Communication
Cell Identity
tissues - highly specialized cell groups found only in multicellular organisms
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each tissue cell performs only the functions of that tissue
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cells gain their identities by controlling the expression of the genes
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only specific sets of genes are turned on
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tissue-specific identity markers - mark cell surfaces as a particular type
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cells of the same tissue type form connections when they recognize each other
glycolipids - lipids w/ carbohydrate heads
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accounts for majority of tissue-specific surface markers
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responsible for differences between blood types
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MHC proteins - distinguishes cells of the organism from foreign cells
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single-pass proteins anchored in the plasma membrane
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immune system cells destroy cells w/o the correct identity markers
intercellular adhesion - cells usually in physical contact w/ each other at all times
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cell junctions - permanent/long-lasting connections between cells
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tissue functions depend on how the cells connect
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3 main types of connections - tight junctions, anchoring junctions, communicating junctions
tight junctions - aka occluding junctions
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connect plasma membranes of adjacent cells in a sheet
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prevent small molecules from leaking between cells
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digestive tracts only 1 cell thick, but still prevents food from passing through due to tight junctions
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prevents certain proteins from drifting from 1 side to another
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food enters the blood stream by going through the transport proteins
anchoring junctions - mechanically attach the cytoskeletons
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most common in muscles and skin
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desmosomes - connect cytoskeletons of adjacent cells
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hemidesmosomes - connect epithelial cells to basement membrane
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connections between proteins not tethered to intermediate filaments not as strong as connections between tethered proteins
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cadherins - mostly single-pass transmembrane glycoproteins
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forms the link in the anchoring junction
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can also connect actin filaments of adjacent cells
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may have a role in determining where migrating cells go during development
adherens junctions - connects actin filaments of neighboring cells or to extracellular matrix
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integrins - proteins that bind to a protein part of extracellular matrix
communicating junctions - direct connections between adjacent cells used for communication
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chemical/electrical signals pass directly from 1 cell to another
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some small molecules/ions can also pass through
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gap junctions - communicating junctions in animals
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made up of connexons (complexes of 6 transmembrane proteins arranged in a circle)
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forms when connexons line up perfectly
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small enough to prevent large molecules like proteins from passing through
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holds plasma membranes of adjacent cells about 4 nm apart
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can open/close in response to environment
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plasmodesmata - communicating junctions in plants
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occurs at holes/gaps in the cell wall
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more complex than gap functions
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lined w/ plasma membrane
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contains a central tubule that connects the ER of 2 cells
Receptors Types
Intracellular receptors - protein receptors within the cell
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signal molecules are usually lipid-soluble or very small in order to pass through the membrane
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gene regulating receptors - has binding site for DNA
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inhibitor protein may prevent DNA from binding
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either activates or suppresses certain genes after binding to DNA
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response varies depending on the cell
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lipid-soluble signal molecules tend to last longer than water-soluble signals
regulators as enzymes - catalyzes reactions when activated
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nitric oxide binds to guanylyl cyclase, catalyzes synthesis of GMP (messenger molecule that relaxes smooth
muscle cells)
cell surface receptors - accounts for the majority of a cell's receptors
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turns extracellular signals into intracellular ones
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water-soluble signals can't pass through the membrane, must bind w/ surface receptors
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chemically gated ion channels - allow ions through
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opens only when a neurotransmitter binds to it
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shape/charge of channel determines what type of ion goes through it
enzymic receptors - activates an enzyme when binding to a signal molecule
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protein kinases - enzymes that add phosphate groups to proteins
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binds to signal molecule outside the school, enzyme activity occurs in the cytoplasm
G-protein linked receptors - uses GTP-binding protein to indirectly act on enzymes/ion channels
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starts a diffusible signal within the cell
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has short duration
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G-protein changes shape, leaves receptor once signal molecule arrives
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GTP can start few events, turns into GDP+phosphate very quickly
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pathway shuts down if signals stop coming in
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threads back and forth across the membrane 7 times (7-pass transmembrane protein)
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more of these surface receptors than any other kind
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may have evolved from sensory receptors of prokaryotes
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Rodbell/Gilman - received Noble prize for work w/ G-proteins
Signaling Between/Through Cells
intercellular communication - lacking in most prokaryotes/protists
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uses many different molecules to communicate
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dissolved gasses like nitric oxide can also be used as signals
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signal molecules either attached to surface, secreted through plasma membrane, or released by exocytosis
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receptor proteins - have 3D shapes that fit the shape of a specific signal molecule
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signal molecule and receptor protein bind, changing the shape of the protein
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change in protein shape >> response within the cell
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hard to find, can make up less than 0.01% of a cell's mass
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immunochemistry - uses antibodies to target/isolate specific molecules/proteins
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molecular genetics - intentionally creates mutations in genes
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receptor malfunction is very evident, more easily seen
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determines relationship between protein structures and cellular functions
types of cell signaling - 4 basic mechanisms for communication between cells
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autocrine signaling - cells sending signals to themselves; may reinforce developmental changes
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direct contact - when cells are actually close enough to touch each other
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paracrine signaling - released molecules that only influence cells in close vicinity
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endocrine signaling - uses hormones, which lasts longer in the circulatory system
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synaptic signaling - used by animals' nervous systems
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neurotransmitters - don't travel through the circulatory system; released by nerve cells to very close target cells
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chemical synapse - association of a neuron and its target cell
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neurotransmitters pass across the synaptic gap, last very briefly
second messengers - substances used to relay message from receptors to inside the cytoplasm
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alter the behavior of certain proteins by binding to them, changing their shape
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cyclic AMP (cAMP) - used by all animal cells
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produced by adenylyl cyclase when started by G-protein
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activates the alpha-kinase enzyme, adding phosphates to certain proteins
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works in muscle cells to make more glucose available
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calcium ion - serves as 2nd messengers though found in low levels inside the cell
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levels are much higher outside the cell
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gated channels controlled by G-proteins allow Ca++ in to start certain activities
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IP3 made from phospholipids and phospholipase binds to ER to let Ca++ into the cytoplasm
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binds to calmodulin (148-amino-acid protein w/ 4 binding sites for C++) to activate other proteins
protein kinase cascades - chains of protein messengers used to relay messages to the nucleus
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usually starts w/ phosphorylating a stage 1 protein
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each stage protein activates a large number of proteins in the next stage, and so forth
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different signals may use some of the same messengers, but ultimately have different targets
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vision amplification cascade - starts w/ light activating rhodopsin (a G-protein)
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rhodopsin activates hundreds of transducin (another G-protein)
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each transducin causes phosphodiesterase enzyme to change thousands of cyclic GMP
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human rod cells sensitive enough to detect brief flashes of just 5 photons
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cell division amplification cascade - starts w/ phosphorylating ras (a protein kinase)
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ras proteins activate series of phosphorylation, leading to division
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1/3 of cancers involve a mutation in the ras protein gene, causing unrestrained growth